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287 lines
14 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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-
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- The contents of this file are subject to the Mozilla Public License Version
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- 1.1 (the "License"); you may not use this file except in compliance with
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- the License. You may obtain a copy of the License at
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- http://www.mozilla.org/MPL/
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-
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- Software distributed under the License is distributed on an "AS IS" basis,
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- WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
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- for the specific language governing rights and limitations under the
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- License.
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-
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- The Original Code is PyXPCOM.
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-
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- The Initial Developer of the Original Code is
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- ActiveState Tool Corporation.
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- Portions created by the Initial Developer are Copyright (C) 2000-2001
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- the Initial Developer. All Rights Reserved.
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-
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- Contributor(s):
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-
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- Alternatively, the contents of this file may be used under the terms of
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- either the GNU General Public License Version 2 or later (the "GPL"), or
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- the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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- in which case the provisions of the GPL or the LGPL are applicable instead
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- of those above. If you wish to allow use of your version of this file only
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- under the terms of either the GPL or the LGPL, and not to allow others to
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- use your version of this file under the terms of the MPL, indicate your
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- decision by deleting the provisions above and replace them with the notice
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- and other provisions required by the LGPL or the GPL. If you do not delete
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- the provisions above, a recipient may use your version of this file under
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- the terms of any one of the MPL, the GPL or the LGPL.
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-
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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 Package Tutorial</title>
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</head>
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<body>
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<h1>Python XPCOM Package Tutorial</h1>
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<p>This is a quick introduction to the Python XPCOM Package. We assume that you have a good understanding of Python and <a href="http://www.mozilla.org/projects/xpcom/">XPCOM</a>,
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and have experience both using and implementing XPCOM objects in some other
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language (e.g., C++ or JavaScript). We <b><i>do not</i></b> attempt to
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provide a tutorial to XPCOM or Python itself, only to using Python <i>and</i>
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XPCOM.</p>
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<p>This tutorial contains the following sections:</p>
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<ul>
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<li><a href="#Using">Using XPCOM Objects and Interfaces</a> - when you wish to
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<i>use</i> a component written by anyone else in any XPCOM supported
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language.</li>
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<li><a href="#Implementing">Implementing XPCOM Objects and Interfaces</a> -
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when you wish to implement a component for use by anyone else in any xpcom
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supported language.</li>
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<li><a href="#Parameters">Parameters and Types</a> - useful information
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regarding how Python translates XPCOM types, and handles byref parameters.</li>
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</ul>
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<p>For anything not covered here, try the <a href="advanced.html">advanced
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documentation</a>, and if that fails, use the source, Luke!</p>
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<h2><a name="Using">Using XPCOM object and interfaces.</a></h2>
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<p>The techniques for using XPCOM in Python have been borrowed from JavaScript -
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thus, the model described here should be quite familiar to existing JavaScript
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XPCOM programmers.</p>
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<h3>xpcom.components module</h3>
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<p>When using an XPCOM object, the primary module used is the <u><i>xpcom.components</i></u>
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module. Using this module, you can get a Python object that supports any
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scriptable XPCOM interface. Once you have this Python object, you can
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simply call XPCOM methods on the object, as normal.</p>
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<p>The <u><i>xpcom.components</i></u> module defines the following public
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members:</p>
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<table border="1" width="100%">
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<tr>
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<td width="16%"><b>Name</b></td>
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<td width="84%"><b>Description</b></td>
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</tr>
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<tr>
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<td width="16%">classes</td>
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<td width="84%">A mapping (dictionary-like object) used to get XPCOM
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"classes". These are indexed by XPCOM contract ID, just
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like the JavaScript object of the same name.
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<p>Example:</p>
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<pre>cls = components.classes["@mozilla.org/sample;1"]
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ob = cls.createInstance() # Now have an nsISupports</pre>
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</td>
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</tr>
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<tr>
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<td width="16%">interfaces</td>
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<td width="84%">An object that exposes all XPCOM interface IDs (IIDs).
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Like the JavaScript object of the same name, this object uses
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"dot" notation, as demonstrated below.
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<p>Example:</p>
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<pre>ob = cls.createInstance(components.interfaces.nsISample)
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# Now have an nsISample</pre>
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</td>
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</tr>
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</table>
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<p>For many people, this is all you need to know. Consider the Mozilla Sample Component. The Mozilla Sample
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Component has a contract ID of <i>@mozilla.org/sample;1</i>,
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and implements the <i>nsISample</i> interface.</p>
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<p>Thus, a complete Python program that uses this component is shown below.</p>
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<pre>from xpcom import components
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cls = components.classes["@mozilla.org/sample;1"]
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ob = cls.createInstance() # no need to specify an IID for most components
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# nsISample defines a "value" property - let's use it!
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ob.value = "new value"
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if ob.value != "new value":
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print "Eeek - what happened?"</pre>
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<p>And that is it - a complete Python program that uses XPCOM.</p>
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<h2><a name="Implementing">Implementing XPCOM Objects and Interfaces.</a></h2>
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<p>Implementing XPCOM objects is almost as simple as using them. The
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basic strategy is this:</p>
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<ol>
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<li>Create a standard Python source file, with a standard Python class.</li>
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<li>Add some special <a href="#Attributes"> attributes</a> to your class for use by the Python XPCOM
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framework. This controls the XPCOM behavior of your object.</li>
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<li>Implement the XPCOM <a href="#Properties"> properties</a> and methods of your classes as normal.</li>
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<li>Put the Python source file in the Mozilla <i> components</i> directory.</li>
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<li>Run <i> regxpcom.</i></li>
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</ol>
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<p>Your component is now ready to be used.</p>
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<h3><a name="Attributes">Attributes</a></h3>
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<p>There are two classes of attributes: those used at runtime to define the object
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behavior and those used at registration time to control object
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registration. Not all objects require registration, thus not all
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Python XPCOM objects will have registration-related attributes.</p>
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<table border="1" width="100%">
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<tr>
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<td width="17%"><b>Attribute</b></td>
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<td width="83%"><b>Description</b></td>
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</tr>
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<tr>
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<td width="17%">_com_interfaces_</td>
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<td width="83%">The interface IDs (IIDs) supported by the component.
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For simplicity, this may be either a single IID, or a list of IIDs.
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There is no need to specify base interfaces, as all parent interfaces are
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automatically supported. Thus, it is never necessary to nominate <i>
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nsISupports</i> in the list of interfaces.
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<p>This attribute is required. Objects without such an attribute are
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deemed unsuitable for use as a XPCOM object.</td>
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</tr>
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<tr>
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<td width="17%">_reg_contractid_</td>
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<td width="83%">The contract ID of the component. Required if the
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component requires registration (i.e., exists in the components directory).</td>
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</tr>
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<tr>
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<td width="17%">_reg_clsid_</td>
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<td width="83%">The Class ID (CLSID) of the component, as a string in the
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standard "{XXX-XXX-XXX-XXX}" format. Required if the
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component requires registration (i.e., exists in the components directory).</td>
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</tr>
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<tr>
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<td width="17%">_reg_registrar_</td>
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<td width="83%">Nominates a function that is called at registration
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time. The default is for no extra function to be called. This can
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be useful if a component has special registration requirements and needs
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to hook into the registration process.</td>
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</tr>
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<tr>
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<td width="17%">_reg_desc_</td>
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<td width="83%">The description of the XPCOM object. This may be used by
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browsers or other such objects. If not specified, the contract ID
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is used.</td>
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</tr>
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</table>
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<h3><a name="Properties">Properties</a></h3>
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<p>A Python class can support XPCOM properties in one of two ways. Either
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a standard Python property of the same name can exist - our sample
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component demonstrates this with the <i>boolean_value</i> property.
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Alternatively, the class can provide the <i>get_propertyName(self)</i> and <i>set_propertyName(self,
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value)</i> functions (with <i>propertyName</i> changed to the appropriate value for the
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property), and these functions will be called instead.</p>
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<h4>Example: The Python XPCOM Test Component</h4>
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<p>As an example, examine the Python XPCOM Test Component. This
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code can be found in <i>py_test_component.py</i>.</p>
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<pre>from xpcom import components
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class PythonTestComponent:
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_com_interfaces_ = components.interfaces.nsIPythonTestInterface
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_reg_clsid_ = "{7EE4BDC6-CB53-42c1-A9E4-616B8E012ABA}"
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_reg_contractid_ = "Python.TestComponent"
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def __init__(self):
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self.boolean_value = 1
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...
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def do_boolean(self, p1, p2):
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ret = p1 ^ p2
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return ret, not ret, ret
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...</pre>
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<p><b>Note:</b> This component only specifies the mandatory attributes - <i>_com_interfaces</i>,
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<i>_reg_clsid_</i> and <i>_reg_contractid_</i>.</p>
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<p>This sample code demonstrates supporting the <i>boolean_value</i> attribute,
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supported implicitly, as it is defined in the IDL and exists as a real Python
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attribute of that name, and a method called <i>do_boolean</i>.</p>
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<h4>Tip: The xpcom/xpt.py Script</h4>
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<p> The xpcom/xpt.py script is a useful script that can generate the skeleton of a class for
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any XPCOM interface. Just specify the interface name on the command-line,
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and paste the output into your source file.</p>
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<p>This is the output of running this program over the <i>nsISample</i>
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interface (i.e., assuming we wanted to implement a component that supported this
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interface):</p>
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<pre>class nsISample:
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_com_interfaces_ = xpcom.components.interfaces.nsISample
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# If this object needs to be registered, the following 2 are also needed.
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# _reg_clsid_ = {a new clsid generated for this object}
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# _reg_contractid_ = "The.Object.Name"
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def get_value( self ):
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# Result: string
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pass
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def set_value( self, param0 ):
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# Result: void - None
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# In: param0: string
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pass
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def writeValue( self, param0 ):
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# Result: void - None
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# In: param0: string
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pass
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def poke( self, param0 ):
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# Result: void - None
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# In: param0: string
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pass</pre>
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<p><b>Note:</b> The types of the parameters and the function itself are included in
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the comments. You need to implement the functions
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themselves. Another advantage of this script is that the <a href="#HiddenParams">hidden
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parameters</a> are handled for you; the comments indicate when parameters
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have been hidden.</p>
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<h2><a name="Parameters">Parameters and Types</a></h2>
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<p>This section briefly describes the XPCOM type support in
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Python.</p>
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<p>All XPCOM interfaces define parameters of a specific type. There is
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currently no concept of a variant, or union of all types. Thus, the
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conversion rules are very straightforward, and generally surprise free: for
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any given XPCOM method, there is only one possible type for a given parameter.</p>
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<h3>Type Conversion Rules:</h3>
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<ul>
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<li>All numeric types will attempt to be coerced to the correct type.
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Thus, you can pass a Python float to an XPCOM method expecting an integer,
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or vice-versa. Specifically, when an integer is required, you can pass
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any Python object for which <i>int()</i> would succeed; for a Python float,
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any object for which <i>float()</i> would succeed is acceptable. This
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means that you can pass a Python string object as an integer, as long as the
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string was holding a valid integer.</li>
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<li>Strings and Unicode objects are interchangeable, but no other automatic
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string conversions are performed. Thus, you can not pass an integer
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where a string is expected, even though the reverse is true.</li>
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<li>Any sequence object can be passed as an array. List objects are
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always returned for arrays.</li>
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<li>Any Python instance suitable for use as a XPCOM object (i.e., with the
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<a href="#Implementing">necessary annotations</a>) can be
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passed as a XPCOM object. No special wrapping step is needed to turn a
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Python instance into a XPCOM object. Note you must pass a class <i>instance</i>,
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not the class itself.</li>
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<li><a name="HiddenParams">Many XPCOM <b> method signatures</b> specify
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"count" or "size" parameters. For example, every
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time an array is passed via XPCOM, the method signature will always specify
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an integer that holds the count of the array. These parameters are
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always hidden in Python. As the size param can be implied from the
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length of the Python sequence passed, the Python programmer need never pass
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these parameters; in contrast, JavaScript requires these redundant parameters.</a></li>
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</ul>
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<h2>Interface Flattening</h2>
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<p>Most people can ignore this information - Python XPCOM objects just
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work. However, if you are familiar with xpcom from C++ and the concept of <i>QueryInterface</i>,
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you may like to read this.</p>
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<p>Most components support the concept of "interface
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flattening". Such objects can report the interfaces they support,
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allowing languages such as Python and Javascript avoid using <i>QueryInterface</i>.
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When you are using an XPCOM object from Python, you can just call methods and
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reference properties without regard for the interface that implements it.</p>
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<p>When multiple interfaces share the same method or property name, you can use
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the name of the interface as a differentiator. Thus, <i>ob.nsIFoo.close()</i>
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will call close on <i>ob</i>'s <i>nsIFoo</i> interface, while <i>ob.nsIBar.close()</i>
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will use the <i>nsIBar</i> interface. <i>ob.close()</i> is not defined.</p>
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</body>
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</html>
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