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488 lines
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488 lines
14 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="GENERATOR" content="Mozilla/4.61 [en] (X11; U; Linux 2.2.5-15 i686) [Netscape]">
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<title>xpcom-review</title>
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</head>
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<body>
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<center>
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<h1>
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XPCOM Brown Bag I</h1></center>
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<center>Suresh Duddi <<a href="mailto:dp@netscape.com">dp@netscape.com</a>>
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<br>August 17, 1999
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<hr WIDTH="100%"></center>
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<h3>
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Terminology</h3>
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<table BORDER WIDTH="100%" NOSAVE >
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<tr NOSAVE>
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<td NOSAVE>Interface</td>
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<td>Contract betwen the implementation and usage. A pure virtual abstract
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base class with no data members. eg. nsIZip</td>
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</tr>
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<tr>
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<td>IID</td>
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<td>Interface ID {xxxxx-xxx-xxx-xxxxxx} a unique number that identifies
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the Interface being talked about</td>
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</tr>
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<tr>
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<td>nsISupports</td>
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<td>The mother of all interfaces. All interfaces ultimately inherit from
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this. Provides refcounting. The IID and definition has been carefully choosen
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so as to be binary compatible with COM.</td>
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</tr>
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<tr>
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<td>Component</td>
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<td>An implementation of a set of interfaces identified by a CLSID</td>
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</tr>
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<tr>
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<td>Object</td>
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<td>An instance of a Component</td>
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</tr>
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<tr>
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<td>CLSID</td>
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<td>Component ID {xxxxx-xxx-xxx-xxxxxx} a unique number that identifies
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a component</td>
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</tr>
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<tr>
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<td>Module</td>
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<td>A packaging of a set of Components (ie) a DLL</td>
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</tr>
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<tr>
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<td>Factory</td>
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<td>An object whose sole purpose is creation of objects of a particular
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component. There exists only one factory for every Component (CLSID). The
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IID and definition has been carefully choosen so as to be binary compatible
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with COM. nsIFactory</td>
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</tr>
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<tr>
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<td>ClassObject</td>
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<td>Same object as factory.</td>
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</tr>
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<tr>
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<td>ContractID</td>
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<td>A String name for a Component.</td>
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</tr>
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<tr>
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<td>ComponentManager</td>
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<td>Central Object of XPCOM that provides API for object creation and management.</td>
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</tr>
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<tr>
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<td>ServiceManager</td>
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<td>A singleton manager. Holds and manages singleton references to objects
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for application. Going to be merged with the componentmanager.</td>
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</tr>
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<tr>
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<td>Service</td>
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<td>A singleton object within an application.</td>
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</tr>
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<tr>
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<td>Registry</td>
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<td>A hierarchical attribute-value pair storage.</td>
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</tr>
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<tr>
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<td>XPIDL</td>
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<td>Cross Platform Interface Definition Language. Language for expressing
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interfaces.</td>
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</tr>
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<tr>
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<td>Typelib</td>
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<td>A storage for meta information about interfaces. Interfaces expressed
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in IDL have their meta information maintained in Typelib automatically.</td>
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</tr>
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<tr>
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<td>XPConnect</td>
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<td>Ability to call c++ implementations of interfaces expressed in IDL
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from javascript and viceversa.</td>
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</tr>
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</table>
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<h3>
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An Interface</h3>
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<ul>
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<li>
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E.g Image Decoder <a href="http://lxr.mozilla.org/seamonkey/source/modules/libimg/public_com/nsIImgDecoder.h">nsIImgDecoder</a></li>
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<br>
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<li>
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Dont put data members</li>
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<li>
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Return value of member function should always be <tt>nsresult using the
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NS_IMETHOD macro.</tt></li>
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<li>
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Use XPIDL for interface header files</li>
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<li>
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Useful Macros if not using XPIDL</li>
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<ul>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_DEFINE_STATIC_IID_ACCESSOR">NS_DEFINE_STATIC_IID_ACCESSOR</a>
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- Defines static GetIID() method for an interface</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IMETHOD">NS_IMETHOD</a>
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- Prototypes return value of interface function declaration</li>
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</ul>
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</ul>
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<h3>
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The Component</h3>
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<ul>
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<li>
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E.g GIF Image Decoder nsGIFDecoder <a href="http://lxr.mozilla.org/seamonkey/source/modules/libimg/gifcom/nsGIFDecoder.h">nsGIFDecoder.h</a>,
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<a href="http://lxr.mozilla.org/seamonkey/source/modules/libimg/gifcom/nsGIFDecoder.cpp">nsGIFDecoder.cpp</a></li>
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<br>
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<li>
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Useful Macros</li>
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<ul>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_DECL_ISUPPORTS">NS_DECL_ISUPPORTS</a>
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- Declares all methods of nsISupports. Used in implementation declaration
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header file.</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_INIT_ISUPPORTS">NS_INIT_ISUPPORTS</a>
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- nitilize the base class ISupports. Used in constructor of implementation.</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IMPL_ISUPPORTS">NS_IMPL_ISUPPORTS</a>
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- Implement the nsISupports functions. Used in implementation file.</li>
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<ul>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IMPL_ADDREF">NS_IMPL_ADDREF</a>
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- Implementation of nsISupports::Release()</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IMPL_RELEASE">NS_IMPL_RELEASE</a>
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- Implementation of nsISupports::AddRef()</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IMPL_QUERY_INTERFACE">NS_IMPL_QUERY_INTERFACE</a>
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- Implementation of nsISupports::QueryInterface() for one IID</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IMPL_ISUPPORTS2">NS_IMPL_ISUPPORTS2</a>
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- Implements the nsISupports function with QueryInterface for two IIDs</li>
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<br> </ul>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_DEFINE_IID">NS_DEFINE_IID</a>
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- Define a variable of type nsIID initialized with a struct representatio
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of an IID</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_GET_IID">NS_GET_IID</a>
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- Use this instead of nsIFoo::GetIID() or NS_DEFINE_IID()...</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_DEFINE_CID">NS_DEFINE_CID</a>
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- Define a variable of type nsCID intialized with a struct representation
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of the CLSID</li>
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<br>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IMETHODIMP">NS_IMETHODIMP</a>
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- Prototypes return value for interface functions implementation. Used
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in implementation file.</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_ADDREF">NS_ADDREF</a>,
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IF_ADDREF">NS_IF_ADDREF</a>
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- Addref an object by client</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_ADDREF_THIS">NS_ADDREF_THIS</a>
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- Addref used in object implementation. Dont need this if using NS_IMPL_ADDREF</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_RELEASE">NS_RELEASE</a>,
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_IF_RELEASE">NS_IF_RELEASE</a>
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- Release an object by client</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_RELEASE_THIS">NS_RELEASE_THIS</a>
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- Released used in object implementation. Dont need this if using NS_IMPL_RELEASE</li>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_RELEASE2">NS_RELEASE2</a>
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- Release and get the refcount. Useful to check if refcount hit Zero.</li>
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<br> </ul>
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<li>
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Generic Factory - A generic factory implementation with callback ability
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for object creation. Simplies factory implementation.</li>
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</ul>
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<h3>
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Object Creation</h3>
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<ul><tt>rv = compMgr->CreateInstance(imgContractID, NULL, <a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_GET_IID">NS_GET_IID</a>(nsIImgDecoder),
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(void **)&imgdec);</tt>
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<br>
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<li>
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It is preferable to use the NS_GET_IID() version of getting iid instead
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of the old way</li>
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<ul>
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<li>
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NS_DEFINE_IID(kImgDecodeIID, NS_IIMGDECODER_IID);</li>
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<br>and then using kImgDecoderIID for the IID
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<li>
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<b>DO NOT USE nsIImgDecoder::GetIID()</b></li>
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</ul>
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<li>
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The component manager does this on a CreateInstance()</li>
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<ul>
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<li>
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map ContractID to CLSID</li>
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<li>
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Get module object for dll which implements component CLSID</li>
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<li>
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nsISupports classObject = module->GetClassObject()</li>
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<li>
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nsIFactory factory = classObject->QueryInterface(NS_GET_IID(nsIFactory));</li>
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<li>
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return factory->CreateInstance(IID);</li>
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</ul>
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<li>
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<b>[hint]</b> Use nsCOMPtr</li>
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<br>
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<ul><tt>nsCOMPtr <nsIImgDecoder> imgdec;</tt>
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<br><tt>rv = compMgr->CreateInstance(imgContractID, NULL, <a href="http://lxr.mozilla.org/seamonkey/ident?i=NS_GET_IID">NS_GET_IID</a>(nsIImgDecoder),
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<font color="#990000">getter_AddRefs(imgdec)</font>);</tt></ul>
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</ul>
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<h3>
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Object Release</h3>
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<blockquote><tt>NS_IF_RELEASE(obj);</tt>
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<p>It should be noted that release and addref are directly talking to the
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object with no intermediaries. The macro NS_*_RELEASE() set the object
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to NULL.</blockquote>
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<h3>
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Component Registration Process</h3>
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<ul>
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<li>
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<a href="http://lxr.mozilla.org/seamonkey/source/xpcom/components/nsIModule.idl">nsIModule.idl</a></li>
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</ul>
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<ul>At startup of apprunner, a call to <tt>nsIComponentManager::AutoRegister(<i>Startup,
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<exedir>/Components</i>)</tt> This will descend recursively looking
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for loadable files (files with a certain externsion and on Mac of certain
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type) If the file had changed since the last time the file was visited
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(ModifiedDate check) registration is initiated on the file.
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<p>Registration involves:
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<ul>
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<li>
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Load the dll</li>
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<li>
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nsIModule module = "NSGetModule"() - Create the module object</li>
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<li>
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module->RegisterSelf(<i>location</i>)</li>
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</ul>
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<p><br>The module registers with the componentmanager every {CLSID, ContractID}
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it implements with a call to <tt>nsIComponentManager::RegisterComponentSpec()</tt>
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These registrations are stored in the registry. Hence the next time around,
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the module is loaded not at startup but only when the first object that
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is implemented by a component in the module is created.
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<p>e.g Registration of GIF Decoder <a href="http://lxr.mozilla.org/seamonkey/source/modules/libimg/gifcom/nsGIFModule.cpp">nsGIFModule.cpp</a>
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<p><b>DO NOT CONVERT YOUR COMPONENT TO nsIModule yet. We are working on
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the sample code which will make the conversion process even simpler.</b></ul>
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<h3>
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Module Unloading</h3>
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<blockquote>Every module implements a CanUnload() function that returns
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a boolean of whether the module can be unloaded. This implies that the
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module needs to be counting all outstanding object and return PR_TRUE only
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if there are no outstanding objects served from within the module.
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<p>Unloading happens either on a Timer or on an explicit call to nsComponentManager::<a href="http://lxr.mozilla.org/seamonkey/ident?i=FreeLibraries">FreeLibraries()</a>.
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The general logic is:
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<ul>
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<li>
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PRBool unloadable = module->CanUnload()</li>
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<li>
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if (unloadable && marked for unload)</li>
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<ul>
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<li>
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Unload()</li>
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</ul>
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<li>
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if (unloadable && not mark for unload)</li>
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<ul>
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<li>
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mark for unload</li>
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</ul>
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</ul>
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No assumptions can be made that the module will be unloaded if nsIModule::CanUnload()
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returns PR_TRUE. It is guaranteed that if nsIModule::CanUnload() returns
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PR_FALSE, the module <b>will not</b> be unloaded.</blockquote>
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<h3>
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The Component Registry</h3>
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<blockquote>Nothing but a persistent hiearchical store of attribute value
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pairs. It is the use of the registry by XPCOM that is very interesting.
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Physical location of the registry is in the <exedir>/component.reg
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<p>The xpcom hierarchy of the registry is considered to be opaque to components.
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Use the nsIComponentManager Api to register as opposed to meddling with
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the xpcom hierarchy in the registry directly.
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<p>mozregistry.dat is NOT the component registry. It is the application
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(apprunner) registry.</blockquote>
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<h3>
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Old Method for Module definition</h3>
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<blockquote>Previously module definition was scattered across many exported
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functions from a dll.
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<br> <a href="http://lxr.mozilla.org/seamonkey/ident?i=NSGetFactory">NSGetFactory(
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)</a>, <a href="http://lxr.mozilla.org/seamonkey/ident?i=NSCanUnload">NSCanUnload(
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)</a>, <a href="http://lxr.mozilla.org/seamonkey/ident?i=NSRegisterSelf">NSRegisterSelf(
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)</a>, <a href="http://lxr.mozilla.org/seamonkey/ident?i=NSUnregisterSelf">NSUnregisterSelf(
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)</a>
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<br>Now these are combined into a <a href="http://lxr.mozilla.org/seamonkey/source/xpcom/components/nsIModule.idl">nsIModule</a>
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<p><b>Please continue to use the old method until sample code using nsIModule
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is available.</b></blockquote>
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<h3>
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ComponentManager vs ServiceManager</h3>
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<blockquote>ServiceManager is a an object that holds instances to objects
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in a cache and manages them. It uses the ComponentManager to create an
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instance the first time an object is queried for. Lifetime of the instance
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is currently the lifetime of the app. We are working on what is required
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here and how to control the lifetime. Possbily merging ServiceManager with
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the ComponentManager.
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<p>From the components perspective there is nothing it does special to
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behave as a service. The application controls the creation of services
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by its usage or call to <tt>nsServiceManager::GetService()</tt> .Some components
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that want to enforce being a service return cached instances on <tt>nsComponentManager::CreateInstance()
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</tt>.
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We are working on a more elegant syntax and semantics for this. Post suggestions
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to newsgroup netscape.public.mozilla.xpcom (or Email <a href="mailto:mozilla-xpcom@mozilla.org">mozilla-xpcom@mozilla.org</a>)
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with suggestions.</blockquote>
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<h3>
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ContractID vs CLSID</h3>
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<blockquote>CLSID is a number identifying a specific implementation.
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<p>ContractID is a like a pointer to a CLSID. It is an opaque string to xpcom.It
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is used for two purposes.
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<ol>
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<li>
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Implementation independence for clients: <tt>@mozilla.org/registry;1</tt></li>
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<br>The CLSID associating itself to a ContractID can be changed. Hence if a
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new implementation with a new CLSID superceeds an older implementation,
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the client wont be tied to the older implementation if it uses the ContractID.
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<li>
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CLSID switching: <tt>@mozilla.org/image/decoder;1?type=image/gif</tt></li>
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<br>A client wants to create different objects based on a switch (the mimetype
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for the case of imagedecoders). A Contractid is formed with a concatenation
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of a BASE_CONTRACTID and the switch (mimetype) and an instance of that created.
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Components, when registering, associate their CLSID to specific contractids.</ol>
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NOTE: Only Clients that are adamant about the use of a specific implementation
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need to create instances using CLSID.
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<p>We encourage use of ContractID. This works well in the scriptable world
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too.
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<p><b>We are working on a ContractID syntax.</b></blockquote>
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<h3>
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Steps for XPCOM Review</h3>
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<ol>
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<li>
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Know your interfaces and objects.</li>
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<br>List the objects your component is serving. List the interfaces that
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it implements.
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<li>
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List the objects that your component uses.</li>
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<li>
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<a href="http://www.mozilla.org/scriptable/interface-rules-we-break.html">http://www.mozilla.org/scriptable/interface-rules-we-break.html</a></li>
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<li>
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Do not switch over to using nsIModule yet. We will publish sample code
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in a week with guidelines.</li>
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<li>
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Use the above stated macros.</li>
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<li>
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<a href="http://www.mozilla.org/projects/xpcom/">http://www.mozilla.org/projects/xpcom/</a>
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- <b>Watch for more documents here</b></li>
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</ol>
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<p><br>
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<hr WIDTH="100%">
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<br>
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</body>
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</html>
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