Bindings can define methods and properties on a bound element using the
implementation tag. A binding implementation provides a new set of methods and properties that can be invoked directly from the bound element.
The methods and properties of an implementation can be defined declaratively using
property tags in XML, or an external implementation (e.g., a binary implementation) can be specified using the
src attribute. When the
src attribute is specified, any children of the
implementation element are ignored.
Methods are defined using the
method element. The name given on the tag using the
name attribute is the name that can be used to invoke the method on the element. A
method with parameters specifies those parameters and their names with
parameter elements declared underneath the
The implementation of the method is contained inside a
body element. The parameters specified are bound to their names in the method body.
<method name="scrollToIndex"> <parameter name="index"/> <body> <![CDATA[ if (index < 0) return; ... ]]> </body> </method>
Properties can also be defined on the bound element using
property tags. There
are two basic types of properties. The first type of property is a raw value that is set
directly on the element itself. The second type of property is one that defines functions
that are invoked when the property is either retrieved or set. These functions are
called getters and setters in XBL.
For properties with raw values, an initial value can be specified as a child of the
property tag. The script is evaluated at the time of binding attachment and
the resulting value is stored on the element. It can be assumed that the anonymous content
of the binding has been fully constructed, although the
will not have fired. Property initialization always takes place after content generation
but before the firing of a binding attachment event, since the
handler needs to be able to assume that all properties will be accessible on the binding.
Properties with getters and setters can define them using
onset attributes, or using the more verbose
elements. If both the attribute form and the element form are used to specify a getter or setter,
then the element form is ignored.
A getter contains script whose return value is handed back when the property is requested. A
setter contains a script that is invoked when a new value is assigned to the property. In
script, the word
val is used to represent the new value. A setter should also
val as a result in order to allow for chained assignment (e.g.,
a = this.b = c;) operations to succeed.
Properties can be designated as constant using the
readonly attribute. When
true, the property's value cannot be altered. If a property is
readonly and a setter is defined, then the setter is ignored.
Properties also support a shorthand syntax for defining getters and setters that forward
requests or assignments to an anonymous content element. The
attribute specifies the ID of anonymous content underneath the bound element that should
be used when obtained the property or setting the property.
On the anonymous content element, the property can be obtained either from an attribute
on the element or from a property on the element. The
property attribute's value specifies the name of a property to use
on the anonymous content element. The
attribute attribute specifies the name
of an attribute to use on the anonymous content element. If either of these attributes
is specified on the
property element, then any defined getters and setters
are ignored. A raw initial value is also ignored. If both a property and attribute are
specified, then the property takes precedence and the attribute is ignored.
Inheritance of Implementations
When two bindings in an inheritance chain both supply implementations, the derived binding's implementation inherits from the base binding's implementation. Method and property lookups are dynamic. Without disambiguating, if two bindings define the same method or property, then the most derived binding's method or property will be used.
Because bindings extend DOM elements, they can override any built-in methods or properties
on the DOM element. For example, a binding could override the
Element or the retrieval of the
value property for
The following is currently not implemented in Mozilla, it seems. See bug #373652 for some details.
Implementations can be named using a
name attribute specified on the
implementation element. When an implementation has a name, that name can
be used to disambiguate the desired method or property.
For example, given a binding with an implementation
that derives from an implementation
ColorPicker where the two implementations
both specify the
setColor method, a caller could invoke
method with the following syntax:
... // myElement is a ColorPickerGrid myElement.ColorPicker.setColor(); // Calls the ColorPicker method. myElement.setColor(); // Calls the ColorPickerGrid method. ...
In addition to being able to specifically name a base class, the name
can be used to specify the method or property on the base binding without necessarily knowing
what the base class is. This situation can occur when bindings implicitly inherit, e.g., through
the use of
myElement.baseBinding.setColor(); // Calls the ColorPicker method.