mozilla

WebIDL bindings

在本文章中
  1. 向一个 class 添加 WebIDL 绑定
  2. C++ reflections of WebIDL constructs
    1. C++ reflections of WebIDL operations (methods)
    2. C++ reflections of WebIDL attributes
    3. C++ reflections of WebIDL constructors
    4. C++ reflections of WebIDL types
      1. any
      2. boolean
      3. Integer types
      4. Floating point types
      5. DOMString
      6. ByteString
      7. object
      8. Interface types
      9. Dictionary types
      10. Enumeration types
      11. Callback function types
      12. Sequences
      13. Arrays
      14. Union types
      15. Date
    5. Stringifiers
    6. Legacy Callers
    7. Named getters
  3. Throwing exceptions from WebIDL methods, getters, and setters
  4. Custom extended attributes
    1. [ChromeOnly]
    2. [Pref=prefname]
    3. [Func="funcname"]
    4. [AvailableIn=Where]
    5. [CheckPermissions="list of permissions"]
    6. [Throws][GetterThrows][SetterThrows]
    7. [Pure]
    8. [Constant]
    9. [NeedNewResolve]
    10. [HeaderFile="path/to/headerfile.h"]
    11. [JSImplementation="@mozilla.org/some-contractid;1"]
    12. [NavigatorProperty="propName"]
    13. [StoreInSlot]
    14. [Cached]
    15. [Frozen]
    16. [ChromeConstructor]
  5. Helper objects
    1. Nullable<T>
    2. Optional<T>
    3. NonNull<T>
    4. OwningNonNull<T>
    5. Typed arrays, arraybuffers, array buffer views
    6. Sequence<T>
    7. CallbackFunction
    8. CallbackInterface
    9. DOMString
    10. GlobalObject
    11. Date
    12. ErrorResult
  6. Bindings.conf details
    1. How to get a JSContext passed to a given method
  7. Implementing WebIDL using Javascript
    1. Creating JS-implemented WebIDL objects
      1. Using the WebIDL constructor
      2. Using a _create method  
      3. By returning a chrome-side object from a JS-implemented WebIDL method
    2. Implementing a WebIDL object in JavaScript
    3. Checking for Permissions or Preferences
    4. Example
    5. Guarantees provided by bindings
    6. Accessing the content object from the implementation
    7. Determining the principal of the caller that invoked the WebIDL API
    8. Throwing exceptions from JS-implemented APIs
    9. Inheriting from interfaces implemented in C++

Notes: Need to document the setup for indexed and named getters/setters/creators/deleters.

在构建时产生 WebIDL 绑定需要两个条件:当前的 WEBIDL 文件和描述 WEBIDL 如何映射为 Gecko 内部代码的元数据配置文件。

所有的 WEBIDL 应该放置在 dom/webidl 中并将文件名添加到目录下的 moz.build 文件中的列表中。

注意如果你添加了新的接口,关于 dom/tests/mochitest/general/test_interfaces.html 很可能会失败。这就表示你需要从 DOM peer 出发 review 一下。 Resist the urge to just add your interfaces to the list without the review, it will just annoy the DOM peers and they'll make you get the review anyway.

配置文件 dom/bindings/Bindings.conf, 只是一个简单的 Python dict ,它会将接口名称与接口信息,descriptor , 相映射。映射时会有各种可能的选项,用来处理各种 edge case, 而绝大多数 descriptors 则是非常简单的。

所有产生出的代码都放置在 mozilla::dom 命名空间中。对每个接口来说, a namespace whose name is the name of the interface with Binding appended is created, and all the things pertaining to that interface's binding go in that namespace.

有许多 dom/bindings 中的 helper 对象和工具方法同样在 mozilla::dom 命名空间中,其头文件也暴露在 mozilla/dom 中。

向一个 class 添加 WebIDL 绑定

要向 MyInterface 接口添加 WebIDL 绑定,并且在 mozilla::dom::MyInterface class 实现这个接口, 需要执行下面的步骤:

  1. If your interface doesn't inherit from any other interfaces, Inherit from nsWrapperCacheand hook up the class to the cycle collector so it will trace the wrapper cache properly, and call SetIsDOMBinding() in the constructor of the derived class.  Note that you may not need to do this if your objects can only be created, never gotten from other objects.  If you also inherit from nsISupports, make sure the nsISupports comes before thensWrapperCache in your list of parent classes.  If your interface does inherit from another interface, just inherit from the C++ type that other interface corresponds to.
  2. Implement a GetParentObject override that, for a given instance of your class, returns the same object every time (unless you write explicit code that handles your parent object changing by reparenting JS wrappers, as nodes do).  The idea is that walking theGetParentObject chain will eventually get you to a Window, so that every WebIDL object is associated with a particular Window.  For example,nsINode::GetParentObject returns the node's owner document.  The return value ofGetParentObject must either singly-inherit from nsISupports or have a corresponding ToSupports() method that can produce an nsISupports from it.  If many instances of MyInterface are expected to be created quicky, the return value ofGetParentObject should itself inherit from nsWrapperCache for optimal performance.  Returning null from GetParentObject is allowed in situations in which it's OK to associate the resulting object with a random global object for security purposes; this is not usually ok for things that are exposed to web content.  Again, if you do not need wrapper caching you don't need to do this.
  3. Add the WebIDL for MyInterface in dom/webidl and to the list indom/webidl/moz.build.
  4. Add an entry to dom/bindings/Bindings.conf that sets some basic information about the implementation of the interface.  If the C++ type is notmozilla::dom::MyInterface, you need to set the 'nativeType' to the right type.  If the type is not in the header file one gets by replacing '::' with '/' and appending '.h', then add a corresponding 'headerFile' annotation (or HeaderFile annotation to the .webidl file).  If you don't have to set any annotations, then you don't need to add an entry either and the code generator will simply assume the defaults here.
  5. Add external interface entries to Bindings.conf for whatever non-WebIDL interfaces your new interface has as arguments or return values.
  6. Implement a WrapObject override on mozilla::dom::MyInterface that just calls through to mozilla::dom::MyInterfaceBinding::Wrap.  Note that if your C++ type is implementing multiple distinct Web IDL interfaces, you need to choose whichmozilla::dom::MyInterfaceBinding::Wrap to call here.  SeeAudioContext::Wrap for example.
  7. Expose whatever methods the interface needs on mozilla::dom::MyInterface.  These can be inline, virtual, have any calling convention, and so forth, as long as they have the right argument types and return types.  You can see an example of what the function declarations should look like by running mach webidl-example MyInterface. This will produce two files in dom/bindings in your objdir:MyInterface-example.h and MyInterface-example.cpp, which show a basic implementation of the interface, using a class that inherits from nsISupports and has a wrapper cache.

See this sample patch that migrates window.performance.* to WebIDL bindings.

注意: If your object can only be reflected into JS by creating it, not by retrieving it from somewhere, you can skip steps 1 and 2 above and instead add 'wrapperCache': Falseto your descriptor.  If your object already has classinfo, it should be using thensNewDOMBindingNoWrapperCacheSH scriptable helper in this case.  You will need to flag the functions that return your object as [NewObject] in the WebIDL.

C++ reflections of WebIDL constructs

C++ reflections of WebIDL operations (methods)


A WebIDL operation is turned into a method call on the underlying C++ object.  The return type and argument types are determined as described below.  In addition to those, all methods that are allowed to throw will get an ErrorResult& argument appended to their argument list.  Methods that use certain WebIDL types like any or object will get a JSContext* argument prepended to the argument list.  Static methods will be passed a const GlobalObject& for the relevant global.  This argument comes before the JSContext* argument, if any.

The name of the C++ method is simply the name of the WebIDL operation with the first letter converted to uppercase.

WebIDL overloads are turned into C++ overloads: they simply call C++ methods with the same name and different signatures.

For example, this webidl:

interface MyInterface { void doSomething(long number); double doSomething(MyInterface? otherInstance); [Throws] MyInterface doSomethingElse(optional long maybeNumber); [Throws] void doSomethingElse(MyInterface otherInstance); void doTheOther(any something); static void staticOperation(any arg); };

will require these method declarations:

class MyClass
{
  void DoSomething(int32_t aNumber);
  double DoSomething(MyClass* aOtherInstance);

  already_AddRefed<MyInterface> DoSomethingElse(Optional<int32_t> aMaybeNumber,
                                                ErrorResult& rv);
  void DoSomethingElse(MyClass& aOtherInstance, ErrorResult& rv);
  
  void DoTheOther(JSContext* cx, JS::Value aSomething);

  static void StaticOperation(const GlobalObject& aGlobal, JSContext* cx, JS::Value aSomething);
}
 
 
 
 
 
 
 
 
 
 
 
 
 

C++ reflections of WebIDL attributes

A WebIDL attribute is turned into a pair of method calls for the getter and setter on the underlying C++ object.  A readonly attribute only has a getter and no setter.

The getter's name is the name of the attribute with the first letter converted to uppercase.  This has Get prepended to it if any of these conditions hold:

  1. The type of the attribute is nullable.
  2. The getter can throw.
  3. The return value of the attribute is returned via an out parameter in the C++.

The method signature for the getter looks just like an operation with no arguments and the attribute's type as the return type.

The setter's name is Set followed by the name of the attribute with the first letter converted to uppercase.  The method signature looks just like an operation with a void return value and a single argument whose type as the attribute's type.

C++ reflections of WebIDL constructors

A WebIDL constructor is turned into a static class method named Constructor.  The arguments of this method will be the arguments of the WebIDL constructor, with a const GlobalObject& for the relevant global prepended.  For the non-worker case, the global is typically the inner window for the DOM Window the constructor function is attached to.  If aJSContext* is also needed due to some of the argument types, it will come after the global.  The return value of the constructor for MyInterface is exactly the same as that of a method returning an instance of MyInterface. Constructors are always allowed to throw.

For example, this IDL:

[Constructor, Constructor(unsigned long someNumber)] interface MyInterface { };

will require the following declarations in MyClass:

class MyClass {
  // Various nsISupports stuff or whatnot
  static
  already_AddRefed<MyClass> Constructor(const GlobalObject& aGlobal,
                                        ErrorResult& rv);
  static
  already_AddRefed<MyClass> Constructor(const GlobalObject& aGlobal,
                                        uint32_t aSomeNumber,
                                        ErrorResult& rv);  
};
 
 
 
 
 
 
 
 
 
 

C++ reflections of WebIDL types

The exact C++ representation for WebIDL types can depend on the precise way that they're being used: e.g. return values, arguments, and sequence or dictionary members might all have different representations.

Unless stated otherwise, a type only has one representation.  Also, unless stated otherwise, nullable types are represented by wrapping Nullable<> around the base type.

In all cases, optional arguments which do not have a default value are represented by wrappingconst Optional<>& around the representation of the argument type.  If the argument type is a C++ reference, it will also become a NonNull<> around the actual type of the object in the process.  Optional arguments which do have a default value are just represented by the argument type itself, set to the default value if the argument was not in fact passed in.

Variadic WebIDL arguments are treated as a const Sequence<>& around the actual argument type.

any

any is represented in three different ways, depending on use:

  • any arguments become JS::Handle<JS::Value>.
  • any return values become a JS::MutableHandle<JS::Value> out param appended to the argument list.  This comes after all IDL arguments, but before the ErrorResult&, if any, for the method.
  • any dictionary members and sequence elements become JS::Value.  The dictionary members and sequence elements are guaranteed to be marked by whoever puts the sequence or dictionary on the stack, using SequenceRooter and DictionaryRooter.

Methods using any always get a JSContext* argument.

For example, this WebIDL:

interface Test { attribute any myAttr; any myMethod(any arg1, sequence<any> arg2, optional any arg3); };

will correspond to these C++ function declarations:

void MyAttr(JSContext* cx, JS::MutableHandle<JS::Value> retval);
void SetMyAttr(JSContext* cx, JS::Handle<JS::Value> value);
void MyMethod(JSContext* cx, JS::Handle<JS::Value> arg1, 
              const Sequence<JS::Value>& arg2,
              const Optional<JS::Handle<JS::Value> >& arg3,
              JS::MutableHandle<JS::Value> retval);
 
 
 
 
 
 

boolean

The boolean WebIDL type is represented as a C++ bool.

For example, this WebIDL:

interface Test { attribute boolean myAttr; boolean myMethod(optional boolean arg); };

will correspond to these C++ function declarations:

bool MyAttr();
void SetMyAttr(bool value);
JS::Value MyMethod(const Optional<bool>& arg);
 
 
 

Integer types

Integer WebIDL types are mapped to the corresponding C99 stdint types.

For example, this WebIDL:

interface Test { attribute short myAttr; long long myMethod(unsigned long? arg); };

will correspond to these C++ function declarations:

int16_t MyAttr();
void SetMyAttr(int16_t value);
int64_t MyMethod(const Nullable<uint32_t>& arg);
 
 
 

Floating point types

Floating point WebIDL types are mapped to the C++ type of the same name.  So float andunrestricted float become a C++ float, while double and unrestricted doublebecome a C++ double.

For example, this WebIDL:

interface Test { float myAttr; double myMethod(unrestricted double? arg); };

will correspond to these C++ function declarations:

float MyAttr();
void SetMyAttr(float value);
double MyMethod(const Nullable<double>& arg);
 
 
 

DOMString

Strings are reflected in three different ways, depending on use:

  • String arguments become const nsAString&.
  • String return values become a mozilla::dom::DOMString& out param appended to the argument list.  This comes after all IDL arguments, but before the ErrorResult&, if any, for the method.  Note that this allows callees to declare their methods as taking annsAString& or nsString& if desired.
  • Strings in sequences, dictionaries, owning unions, and variadic arguments becomensString.

Nullable strings are represented by the same types as non-nullable ones, but the string will return true for DOMStringIsNull().  Returning null as a string value can be done usingSetDOMStringToNull on the out param if it's an nsAString or calling SetNull() on aDOMString.

For example, this WebIDL:

interface Test { DOMString myAttr; [Throws] DOMString myMethod(sequence<DOMString> arg1, DOMString? arg2, optional DOMString arg3); };

will correspond to these C++ function declarations:

void GetMyAttr(nsString& retval);
void SetMyAttr(const nsAString& value);
void MyMethod(const Sequence<nsString>& arg1, const nsAString& arg2,
              const Optional<nsAString>& arg3, nsString& retval, ErrorResult& rv);
 
 
 
 

ByteString

ByteString is reflected in three different ways, depending on use:

  • ByteString arguments become const nsACString&.
  • ByteString return values become an nsCString& out param appended to the argument list.  This comes after all IDL arguments, but before the ErrorResult&, if any, for the method.
  • ByteString in sequences, dictionaries, owning unions, and variadic arguments becomes nsCString.

Nullable ByteString are represented by the same types as non-nullable ones, but the string will return true for IsVoid().  Returning null as a string value can be done using SetIsVoid()on the out param.

object

object is represented in three different ways, depending on use:

  • object arguments become JS::Handle<JSObject*>.
  • object return values become a JS::MutableHandle<JSObject*> out param appended to the argument list.  This comes after all IDL arguments, but before theErrorResult&, if any, for the method.
  • object dictionary members and sequence elements become JSObject*.  The dictionary members and sequence elements are guaranteed to be marked by whoever puts the sequence or dictionary on the stack, using SequenceRooter andDictionaryRooter.

Methods using object always get a JSContext* argument.

For example, this WebIDL:

interface Test { object myAttr; object myMethod(object arg1, object? arg2, sequence<object> arg3, optional object arg4, optional object? arg5); };

will correspond to these C++ function declarations:

void GetMyAttr(JSContext* cx, JS::MutableHandle<JSObject*> retval);
void SetMyAttr(JSContext* cx, JS::Handle<JSObject*> value);
void MyMethod(JSContext* cx, JS::Handle<JSObject*> arg1, JS::Handle<JSObject*> arg2,
              const Sequence<JSObject*>& arg3,
              const Optional<JS::Handle<JSObject*> >& arg4,
              const Optional<JS::Handle<JSObject*> >& arg5,
              JS::MutableHandle<JSObject*> retval);
 
 
 
 
 
 
 

Interface types

There are four kinds of interface types in the WebIDL bindings.  Callback interfaces are used to represent script objects that browser code can call into.  External interfaces are used to represent objects that have not been converted to the WebIDL bindings yet.  WebIDL interfaces are used to represent WebIDL binding objects.  "SpiderMonkey" interfaces are used to represent objects that are implemented natively by the JavaScript engine (e.g. typed arrays).

Callback interfaces

Callback interfaces are represented in C++ as objects inheriting frommozilla::dom::CallbackInterface, whose name, in the mozilla::dom namespace, matches the name of the callback interface in the WebIDL.  The exact representation depends on how the type is being used.

  • Nullable arguments become Foo*.
  • Non-nullable arguments become Foo&.
  • Return values become already_AddRefed<Foo> or Foo* as desired.  The pointer form is required if the method or property involved is flagged as resultNotAddRefed inBindings.conf.
  • WebIDL callback interfaces in sequences, dictionaries, owning unions, and variadic arguments are represented by nsRefPtr<Foo> if nullable and OwningNonNull<Foo>otherwise.

If the interface is a single-opertion interface, the object exposes two methods that both invoke the same underlying JS callable.  The first of these methods allows the caller to pass in a thisobject, while the second defaults to undefined as the this value.  In either case, the thisvalue is only used if the callback interface is implemented by a JS callable.  If it's implemented by an object with a property whose name matches the operation, the object itself is always used as this.

If the interface is not a single-operation interface, it just exposes a single method for every IDL method/getter/setter.

The signatures of the methods correspond to the signatures for throwing IDL methods/getters/setters with an additional trailing "mozilla::dom::CallbackObject::ExceptionHandling aExceptionHandling" argument, defaulting to eReportExceptions.  If aReportExceptions is set toeReportExceptions, the methods will report JS exceptions before returning.  IfaReportExceptions is set to eRethrowExceptions, JS exceptions will be stashed in theErrorResult and will be reported when the stack unwinds to wherever the ErrorResult was set up.

For example, this WebIDL:

callback interface MyCallback { attribute long someNumber; short someMethod(DOMString someString); }; callback interface MyOtherCallback { // single-operation interface short doSomething(Node someNode); }; interface MyInterface { attribute MyCallback foo; attribute MyCallback? bar; };

will lead to these C++ class declarations, in the mozilla::dom namespace:

class MyCallback : public CallbackInterface
{
  int32_t GetSomeNumber(ErrorResult& rv, ExceptionHandling aExceptionHandling = eReportExceptions);
  void SetSomeNumber(int32_t arg, ErrorResult& rv,
                     ExceptionHandling aExceptionHandling = eReportExceptions);
  int16_t SomeMethod(const nsAString& someString, ErrorResult& rv,
                     ExceptionHandling aExceptionHandling = eReportExceptions);
};

class MyOtherCallback : public CallbackInterface
{
public:
  int16_t
  DoSomething(nsINode& someNode, ErrorResult& rv,
              ExceptionHandling aExceptionHandling = eReportExceptions);

  template<typename T>
  int16_t
  DoSomething(const T& thisObj, nsINode& someNode, ErrorResult& rv,
              ExceptionHandling aExceptionHandling = eReportExceptions);
};
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

and these C++ function declarations on the implementation of MyInterface:

already_AddRefed<MyCallback> GetFoo(); void SetFoo(MyCallback&); already_AddRefed<MyCallback> GetBar(); void SetBar(MyCallback*);
External interfaces

External interfaces are represented in C++ as objects that XPConnect knows how to unwrap to.  This can mean XPCOM interfaces (whether declared in XPIDL or not) or it can mean some type that there's a castable native unwrapping function for.  The C++ type to be used should be thenativeType listed for the external interface in the Bindings.conf file.  The exact representation depends on how the type is being used.

  • Arguments become nsIFoo*.
  • Return values become already_AddRefed<nsIFoo> or nsIFoo* as desired.  The pointer form is required if the method or property involved is flagged asresultNotAddRefed in Bindings.conf.
  • External interfaces in sequences, dictionaries, owning unions, and variadic arguments are represented by nsRefPtr<nsIFoo>.
WebIDL interfaces

WebIDL interfaces are represented in C++ as C++ classes.  The class involved must either be refcounted or must be explicitly annotated in Bindings.conf as being directly owned by the JS object.  If the class inherits from nsISupports, then the canonical nsISupports must be on the primary inheritance chain of the object.  If the interface has a parent interface, the C++ class corresponding to the parent must be on the primary inheritance chain of the object.  This guarantees that a void* can be stored in the JSObject which can then be reinterpret_castto any of the classes that correspond to interfaces the object implements.  The C++ type to be used should be the nativeType listed for the interface in the Bindings.conf file, ormozilla::dom::InterfaceName if none is listed.  The exact representation depends on how the type is being used.

  • Nullable arguments become Foo*.
  • Non-nullable arguments become Foo&.
  • Return values become already_AddRefed<Foo> or Foo* as desired.  The pointer form is required if the method or property involved is flagged as resultNotAddRefed inBindings.conf.
  • WebIDL interfaces in sequences, dictionaries, owning unions, and variadic arguments are represented by nsRefPtr<Foo> if nullable and OwningNonNull<Foo> otherwise.

For example, this WebIDL:

interface MyInterface { attribute MyInterface myAttr; void passNullable(MyInterface? arg); MyInterface? doSomething(sequence<MyInterface> arg); MyInterface doTheOther(sequence<MyInterface?> arg); readonly attribute MyInterface? nullableAttr; readonly attribute MyInterface someOtherAttr; // Marked as resultNotAddRefed readonly attribute MyInterface someYetOtherAttr; };

Would correspond to these C++ function declarations:

already_AddRefed<MyClass> MyAttr();
void SetMyAttr(MyClass& value);
void PassNullable(MyClass* arg);
already_AddRefed<MyClass> doSomething(const Sequence<OwningNonNull<MyClass> >& arg);
already_AddRefed<MyClass> doTheOther(const Sequence<nsRefPtr<MyClass> >& arg);
already_Addrefed<MyClass> GetMyAttr();
MyClass* SomeOtherAttr();
MyClass* SomeYetOtherAttr(); // Don't have to return already_AddRefed!
 
 
 
 
 
 
 
 
"SpiderMonkey" interfaces

Typed array, array buffer, and array buffer view arguments are represented by the objects inTypedArray.h.  For example, this WebIDL:

interface Test { void passTypedArrayBuffer(ArrayBuffer arg); void passTypedArray(ArrayBufferView arg); void passInt16Array(Int16Array? arg); }

will correspond to these C++ function declarations:

void PassTypedArrayBuffer(const ArrayBuffer& arg);
void PassTypedArray(const ArrayBufferView& arg);
void PassInt16Array(const Nullable<Int16Array>& arg);
 
 
 

Typed array return values are represented by JSObject*.

Typed arrays store a JSObject* and hence need to be rooted properly.  On-stack typed arrays can be declared as RootedTypedArray<TypedArrayType> (e.g.RootedTypedArray<Int16Array>).  Typed arrays on the heap need to be traced.

Dictionary types

A dictionary argument is represented by a const reference to a struct whose name is the dictionary name in the mozilla::dom namespace.  The struct has one member for each of the dictionary's members with the same name except the first letter uppercased and prefixed with "m". The members that have default values have types as described under the corresponding WebIDL type in this document.  The members that don't have default values have those types wrapped in Optional<>.

Dictionary return values are represented by an out parameter whose type is a non-const reference to the struct described above, with all the members that have default values preinitialized to those default values.

Note that optional dictionary arguments are always considered to have a default value of nullso dictionary arguments are never wrapped in Optional<>.

If necessary, dictionaries can be directly initialized from a JS::Value in C++ code by invoking their Init() method.  Consumers doing this should declare their dictionary asRootedDictionary<DictionaryName>.  When this is done, passing in a null scope object and even a null JSContext* is allowed if the passed-in JS::Value is JS::NullValue().  Likewise, a dictionary struct can be converted to a JS::Value in C++ by calling ToJSValuewith the dictionary as the second argument.  If Init() or ToJSValue() return false, they will generally set a pending exception on the JSContext; reporting those is the responsibility of the caller.

For example, this WebIDL:

dictionary Dict { long foo = 5; DOMString bar; }; interface Test { void initSomething(optional Dict arg); };

will correspond to this C++ function declaration:

void InitSomething(const Dict& arg);
 

and the Dict struct will look like this:

struct Dict {
  bool Init(JSContext* aCx, JS::Handle<JS::Value> aVal, const char* aSourceDescription = "value");

  Optional<nsString> mBar;
  int32_t mFoo;
}
 
 
 
 
 
 

Note that the dictionary members are sorted in the struct in alphabetical order.

Enumeration types

WebIDL enumeration types are represented as C++ enums.  The values of the C++ enum are named by taking the strings in the WebIDL enumeration, replacing all non-alphanumerics with underscores, and uppercasing the first letter, with a special case for the empty string, which becomes the value _empty.

For a WebIDL enum named MyEnum, the C++ enum is named MyEnum and placed in themozilla::dom namespace, while the values are placed in the mozilla::dom::MyEnumnamespace.  There is also a mozilla::dom::MyEnumValues::strings which is an array ofmozilla::dom::EnumEntry structs that gives access to the string representations of the values.

For example, this WebIDL:

enum MyEnum { "something", "something-else", "", "another" };

would lead to this C++ enum declaration:

MOZ_BEGIN_ENUM_CLASS(MyEnum, uint32_t)
  Something,
  Something_else,
  _empty,
  Another
MOZ_END_ENUM_CLASS(MyEnum)

namespace MyEnumValues {
extern const EnumEntry strings[10];
} // namespace MyEnumValues
 
 
 
 
 
 
 
 
 
 

Callback function types

Callback functions are represented as an object, inheriting frommozilla::dom::CallbackFunction, whose name, in the mozilla::dom namespace, matches the name of the callback function in the WebIDL.  If the type is nullable, a pointer is passed in; otherwise a reference is passed in.

The object exposes two Call methods, which both invoke the underlying JS callable.  The firstCall method has the same signature as a throwing method declared just like the callback function, with an additional trailing "mozilla::dom::CallbackObject::ExceptionHandling aExceptionHandling" argument, defaulting to eReportExceptions, and calling it will invoke the callable withundefined as the this value.  The second Call method allows passing in an explicit thisvalue as the first argument.  This second call method is a template on the type of the first argument, so the this value can be passed in in whatever form is most convenient, as long as it's either a type that can be wrapped by XPConnect or a WebIDL interface type.

If aReportExceptions is set to eReportExceptions, the Call methods will report JS exceptions before returning.  If aReportExceptions is set to eRethrowExceptions, JS exceptions will be stashed in the ErrorResult and will be reported when the stack unwinds to wherever the ErrorResult was set up.

For example, this WebIDL:

callback MyCallback = long (MyInterface arg1, boolean arg2); interface MyInterface { attribute MyCallback foo; attribute MyCallback? bar; };

will lead to this C++ class declaration, in the mozilla::dom namespace:

class MyCallback : public CallbackFunction
{
public:
  int32_t
  Call(MyInterface& arg1, bool arg2, ErrorResult& rv,
       ExceptionHandling aExceptionHandling = eReportExceptions);

  template<typename T>
  int32_t
  Call(const T& thisObj, MyInterface& arg1, bool arg2, ErrorResult& rv,
       ExceptionHandling aExceptionHandling = eReportExceptions);
};
 
 
 
 
 
 
 
 
 
 
 
 

and these C++ function declarations in the MyInterface class:

already_AddRefed<MyCallback> GetFoo(); void SetFoo(MyCallback&); already_AddRefed<MyCallback> GetBar(); void SetBar(MyCallback*);

Sequences

Sequence arguments are represented by const Sequence<T>&, where T depends on the type of elements in the WebIDL sequence.

Sequence return values are represented by an nsTArray<T> out param appended to the argument list, where T is the return type for the elements of the WebIDL sequence.  This comes after all IDL arguments, but before the ErrorResult&, if any, for the method.

Arrays

IDL array objects are not supported yet.  The spec on these is likely to change drastically anyway.

Union types

Union types are reflected as a struct in the mozilla::dom namespace.  There are two kinds of union structs: one kind does not keep its members alive (is "non-owning"), and the other does (is "owning").  Const references to non-owning unions are used for plain arguments.  Owning unions are used in dictionaries, sequences, and for variadic arguments.  Union return values become a non-const owning union out param.  The name of the struct is the concatenation of the names of the types in the union, with "Or" inserted between them, and for an owning struct "Owning" prepended.  So for example, this IDL:

void passUnion((object or long) arg); (object or long) receiveUnion(); void passSequenceOfUnions(sequence<(object or long)> arg); void passOtherUnion((HTMLDivElement or ArrayBuffer or EventInit) arg);

would correspond to these C++ function declarations:

void PassUnion(const ObjectOrLong& aArg);
void ReceiveUnion(OwningObjectObjectOrLong& aArg);
void PassSequenceOfUnions(const Sequence<OwningObjectOrLong>& aArg);
void PassOtherUnion(const HTMLDivElementOrArrayBufferOrEventInit& aArg);
 
 
 
 

Union structs expose accessors to test whether they're of a given type and to get hold of the data of that type.  They also expose setters that set the union as being of a particular type and return a reference to the union's internal storage where that type could be stored.  The one exception is the object type, which uses a somewhat different form of setter where theJSObject* is passed in directly. For example, ObjectOrLong would have the following methods:

bool IsObject() const;
JSObject* GetAsObject() const;
void SetToObject(JSContext*, JSObject*);
bool IsLong() const;
int32_t GetAsLong() const;
int32_t& SetAsLong()
 
 
 
 
 
 

Owning unions used on the stack should be declared as a RootedUnion<UnionType>.  For example, RootedUnion<OwningObjectOrLong>.

Date

WebIDL Date types are represented by a mozilla::dom::Date struct.

Stringifiers

Named stringifiers operations in WebIDL will just invoke the corresponding C++ method.

Anonymous stringifiers in WebIDL will invoke the C++ method called Stringify.  So for example given this IDL:

interface FirstInterface { stringifier; }; interface SecondInterface { stringifier DOMString getStringRepresentation(); };

the corresponding C++ would be:

class FirstInterface { public: void Stringify(nsAString& aResult); }; class SecondInterface { public: void GetStringRepresentation(nsAString& aResult); };

Legacy Callers

Only anonymous legacy callers are supported, and will invoke the C++ method calledLegacyCall.  This will be passed the JS "this" value as the first argument, then the arguments to the actual operation.  A JSContext will be passed if any of the operation arguments need it.  So for example given this IDL:

interface InterfaceWithCall { legacycaller long (float arg); };

the corresponding C++ would be:

class InterfaceWithCall {
public:
  int32_t LegacyCall(JS::Handle<JS::Value> aThisVal, float aArgument);
};
 
 
 
 

Named getters

If the interface has a named getter, the binding will expect several methods on the C++ implementation:

  • NamedGetter method.  This takes a property name and returns whatever type the named getter is declared to return.  It also has a boolean out param for whether a property with that name should exist at all.
  •  A NameIsEnumerable method.  This takes a  that takes a property name and returns a boolean that indicates whether the property is enumerable.
  • GetSupportedNames method.  This takes an unsigned integer which corresponds to the flags passed to the iterate proxy trap and returns a list of property names.  For implementations of this method, the important flags is JSITER_HIDDEN.  If that flag is set, the call needs to return all supported property names.  If it's not set, the call needs to return only the enumerable ones.

The NameIsEnumerable and GetSupportedNames methods need to agree on which names are and are not enumerable.  The NamedGetter and GetSupportedNames methods need to agree on which names are supported.

So for example, given this IDL:

interface InterfaceWithNamedGetter { getter long(DOMString arg); };

the corresponding C++ would be:

class InterfaceWithNamedGetter 
{
public:
  int32_t NamedGetter(const nsAString& aName, bool& aFound);
  bool NameIsEnumerable(const nsAString& aName);
  void GetSupportedNames(unsigned aFlags, nsTArray<nsString>& aNames);
};
 
 
 
 
 
 
 

Throwing exceptions from WebIDL methods, getters, and setters

WebIDL methods, getters, and setters that are explicitly marked as allowed to throw have anErrorResult& argument as their last argument.  To throw an exception, simply call Throw()on the ErrorResult& and return from your C++ back into the binding code.

In cases when the specification calls for throwing a TypeError, you should useErrorResult::ThrowTypeError() instead of calling Throw().

Custom extended attributes

Our WebIDL parser and code generator recognize several extended attributes that are not present in the WebIDL spec.

[ChromeOnly]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole.

Interface members flagged as [ChromeOnly] are only exposed in chrome Windows (and in particular, are not exposed to webpages).  From the point of view of web content, it's as if the interface member were not there at all.  These members are exposed to chrome script working with a content object via Xrays.

If specified on an interface as a whole, this functions like [Func] except that the binding code will automatically check whether the caller script has the system principal (is chrome or a worker started from a chrome page) instead of calling into the C++ implementation to determine whether to expose the interface object on the global.   This means that accessing a  content global via Xrays will show [ChromeOnly] interface objects on it.

This extended attibute can be specified together with [AvailableIn][CheckPermissions],[Func],  and [Pref].  If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

[Pref=prefname]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole. It takes a value, which must be the name of a boolean preference.

If specified on an interface member, the interface member involved is only exposed if the preference is set to true. An example of how this can be used:

interface MyInterface { attribute long alwaysHere; [Pref="my.pref.name"] attribute long onlyHereIfEnabled; };

If specifed on an interface as a whole, this functions like [Func] except that the binding will check the value of the preference directly without calling into the C++ implementation of the interface at all. This is useful when the enable check is simple and it's desirable to keep the prefname with the WebIDL declaration. The implementation can callMyInterfaceBinding::PrefEnabled() to check whether it is enabled or not.  An example of how this can be used:

[Pref="my.pref.name"] interface MyConditionalInterface { };

This extended attibute can be specified together with [AvailableIn][CheckPermissions],[ChromeOnly], and[Func].  If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

[Func="funcname"]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole.  It takes a value, which must be the name of a static function. 

If specified on an interface member, the interface member involved is only exposed if the specified function returns true.   An example of how this can be used:

interface MyInterface { attribute long alwaysHere; [Func="MyClass::StuffEnabled"] attribute long onlyHereIfEnabled; };

The function is invoked with two arguments: the JSContext that the operation is happening on and the JSObject for the global of the object that the property will be defined on if the function returns true.  In particular, in the Xray case the JSContext is in the caller compartment (typically chrome) but the JSObject is in the target compartment (typically content).  This allows the method implementation to select which compartment it cares about in its checks.

The above IDL would also require the following C++:

class MyClass {
  static bool StuffEnabled(JSContext* cx, JSObject* obj);
};
 
 
 

If specified on an interface as a whole, then lookups for the interface object for this interface on a DOM Window will only find it if the specified function returns true.  For objects that can only be created via a constructor, this allows disabling the functionality altogether and making it look like the feature is not implemented at all.

An example of how [Func] can be used:

[Func="MyClass::MyConditionalInterfaceEnabled"] interface MyConditionalInterface { };

In this case, the C++ function is passed a JS::Handle<JSObject*>.  So the C++ in this case would look like this:

class MyClass {
  static bool MyConditionalInterfaceEnabled(JSContext* cx, JS::Handle<JSObject*> obj);
};
 
 
 

Just like in the interface member case, the JSContext is in the caller compartment but theJSObject is the actual object the property would be defined on.  In the Xray case that mean obj is in the target compartment (typically content) and cx is typically chrome.

This extended attibute can be specified together with [AvailableIn][CheckPermissions],[ChromeOnly], and [Pref].  If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

[AvailableIn=Where]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole. It takes a value, which must be either PrivilegedApps orCertifiedApps.  This will make the interface or interface member only visible in privileged and certified apps respectively on Firefox OS.

This extended attibute can be specified together with [ChromeOnly][CheckPermissions],[Func], and  [Pref].  If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

[CheckPermissions="list of permissions"]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole. It takes a whitespace-separated list of permissions to be checked before making the interface or interface member visible to a page or app. When multiple permission names are specified, at least one of them will need to be set to nsIPermissionManager::ALLOW_ACTION for the interface or interface member to be exposed.

This extended attribute can be specified together with [AvailableIn][ChromeOnly],[Func] and [Pref]. If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

[Throws][GetterThrows][SetterThrows]

Used to flag methods or attributes as allowing the C++ callee to throw.  This causes the binding generator, and in many cases the JIT, to generate extra code to handle possible exceptions.  Possibly-throwing methods and attributes get an ErrorResult& argument.

[Throws] applies to both methods and attributes; for attributes it means both the getter and the setter can throw.  [GetterThrows] applies only to attributes.  [SetterThrows] applies only to non-readonly attributes.

For bindings that involve workers, the above can all be specified with MainThread or Workersas a value.  When doing this, if [Throws] is specified on an attribute, no matter what its value, then [GetterThrows] and [SetterThrows] will be ignored.  So to have an attribute which can throw both when getting and setting on main thread but can only throw from the setter in workers, use [SetterThrows, GetterThrows=MainThread].

For interfaces flagged with [JSImplementation], all methods and properties are assumed to be able to throw and do not need to be flagged as throwing.

[Pure]

Used to flag attributes whose getter has no side-effects or methods that have no side-effects in the DOM.  Attributes/methods flagged in this way promise that they will keep returning the same value as long as no DOM setters or non-[Pure] DOM methods executed.  This allows the JIT to perform loop-hoisting and common subexpression elimination on the return values of these attributes/methods in some cases.  [Pure] things are allowed to throw exceptions as long as they do so deterministically.  This extended attribute can be used on writable attributes as long as the getter obeys the above rules.

[Constant]

Used to flag readonly attributes that could have been annotated with [Pure] and also always return the same value.  This allows the JIT to do even more aggressive optimization of getters for such attributes.  This should only be used when it's absolutely guaranteed that the return value of the attribute getter will always be the same from the JS engine's point of view.  This extended attribute implies [Pure] as far as the JIT is concerned.

[NeedNewResolve]

Used to flag interfaces which have a custom resolve hook.  This annotation will cause theDoNewResolve method to be called on the underlying C++ class when a property lookup happens on the object.  The signature of this method is: bool DoNewResolve(JSContext*, JS::Handle<JSObject*>, JS::Handle<jsid>, JS::MutableHandle<JS::Value>).  Here the passed-in object is the object the property lookup is happening on (which may be an Xray for the actual DOM object) and the jsid is the property name.  The value that the property should have is returned in the MutableHandle<Value>, with UndefinedValue() indicating that the property does not exist.

If this extended attribute is used, then the underlying C++ class must also implement a method called GetOwnPropertyNames with the signature void GetOwnPropertyNames(JSContext* aCx, nsTArray<nsString>& aNames, ErrorResult& aRv).  This method wil be called by the JS engine's enumerate hook and must provide a superset of all the property names that DoNewResolve might resolve.  Providing names that DoNewResolve won't actually resolve is OK.

[HeaderFile="path/to/headerfile.h"]

Indicates where the implementation can be found. Similar to the headerFile annotation in Bindings.conf.

[JSImplementation="@mozilla.org/some-contractid;1"]

Used on an interface to provide the contractid of the JavaScript component implementing the interface.

Setting this extended attribute to propName on an interface causeswindow.navigator.propName to be an instance of the interface.

[StoreInSlot]

Used to flag attributes that can be gotten very quickly from the JS object by the JIT.  Such attributes will have their getter called immediately when the JS wrapper for the DOM object is created, and the returned value will be stored directly on the JS object.  Later gets of the attribute will not call the C++ getter and instead use the cached value.  If the value returned by the attribute needs to change, the C++ code should call the ClearCachedFooValue method in the namespace of the relevant binding, where foo is the name of the attribute.  This will immediately call the C++ getter and cache the value it returns, so it needs a JSContext to work on.  This extended attribute can only be used in on attributes whose getters are [Pure] or[Constant] and which are not [Throws] or [GetterThrows].

So for example, given this IDL:

interface MyInterface { [Pure, StoreInSlot] attribute long myAttribute; };

the C++ implementation of MyInterface would clear the cached value by callingmozilla::dom::MyInterfaceBinding::ClearCachedMyAttributeValue(cx, this).

If the attribute is not readonly, setting it will automatically clear the cached value and reget it again before the setter returns.

[Cached]

Used to flag attributes that, when their getter is called, will cache the returned value on the JS object.  This can be used to implement attributes whose value is a sequence or dictionary (which would otherwise end up returning a new object each time and hence not be allowed in WebIDL).

Unlike [StoreInSlot] this does not cause the getter to be eagerly called at JS wrapper creation time; the caching is lazy.  [Cached] attributes must be [Pure] or [Constant], because otherwise not calling the C++ getter would be observable, but are allowed to have throwing getters.  Their cached value can be cleared by calling the ClearCachedFooValue method in the namespace of the relevant binding, where foo is the name of the attribute.  Unlike [StoreInSlot] attributes, doing so will not immediately invoke the getter, so does not need a JSContext.

So for example, given this IDL:

interface MyInterface { [Pure, StoreInSlot] attribute long myAttribute; };

the C++ implementation of MyInterface would clear the cached value by callingmozilla::dom::MyInterfaceBinding::ClearCachedMyAttributeValue(this).

If the attribute is not readonly, setting it will automatically clear the cached value.

[Frozen]

Used to flag attributes that, when their getter is called, will call Object.freeze on the return value before returning it.  This extended attribute is only allowed on attributes that return sequences, and corresponds to returning a frozen Array.

[ChromeConstructor]

[ChromeConstructor] has the same behaviour as [Constructor], but the constructor will throw if it's not called from chrome code. The usage rules and restrictions as those for[Constructor] apply. Note that [Constructor] and [ChromeConstructor] are mutually exclusive, while there can be multiple of either, there can never be both on the same interface.

Helper objects

The C++ side of the bindings uses a number of helper objects.

Nullable<T>

Nullable<> is a struct declared in Nullable.h and exported tomozilla/dom/Nullable.h that is used to represent nullable values of types that don't have a natural way to represent null.

Nullable<T> has an IsNull() getter that returns whether null is represented and a Value()getter that returns a const T& and can be used to get the value when it's not null.

Nullable<T> has a SetNull() setter that sets it as representing null and two setters that can be used to set it to a value: "void SetValue(T)" (for setting it to a given value) and "T& SetValue()" for directly modifying the underlying T&.

Optional<T>

Optional<> is a struct declared in BindingDeclarations.h and exported tomozilla/dom/BindingDeclarations.h that is used to represent optional arguments and dictionary members, but only those that have no default value.

Optional<T> has a WasPassed() getter that returns true if a value is available.  In that case, the Value() getter can be used to get a const T& for the value.

NonNull<T>

NonNull<T> is a struct declared in BindingUtils.h and exported tomozilla/dom/BindingUtils.h that is used to represent non-null C++ objects.  It has a conversion operator that produces T&.

OwningNonNull<T>

OwningNonNull<T> is a struct declared in BindingUtils.h and exported tomozilla/dom/BindingUtils.h that is used to represent non-null C++ objects and holds a strong reference to them.  It has a conversion operator that produces T&.

Typed arrays, arraybuffers, array buffer views

TypedArray.h is exported to mozilla/dom/TypedArray.h and exposes structs that correspond to the various typed array types, as well as ArrayBuffer and ArrayBufferView, all in the mozilla::dom namespace.  Each struct has an Data() method that returns a pointer to the relevant type (uint8_t for ArrayBuffer and ArrayBufferView) and a Length()method that returns the length in units of *Data().  So for example, Int32Array has aData() returning int32_t* and a Length() that returns the number of 32-bit ints in the array..

Sequence<T>

Sequence<> is a type declared in BindingDeclarations.h and exported tomozilla/dom/BindingDeclarations.h that is used to represent sequence arguments.  It's some kind of typed array, but which exact kind is opaque to consumers.  This allows the binding code to change the exact definition (e.g. to use auto arrays of different sizes and so forth) without having to update all the callees.

CallbackFunction

CallbackFunction is a type declared in CallbackFunction.h and exported tomozilla/dom/CallbackFunction.h that is used as a common base class for all the generated callback function representations.  This class inherits from nsISupports, and consumers must make sure to cycle-collect it, since it keeps JS objects alive.

CallbackInterface

CallbackInterface is a type declared in CallbackInterface.h and exported tomozilla/dom/CallbackInterface.h that is used as a common base class for all the generated callback interface representations.  This class inherits from nsISupports, and consumers must make sure to cycle-collect it, since it keeps JS objects alive.

DOMString

DOMString is a class declared in BindingDeclarations.h and exported tomozilla/dom/BindingDeclarations.h that is used for WebIDL DOMString return values.  It has a conversion operator to nsString& so that it can be passed to methods that take that type or nsAString&, but callees that care about performance, have an nsStringBufferavailable, and promise to hold on to the nsStringBuffer at least until the binding code comes off the stack can also take a DOMString directly for their string return value and call itsSetStringBuffer method with the nsStringBuffer and its length.  This allows the binding code to avoid extra reference-counting of the string buffer in many cases, and allows it to take a faster codepath even if it does end up having to addref the nsStringBuffer.

GlobalObject

GlobalObject is a class declared in BindingDeclarations.h and exported tomozilla/dom/BindingDeclarations.h that is used to represent the global object for static attributes and operations (including constructors).  It has a Get() method that returns theJSObject*  for the global and a GetAsSupports() method that returns an nsISupports*for the global on the main thread, if such is available. It also has a GetContext() method that returns the JSContext* the call is happening on.  A caveat: the compartment of theJSContext may not match the compartment of the global!

Date

Date is a class declared in BindingDeclarations.h and exported tomozilla/dom/BindingDeclarations.h that is used to represent WebIDL Dates.  It has aTimeStamp() method returning a double which represents a number of milliseconds since the epoch, as well as SetTimeStamp() methods that can be used to initialize it with a double timestamp or a JS Date object.  It also has a ToDateObject() method that can be used to create a new JS Date.

ErrorResult

ErrorResult is a class declared in ErrorResult.h and exported to mozilla/ErrorResult.hthat is used to represent exceptions in WebIDL bindings.  This has the following methods:

  • Throw: allows throwing an nsresult.  The nsresult must be a failure code.
  • ThrowTypeError: allows throwing a TypeError with the given error message.  The list of allowed TypeErrors and corresponding messages is indom/bindings/Errors.msg.
  • ThrowJSException: allows throwing a preexisting JS exception value. However, theMightThrowJSException() method must be called before any such exceptions are thrown (even if no exception is thrown).
  • Failed: checks whether an exception has been thrown on this ErrorResult.
  • ErrorCode: returns a failure nsresult representing (perhaps incompletely) the state of this ErrorResult.
  • operator=: takes an nsresult and acts like Throw if the result is an error code, and like a no-op otherwise (unless an exception has already been thrown, in which case it asserts).  This should only be used for legacy code that has nsresult everywhere; we would like to get rid of this operator at some point.

Bindings.conf details

XXXbz write me.  In particular, need to describe at least use of concreteprefable, andaddExternalInterface

How to get a JSContext passed to a given method

In some rare cases you may need a JSContext* argument to be passed to a C++ method that wouldn't otherwise get such an argument. To see how to achieve this, search forimplicitJSContext in dom/bindings/Bindings.conf.

Implementing WebIDL using Javascript

There is support for implementing WebIDL interfaces in JavaScript.  When this is done, there are actually two objects created: the implementation object (running as a chrome-privileged script) and the content-exposed object (which is what the web page sees).  This allows the implementation object to have various APIs that the content-exposed object does not.  It also means that consumers have to be careful about which object they are creating.

Creating JS-implemented WebIDL objects

To create a JS-implemented WebIDL object, one must create both the chrome-side implementation object and the content-side page-exposed object. There are three ways to do this.

Using the WebIDL constructor

If the interface has a constructor, a content-side object can be created by getting that constructor from the relevant content window and invoking it. For example:

var contentObject = new contentWin.RTCPeerConnection();
 

The returned object will be an Xray wrapper for the content-side object. Creating the object this way will automatically create the chrome-side object using its contractID.

This method is limited to the constructor signatures exposed to webpages. Any additional configuration of the object needs to be done via [ChromeOnly] methods on the interface. 

Creating many objects this way can be slow due to the createInstance overhead involved. 

Using a _create method  

 A content-side object can be created for a given chrome-side object by invoking the static_create method on the interface. This method takes two arguments: the content window in which to create the object and the chrome-side object to use. For example:

var contentObject = RTCPeerConnection._create(contentWin,
                                              new MyPeerConnectionImpl());
 
 

However, if you are in a JS component, you may only be able to get to the correct interface object via some window object. In this case, the code would look more like:

var contentObject = contentWin.RTCPeerConnection._create(contentWin,
                                                         new MyPeerConnectionImpl());
 
 

Creating the object this way will not invoke its __init method or init method.

By returning a chrome-side object from a JS-implemented WebIDL method

If a JS-implemented WebIDL method is declared as returning a JS-implemented interface, then a non-WebIDL object returned from that method will be treated as the chrome-side part of a JS-implemented WebIdL object and the content-side part will be automatically created.

Creating the object this way will not invoke its __init method or init method.

Implementing a WebIDL object in JavaScript

To implement a WebIDL interface in JavaScript, first add a WebIDL file, in the same way as you would for a C++-implemented interface.  To support implementation in JS, you must add an extended attribute JSImplementation="CONTRACT_ID_STRING" on your interface, where CONTRACT_ID_STRING is the XPCOM component contract ID of the JS implementation.  Here's an example:

[Constructor(optional long firstNumber), JSImplementation="@mozilla.org/my-number;1"] interface MyNumber { attribute long value; readonly attribute long otherValue; void doNothing(); };

Next, create an XPCOM component that implements this interface.  Basic directions for how to do this can be found elsewhere on MDN.  Use the same contract ID as you specified in the WebIDL file.  The class ID doesn't matter, except that it should be a newly generated one.  ForQueryInterface, you only need to implement nsISupports, not anything corresponding to the WebIDL interface.  The name you use for the XPCOM component should be distinct from the name of the interface, to avoid confusing error messages.

WebIDL attributes are implemented as properties on the JS object or its prototype chain, whereas WebIDL methods are implemented as methods on the object or prototype.  Note that any other instances of the interface that you are passed in as arguments are the full web-facing version of the object, and not the JS implementation, so you currently cannot access any private data.

The WebIDL constructor invocation will first create your object.  If the XPCOM component implements nsIDOMGlobalPropertyInitializer, then the object's init method will be invoked with a single argument: the content window the constructor came from.  This allows the JS implementation to know which content window it's associated with.  The init method should not return anything.  After this, the content-side object will be created. Then, if there are any constructor arguments, the object's __init method will be invoked, with the constructor arguments as its arguments.

If you want an instance of the class to be added to window.navigator, add an extended attribute NavigatorProperty="PropertyName" which will make the instance available aswindow.navigator.PropertyName.

Checking for Permissions or Preferences

When implementing an XPIDL interface using Javascript, the init method may check if the caller has the proper permissions, or if the appropriate preference is set. If this check fails, then it will return null to indicate that the object should not be created.  JS-implemented WebIDL does NOT work like that.  In JS-implemented WebIDL, the init method should only return undefined.  If any other value, such as null, is returned, the bindings code will assert or crash.  In other words, it acts like it has a "void" return type.

Instead, preference or permission checking should be implemented by adding an extended attribute to the WebIDL interface. This has the advantage that if the check fails, the constructor or object will not show up at all.

For preference checking, add an extended attribute Pref="myPref.enabled" wheremyPref.enabled is the preference that should be checked.  SettingsLock is an example of this.

For permissions or other kinds of checking, add an extended attributeFunc="MyPermissionChecker" where MyPermissionChecker is a function implemented in C++ that returns true if the interface should be enabled.  This function can do whatever checking is needed.  One example of this is PushManager.

Example

Here's an example JS implementation of the above interface. The invisibleValue field will not be accessible to web content, but is usable by the doNothing() method.

Components.utils.import("resource://gre/modules/XPCOMUtils.jsm"); function MyNumberInner() { this.value = 111; this.invisibleValue = 12345; } MyNumberInner.prototype = { classDescription: "Get my number XPCOM Component", classID: Components.ID("{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}"), // dummy UUID contractID: "@mozilla.org/my-number;1", QueryInterface: XPCOMUtils.generateQI([Components.interfaces.nsISupports]), doNothing: function() {}, get otherValue() { return this.invisibleValue - 4; }, __init: function(firstNumber) { if (arguments.length > 0) { this.value = firstNumber; } } } var components = [MyNumberInner]; var NSGetFactory = XPCOMUtils.generateNSGetFactory(components);

Finally, add a component and a contract and whatever other manifest stuff you need to implement an XPCOM component.

Guarantees provided by bindings

When implementing a WebIDL interface in JavaScript, certain guarantees will be provided by the binding implementation.  For example, string or numeric arguments will actually be primitive strings or numbers.  Dictionaries will contain only the properties that they are declared to have, and they will have the right types.  Interface arguments will actually be objects implementing that interface.

What the bindings will NOT guarantee is much of anything about object and any arguments.  They will get cross-compartment wrappers that make touching them from chrome code not be an immediate security bug, but otherwise they can have quite surprising behavior if the page is trying to be malicious.  Try to avoid using these types if possible.

Accessing the content object from the implementation

If the JS implementation of the WebIDL interface needs to access the content object, it is available as a property called __DOM_IMPL__ on the chrome implementation object.  This property only appears after the content-side object has been created. So it is available in__init but not in init.

Determining the principal of the caller that invoked the WebIDL API

This can be done by calling Component.utils.getWebIDLCallerPrincipal().

Throwing exceptions from JS-implemented APIs

There are two reasons a JS implemented API might throw.  The first reason is that some unforeseen condition occurred and the second is that a specification requires an exception to be thrown.

When throwing for an unforeseen condition, the exception will be reported to the console, and a sanitized NS_ERROR_UNEXPECTED exception will be thrown to the calling content script, with the file/line of the content code that invoked your API.  This will avoid exposing chrome URIs and other implementation details to the content code.

When throwing because a specification requires an exception, you need to communicate to the binding code that this is what you're doing.  Right now this is done by throwing a DOMErrorfrom the window your WebIDL object is associated with (the one that was passed to your initmethod).  The binding code will then rethrow just the message string of that DOMError to the web page, as a plain JS Error.  This does not allow implementing exceptions per spec (e.g. there is no way to explicitly throw a TypeError or other Error subclass), unfortunately; we're still working on that.  Since you know for this case the exception is being thrown because a spec requires it, you know you need to create the DOMError.  An example of how this could work:

if (!isValid(passedInObject)) {
  throw new this.contentWindow.DOMError("Error", "Object is invalid");
}
 
 
 

In some cases you may need to perform operations whose exception message you just want to propagate to the content caller.  This can be done like so:

try {
  someOperationThatCanThrow();
} catch (e) {
  throw new this.contentWindow.DOMError(e.name, e.message);
}
 
 
 
 
 

Inheriting from interfaces implemented in C++

It's possible to have an interface implemented in JavaScript inherit from an interface implemented in C++.  To do so, simply have one interface inherit from the other and the bindings code will auto-generate a C++ object inheriting from the implementation of the parent interface.  The class implementing the parent interface will need a constructor that takes annsPIDOMWindow* (though it doesn't have to do anything with that argument).

If the class implementing the parent interface is abstract and you want to use a specific concrete class as the implementation to inherit from, you will need to add a defaultImpl annotation to the descriptor for the parent interface in Bindings.conf.  The value of the annotation is the C++ class to use as the parent for JS-implemented descendants; if defaultImpl is not specified, the nativeType will be used.

For example, consider this interface that we wish to implement in JavaScript:

[JSImplementation
 

="some-contract"] interface MyEventTarget : EventTarget { attribute EventHandler onmyevent; void dispatchTheEvent(); // Sends a "myevent" event to this EventTarget }

The implementation would look something like this, ignoring the XPCOM boilerplate:

function MyEventTargetImpl() { } MyEventTargetImpl.prototype = { init: function(contentWindow) { // XXXbz need to document how to get this called on you! this.contentWindow = contentWindow; } get onmyevent() { return this.__DOM_IMPL__.getEventHandler("onmyevent"); } set onmyevent(handler) { this.__DOM_IMPL__.setEventHandler("onmyevent", handler); } dispatchTheEvent: function() { var event = new this.contentWindow.Event("myevent"); this.__DOM_IMPL__.dispatchEvent(event); } };

The implementation would automatically support the API exposed on EventTarget (so for example addEventListener).  Calling the dispatchTheEvent method would cause dispatch of an event that content script can see via listeners it has added.

Note that in this case the chrome implementation is relying on some [ChromeOnly] methods on EventTarget that were added specifically to make it possible to easily implement event handlers.  Other cases can do similar things as needed

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