XPCNativeWrapper

  • Revision slug: XPCNativeWrapper
  • Revision title: XPCNativeWrapper
  • Revision id: 93002
  • Created:
  • Creator: Bzbarsky
  • Is current revision? No
  • Comment /* Deep Wrappers */

Revision Content

bz: Note that this is a work in progress. Some major rewriting may need to happen pending the outcome of and . Please let me know if you plan to make any major changes to this document before doing so. --Bzbarsky 22:51, 29 May 2005 (PDT)

XPCNativeWrapper is a way to wrap up an object so that it's safe to access from privileged code. It can be used in all Firefox versions, though the behavior changed somewhat starting with Firefox 1.1.

XPCNativeWrapper in Firefox versions prior to 1.1

In Firefox versions prior to 1.1, use of XPCNativeWrapper requires manually constructing an XPCNativeWrapper and passing it the object to be wrapped and the names of the methods/properties to be exposed as arguments. The resulting object exposes ONLY the methods/properties whose methods were passed as arguments. This is described in more detail in the the entry for XPCNativeWrapper at the MozillaZine KnowledgeBase.

XPCNativeWrapper in Firefox versions starting with 1.1

bz: Three types, or two? Still sorting this out. See https://bugzilla.mozilla.org/show_bug.cgi?id=295782

There are three slightly different types of XPCNativeWrapper in Firefox 1.1. All three types wrap a possibly-unsafe object and provide safe access to all of its properties and methods (unlike XPCNativeWrapper in versions before 1.1, which only provided safe access to the properties and methods listed in its constructor). If unsafe access to a property is required for some reason, this can be accomplished via the wrappedJSObject property of the wrapper. For example, if docWrapper is a wrapper for doc, then

docWrapper.wrappedJSObject.prop

is the same as

doc.prop

One limitation of XPCNativeWrapper is that assigning to the location property of a wrapped document or window will not work. Code that wishes to set the location should assign to the href property of the location object (after wrapping the location object itself, as needed).

Explicit XPCNativeWrapper with string arguments

For example:

var contentWinWrapper = new XPCNativeWrapper(content,
                                             "document");

This syntax has been kept for compatibility with versions prior to Firefox 1.1. While all properties of the contentWinWrapper object can now be safely accessed, the return values of these properties are NOT safe to access (just like in versions prior to Firefox 1.1). So to compare the content document title to the current content selection, one must do:

var winWrapper = new XPCNativeWrapper(content, "document",
                                      "getSelection()");
var docWrapper = new XPCNativeWrapper(winWrapper.document,
                                      "title");
return docWrapper.title == winWrapper.getSelection();

Note that the "getSelection()" argument is not strictly needed here; if the code is not intended for use with Firefox versions before 1.1 it can be removed. A single string argument after the object being wrapped is all that is required for Firefox 1.1.

Explicit XPCNativeWrapper with no string arguments

For example:

var contentWinWrapper = new XPCNativeWrapper(content);

The resulting object behaves just like an implicit wrapper (most importantly, it will be deep), except that properties set on it will not be visible on either other explicit wrappers or implicit wrappers. Note that this means that for code not using xpcnativewrappers=yes this type of XPCNativeWrapper is useless at the moment.

bz: Again, see https://bugzilla.mozilla.org/show_bug.cgi?id=295782 -- that might introduce more differences between this type of wrapper and implicit wrappers.

Implicit XPCNativeWrapper

This type of wrapper can be created in one of two ways: either by accessing a content object from code that has xpcnativewrappers=yes set or by accessing a property of a deep wrapper.

bz: The latter may not always be true pending outcome of https://bugzilla.mozilla.org/show_bug.cgi?id=295782

The wrapper created in this way is always a deep wrapper. Furthermore, a wrapper created in this way will stick around as long as the object being wrapped does.

bz: except for https://bugzilla.mozilla.org/show_bug.cgi?id=295937 -- sort that out before trying to explain the exact behavior here.

Deep Wrappers

If a wrapper is deep, that means that the return value of any property access or method call on the wrapper will be implicitly wrapped.

If a deep wrapper is accessed from code that doesn't use xpcnativewrappers=yes, it will NOT provide safe access to properties. Instead, it will automatically forward all accesses to the underlying wrappedJSObject.

bz: This may not be the case for all deep wrappers pending https://bugzilla.mozilla.org/show_bug.cgi?id=295782

Revision Source

<p><i>bz: Note that this is a work in progress. Some major rewriting may need to happen pending the outcome of <a class="external" href="https://bugzilla.mozilla.org/show_bug.cgi?id=295782"> and </a><a class="external" href="https://bugzilla.mozilla.org/show_bug.cgi?id=295937">. Please let me know if you plan to make any major changes to this document before doing so.	 </a></i><a class="external" href="https://bugzilla.mozilla.org/show_bug.cgi?id=295937">
--</a><a href="User:Bzbarsky">Bzbarsky</a> 22:51, 29 May 2005 (PDT)
</p><p><code>XPCNativeWrapper</code> is a way to wrap up an object so that it's safe to access from privileged code.  It can be used in all Firefox versions, though the 
behavior changed somewhat starting with Firefox 1.1.
</p>
<h3 name="XPCNativeWrapper_in_Firefox_versions_prior_to_1.1"> <code>XPCNativeWrapper</code> in Firefox versions prior to 1.1 </h3>
<p>In Firefox versions prior to 1.1, use of <code>XPCNativeWrapper</code> requires manually constructing an <code>XPCNativeWrapper</code> and passing it the object to be wrapped and the names of the methods/properties to be exposed as arguments.  The resulting object exposes ONLY the methods/properties whose methods were passed as arguments.  This is described in more detail in the <a class="external" href="http://kb.mozillazine.org/XPCNativeWrapper">the entry for <code>XPCNativeWrapper</code> at the MozillaZine KnowledgeBase</a>.
</p>
<h3 name="XPCNativeWrapper_in_Firefox_versions_starting_with_1.1"> <code>XPCNativeWrapper</code> in Firefox versions starting with 1.1 </h3>
<p><i>bz: Three types, or two?  Still sorting this out.  See https://bugzilla.mozilla.org/show_bug.cgi?id=295782</i>
</p><p>There are three slightly different types of <code>XPCNativeWrapper</code> in Firefox 1.1.  All three types wrap a possibly-unsafe object and provide safe access to all of its properties and methods (unlike <code>XPCNativeWrapper</code> in versions before 1.1, which only provided safe access to the properties and methods listed in its constructor).  If unsafe access to a property is required for some reason, this can be accomplished via the <code>wrappedJSObject</code> property of the wrapper.  For example, if <code>docWrapper</code> is a wrapper for <code>doc</code>, then
</p>
<pre class="eval">docWrapper.wrappedJSObject.prop
</pre>
<p>is the same as
</p>
<pre class="eval">doc.prop
</pre>
<p>One limitation of <code>XPCNativeWrapper</code> is that assigning to the <code>location</code> property of a wrapped document or window will not work.  Code that wishes to set the location should assign to the <code>href</code> property of the location object (after wrapping the location object itself, as needed).
</p>
<h4 name="Explicit_XPCNativeWrapper_with_string_arguments"> Explicit <code>XPCNativeWrapper</code> with string arguments </h4>
<p>For example:
</p>
<pre class="eval">var contentWinWrapper = new XPCNativeWrapper(content,
                                             "document");
</pre>
<p>This syntax has been kept for compatibility with versions prior to Firefox 1.1.  While all properties of the <code>contentWinWrapper</code> object can now be safely accessed, the return values of these properties are NOT safe to access (just like in versions prior to Firefox 1.1).  So to compare the content document title to the current content selection, one must do:
</p>
<pre class="eval">var winWrapper = new XPCNativeWrapper(content, "document",
                                      "getSelection()");
var docWrapper = new XPCNativeWrapper(winWrapper.document,
                                      "title");
return docWrapper.title == winWrapper.getSelection();
</pre>
<p>Note that the <code>"getSelection()"</code> argument is not strictly needed here; if the code is not intended for use with Firefox versions before 1.1 it can be removed.  A single string argument after the object being wrapped is all that is required for Firefox 1.1.
</p>
<h4 name="Explicit_XPCNativeWrapper_with_no_string_arguments"> Explicit <code>XPCNativeWrapper</code> with no string arguments </h4>
<p>For example:
</p>
<pre class="eval">var contentWinWrapper = new XPCNativeWrapper(content);
</pre>
<p>The resulting object behaves just like an implicit wrapper (most importantly, it will be <a href="#Deep_Wrappers">deep</a>), except that properties set on it will not be visible on either other explicit wrappers or implicit wrappers.  Note that this means that for code not using <code>xpcnativewrappers=yes</code> this type of <code>XPCNativeWrapper</code> is useless at the moment.
</p><p><i>bz: Again, see https://bugzilla.mozilla.org/show_bug.cgi?id=295782 -- that might introduce more differences between this type of wrapper and implicit wrappers.</i>
</p>
<h4 name="Implicit_XPCNativeWrapper"> Implicit <code>XPCNativeWrapper</code> </h4>
<p>This type of wrapper can be created in one of two ways: either by accessing a content object from code that has <code>xpcnativewrappers=yes</code> set or by accessing a property of a <a href="#Deep_Wrappers">deep</a> wrapper.
</p><p><i>bz: The latter may not always be true pending outcome of https://bugzilla.mozilla.org/show_bug.cgi?id=295782</i>
</p><p>The wrapper created in this way is always a <a href="#Deep_wrappers">deep</a> wrapper.  Furthermore, a wrapper created in this way will stick around as long as the object being wrapped does.
</p><p><i>bz: except for https://bugzilla.mozilla.org/show_bug.cgi?id=295937 -- sort that out before trying to explain the exact behavior here.</i>
</p>
<h5 name="Deep_Wrappers"> Deep Wrappers </h5>
<p>If a wrapper is deep, that means that the return value of any property access or method call on the wrapper will be <a href="#Implicit_XPCNativeWrapper">implicitly wrapped</a>.
</p><p>If a deep wrapper is accessed from code that doesn't use <code>xpcnativewrappers=yes</code>, it will NOT provide safe access to properties.  Instead, it will automatically forward all accesses to the underlying wrappedJSObject.
</p><p><i>bz: This may not be the case for all deep wrappers pending https://bugzilla.mozilla.org/show_bug.cgi?id=295782</i>
</p>
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