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117 lines
6.0 KiB
HTML
117 lines
6.0 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>Architecture</title>
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</head>
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<body>
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<h1>Python XPCOM Package Architecture</h1>
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<h2><a name="Architecture">Architecture</a></h2>
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<p>Much of the design for the Python XPCOM Package has been borrowed from the Python MS-COM
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extensions in <i>win32com</i>. Most of the major limitations and drawbacks in the <i>win32com</i>
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design have been addressed, mainly "auto-wrapping" of
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interface objects, which is not supported by <i>win32com</i>.</p>
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<p>Like <i>win32com</i>, this architecture includes the concept of <i>client COM</i> and <i>server
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COM.</i> </p>
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<p>Client COM:</p>
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<ul>
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<li>calls other interfaces</li>
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<li>is supported by <i>PyInterfaces</i> implemented in C++, which assists
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in making the COM calls</li>
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<li>is supported by <i>PyGateways</i>, which assists in receiving
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external COM calls and dispatching them to the correct Python object</li>
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<li> is supported in the <i>xpcom/client</i> package</li>
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</ul>
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<p>Server COM:</p>
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<ul>
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<li>implements interfaces for use by other XPCOM applications or components</li>
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<li> is
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supported in the <i>xpcom/server</i> package</li>
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</ul>
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<p>The XPConnect framework is very powerful, and far exceeds what COM's <i>
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IDispatch</i> can offer. Thus, we are able to get by with far fewer interfaces
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supported in the C++ level, and defer most things to the Python code that uses
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XPConnect. As a result, the requirement for a huge number of interfaces to
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exist in the <i>.pyd</i> does not exist. There are, however, a number of
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interfaces that do require native C++ support: these are interfaces
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required to "boot" the XPConnect support (i.e., the interfaces that are
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used to get information about interfaces), and also two gateways that need to
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work without interface information available. This last requirement is
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due to the XPCOM shutdown-ordering - it may be a bug, but is not an unreasonable
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amount of code anyway.</p>
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<p><b>Auto-wrapping</b> of COM objects is supported by both client COM and
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server COM. For client COM, auto-wrapping means that the
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Python programmer always sees Python "component" objects, rather than
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raw C++ interface objects; to the user, it all appears to "just
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work". This is a major source of frustration in the <i>win32com</i>
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framework.</p>
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<p>For server COM, auto-wrapping means that you can
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pass Python instances wherever a COM object is expected. If the Python
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instance supports COM interfaces, by virtue of having a <i>_com_interfaces_</i>
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attribute that lists the interface requested, it will be automatically wrapped
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in the correct COM object. </p>
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<p><b>Error Handling:</b> The C++ framework has good error handling support,
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and uses the XPCOM console service to log debug messages, Python exceptions and
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tracebacks. <i>win32com</i> does not have good exception/traceback support
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at the C++ level, mainly because COM does not define a service like
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the console where debug messages can go. This does mean that in Mozilla
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release builds, these debug messages are likely to be lost, but the <i>--console</i>
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command line option to a release Mozilla will get them back. Therefore,
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the other error-support utilities, such as the error callbacks made on the
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policy object, may be used.</p>
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<p><b>Component Loader, Modules and Factories:</b> XPCOM has the concept
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of a component loader - a module used to load all components of a
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particular type. For example, the <i>moz.jsloader.1</i> component loads all
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the JavaScript components. Similarly, the <i>moz.pyloader.1</i>
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component loads all Python components. However, unlike
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JavaScript, the Python component loader is actually implemented in Python
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itself! Since the Python component loader can not be used to load
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itself, this component has some special code, <i>pyloader.dll,</i> to boot-strap itself.</p>
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<p>This means is that all XPCOM components, including the Python loader itself and all
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XPCOM module and factory interfaces, are implemented in
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Python. <b>There are no components or interfaces implemented purely in C++
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in this entire package!</b></p>
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</body>
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</html>
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