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177 lines
9.3 KiB
HTML
<html>
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<!-- ***** BEGIN LICENSE BLOCK *****
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- Version: MPL 1.1/GPL 2.0/LGPL 2.1
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- The contents of this file are subject to the Mozilla Public License Version
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- The Original Code is PyXPCOM.
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- ActiveState Tool Corporation.
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- ***** END LICENSE BLOCK ***** -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
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<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
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<meta name="ProgId" content="FrontPage.Editor.Document">
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<title>Python XPCOM Advanced Topics</title>
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</head>
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<body>
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<h1>Python XPCOM Advanced Topics</h1>
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<p>This document contains a series of tidbits that don't fit
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anywhere else. As the Python XPCOM Package documentation matures, most of
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these topics will have another home.</p>
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<h2>XPCOM Services</h2>
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<p>An XPCOM service is simply a singleton registered by name. Python has
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full support for both using and implementing XPCOM services. To use a
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service, use <i>xpcom.components.services</i> just like the JavaScript
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counterpart. There is nothing special about implementing a service in
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Python; see the standard XPCOM documentation on services for more information.</p>
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<h2>nsIVariant</h2>
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<p>There is (almost) full support for <i>nsIVariant</i>. Any <i>nsIVariant</i>
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parameters will automatically be translated to and from regular Python objects
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giving, in effect, a multi-type parameter. This should be automatic, so
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there is not much else to say! Note that if you really want, you can
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create and pass your own <i>nsIVariant</i> object instead of a regular Python
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object, thereby allowing explicit control over the type of variant created.</p>
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<h2>nsISupports Primitives.</h2>
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<p>There is a set of interfaces described in <i>nsISupportsPrimitives.idl</i>, which I
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term collectively the <i>nsISupports Primitives Interfaces</i>. These
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are a set of interfaces a component can support to allow automatic conversion to
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and from many basic types. For example, an interface can define that it
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supports the <i>nsISupportsCString</i> interface, and this could be used by any
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program that wishes to get a string representation of the object. If an
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interface wishes to expose itself as a "boolean value", it may choose
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to support the <i>nsISupportsPRBool</i> interface.</p>
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<p>When you call an XPCOM object (i.e., you have an XPCOM interface you are
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calling), you can use
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the builtin functions <i>str()</i>, <i>int()</i>, <i>long()</i> etc., on the
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object<i>.</i> In the
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case of <i>str()</i>, if the object does not support the <i>nsISupportsCString</i>
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or <i>nsISupportsString</i> interfaces, the default string <i>str()</i> for the
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object will be returned (i.e., what is normally returned for most XPCOM objects -
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support for these interface is not very common!). In the case of the numeric functions, a <i>ValueError</i>
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exception will be raised if the objects do not support any interface that can be
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used for the conversion. <i>ValueError</i> is used instead of <i>TypeError</i>,
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as the type itself (i.e., an XPCOM object) can sometimes be used in this context -
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hence it is the specific <i>value</i> of the object that is the problem.</p>
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<p>The use of <i>repr()</i> on an XPCOM interface object prevents support
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attempts for these interfaces, and allows you to see the
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"real" object, rather than what the object wants you to see!</p>
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<p>When you implement an XPCOM object, you have two choices for implementation
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of these interfaces:</p>
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<ul>
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<li>You can explicitly handle these interfaces like any other interface.
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In this case, you have full control. However, if you
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implement only one of these standard interfaces, then you are only
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overriding the default behavior for that specific interface - all other
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interfaces not explicitly listed in your class will still get the behavior
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described below.<br>
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</li>
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<li>If your class does not define support for these interfaces, the framework
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will use standard Python class semantics to implement them - i.e., if your
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class provides a <i>__str__</i> method, it will be used to implement <i>nsISupportsCString</i>
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and <i>nsISupportsString</i>, if you provide <i>__int__</i>, <i>__long__</i>,
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<i>__float__</i> etc., methods, they will be used to implement the numeric
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interfaces. If your class defines no such special methods, then the <i>
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QueryInterface()</i> for those interfaces fails (rather than the QI succeeding
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and the operation to fetch the data failing).</li>
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</ul>
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<blockquote>
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<p>This allows for an interesting feature that would not normally be
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possible. Consider Python code that does a <i>str()</i> on an XPCOM
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interface, and where the XPCOM interface itself is implemented in Python and
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provides a <i>__str__</i> method. The <i>str()</i> on the original
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interface queries for the <i>nsISupportsCString</i> interface. The
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Python implemented object responds to this interface and delegates to the <i>__str__</i>
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method. At the end of all this, <i>str()</i> returns the same result
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as if the objects were native Python objects with no XPCOM layer in between.</p>
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</blockquote>
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<h2>Enumerators</h2>
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<p>The primary enumerator used by XPCOM is <i>nsISimpleEnumerator</i>.
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Although the Python XPCOM package has full support for <i>nsIEnumerator</i>,
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since this interface is not "scriptable", you should avoided using it in interfaces
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you design.</p>
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<p>When you use <i>nsISimpleEnumerator</i> from Python, the following enhancements
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are available:</p>
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<ul>
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<li>The <i>GetNext()</i> method takes an optional IID as a parameter. If
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this is specified, the returned object will be of this interface. This
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prevents the manual <i>QueryInterface()</i> generally required from other
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languages.</li>
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<li>There is a <i>FetchBlock(num, [iid])</i> method, which fetches the
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specified number of elements in one operation and returns a Python
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list. This can be useful for large enumerator sets, so the loop
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iterating the elements runs at full C++ speed.</li>
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</ul>
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<p><i>nsIEnumerator</i> has similar enhancements.</p>
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<p>When implementing a Python XPCOM object, the Python class <i>xpcom.server.enumerator.SimpleEnumerator()</i>
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can be used. You can pass a standard Python sequence (list, etc), and it
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will be correctly wrapped in an <i>nsISimpleEnumerator</i> interface.</p>
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<h2>Files</h2>
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<p>The Python XPCOM package provides an <i> xpcom.file</i> module. This implements
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a Python-like file object on top of the XPCOM/Mozilla stream interfaces.
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When run from within the Mozilla environment, this allows you to open almost any
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URL supported by Mozilla (including "chrome://" etc.,).</p>
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<p>See this module for more information, including test code.</p>
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<h2>XPCOM Object Identity</h2>
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<p>XPCOM has defined rules for object identity and for how objects must behave
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in their <i> QueryInterface()</i> implementations. The Python XPCOM framework
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manages this for you; your code can return new Python instances etc., when
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responding to new interfaces, and the framework itself will ensure the XPCOM
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semantics are followed. Critically, the framework provides no mechanism
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for breaking these rules.</p>
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<h2>Policies</h2>
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<p>The Python XPCOM framework has the concept of "policies" that
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define how XPCOM semantics are mapped to Python objects. It is the policy
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that implements delegation of <i> QueryInterface()</i>, translates property
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references into direct property references, and failing that, "get_name"
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and "set_name" calls, decides how to handle exceptions in the
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component, and so on.</p>
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<p>The default policy is very flexible and suitable for most purposes.
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Indeed, the Komodo project has never had to implement a custom policy.
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However, you should be aware the feature exists should you wish to do some
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bizarre things, such as using Python as a bridge between XPCOM and some other
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component technology.</p>
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</body>
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</html>
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