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authorMatt A. Tobin <email@mattatobin.com>2019-11-03 00:17:46 -0400
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Issue #1258 - Part 1: Import mailnews, ldap, and mork from comm-esr52.9.1
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+<!DOCTYPE html>
+<html>
+ <head>
+ <meta charset="utf-8" />
+ <title>JsAccount Usage and Architecture</title>
+ </head>
+ <body>
+ <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.</p>
+ <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,
+ NS_MSGINCOMINGSERVER_CONTRACTID_PREFIX
+ "@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.</p>
+ <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.</p>
+ <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.</p>
+ <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.</p>
+ <p><br>
+ </p>
+ </body>
+</html>