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    <title>JsAccount Usage and Architecture</title>

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    <h1>Overview</h1>

<p>

      JsAccount is a technology that allows message account types to be created

      in Mozilla Mailnews code using JavaScript. Although this is primarily

      targeted at allowing extensions to create new accounts, it might also be

      useful as a bridge to convert existing account types from being C++ based

      to JavaScript based.

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    <h2>Existing C++-based architecture of mailnews accounts</h2>

<p>

      In mailnews code, an account type is a set of classes that allow

      implementation of a messaging particular protocol. The account type is

      given a short string identifier ("imap", "news", "pop3") and is then used

      to create objects of the appropriate type by appending that string to the

      end of a base XPCOM contractID. So, for example, to create an imap server,

      you generate a contractID using a base ID,

      "@mozilla.org/messenger/server;1?type=", then append "imap" to get:

    </p>

    <p>@mozilla.org/messenger/server;1?type=imap</p>

<p>

      In the C++ code, there is a base object implementing shared functionality.

      An account-specific class inherits that base functionality, then extends

      it to represent the account-specific behavior that is needed. This same

      basic concept is used to represent a whole series of classes that are

      necessary to implement a specific mailnews account type.

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<p>

      For the server example, there is a base class named

      nsMsgIncomingServer.cpp that implements that base interface

      nsIMsgIncomingServer.idl. For imap, there is a specific class

      nsImapIncomingServer.cpp that inherits from nsMsgIncomingServer.cpp,

      overrides some of the methods in nsIMsgIncomingServer.idl, and also

      implements an imap-specific interface nsIImapIncomingServer.idl. All of

      this works fine using C++ inheritance and polymorphism.

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<p>

      Although JsAccount is intended mostly for mailnews accounts, the same

      basic method of using a base class extended for specific types is also

      used in other ways in mailnews code, including for addressbook types and

      views. The technology may also be applied to those other object types as

      well.

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    <h2>Role of JsAccount</h2>

<p>

      The JavaScript class system works very differently than the C++ system,

      and you cannot use normal language constructs to override a C++ class with

      a JavaScript class. What JsAccount allows you to do is to create XPCOM

      objects in JavaScript, and use those objects to override or extend the

      methods from the C++ base class in a way that will function correctly

      whether those objects are executed from within C++ code or JavaScript

      code. This allows you to create a new account using JavaScript code, while

      using the same base class functionality that is used by the core C++

      account types. Thus a new account type may be created in JavaScript-based

      extension. The technology may also be used to create JavaScript versions

      of existing account types in an incremental manner, slowly converting

      methods from C++ to JavaScript.

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