gecko/toolkit/content/tests/browser/common/mockObjects.js

/* ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Mozilla XUL Toolkit Testing Code.
 *
 * The Initial Developer of the Original Code is
 * Paolo Amadini <http://www.amadzone.org/>.
 * Portions created by the Initial Developer are Copyright (C) 2009
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *
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 * either the GNU General Public License Version 2 or later (the "GPL"), or
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/**
 * Allows registering a mock XPCOM component, that temporarily replaces the
 *  original one when an object implementing a given ContractID is requested
 *  using createInstance.
 *
 * @param aContractID
 *        The ContractID of the component to replace, for example
 *        "@mozilla.org/filepicker;1".
 *
 * @param aReplacementCtor
 *        The constructor function for the JavaScript object that will be
 *        created every time createInstance is called. This object must
 *        implement QueryInterface and provide the XPCOM interfaces required by
 *        the specified ContractID (for example
 *        Components.interfaces.nsIFilePicker).
 */
function MockObjectRegisterer(aContractID, aReplacementCtor)
{
  this._contractID = aContractID;
  this._replacementCtor = aReplacementCtor;
}

MockObjectRegisterer.prototype = {
  /**
   * Replaces the current factory with one that returns a new mock object.
   *
   * After register() has been called, it is mandatory to call unregister() to
   * restore the original component. Usually, you should use a try-catch block
   * to ensure that unregister() is called.
   */
  register: function MOR_register() {
    if (this._originalFactory)
      throw new Exception("Invalid object state when calling register()");

    // Define a factory that creates a new object using the given constructor.
    var providedConstructor = this._replacementCtor;
    this._mockFactory = {
      createInstance: function MF_createInstance(aOuter, aIid) {
        if (aOuter != null)
          throw Cr.NS_ERROR_NO_AGGREGATION;
        return new providedConstructor().QueryInterface(aIid);
      }
    };

    // Preserve the original factory.
    this._originalFactory = Cm.getClassObjectByContractID(this._contractID,
                                                          Ci.nsIFactory);

    // Replace the original factory with the mock one.
    var classInfo = this._originalFactory.QueryInterface(Ci.nsIClassInfo);
    var componentRegistrar = Cm.QueryInterface(Ci.nsIComponentRegistrar);
    componentRegistrar.registerFactory(classInfo.classID,
                                       "Mock " + classInfo.classDescription,
                                       this._contractID,
                                       this._mockFactory);
  },

  /**
   * Restores the original factory.
   */
  unregister: function MOR_unregister() {
    if (!this._originalFactory)
      throw new Exception("Invalid object state when calling unregister()");

    // Free references to the mock factory.
    var classInfo = this._originalFactory.QueryInterface(Ci.nsIClassInfo);
    var componentRegistrar = Cm.QueryInterface(Ci.nsIComponentRegistrar);
    componentRegistrar.unregisterFactory(classInfo.classID,
                                         this._mockFactory);

    // Restore the original factory.
    componentRegistrar.registerFactory(classInfo.classID,
                                       classInfo.classDescription,
                                       this._contractID,
                                       this._originalFactory);

    // Allow registering a mock factory again later.
    this._originalFactory = null;
    this._mockFactory = null;
  },

  // --- Private methods and properties ---

  /**
   * The original nsIFactory for the component being replaced, or null when the
   *  original component is in place.
   */
  _originalFactory: null,

  /**
   * The nsIFactory that was automatically generated by this object.
   */
  _mockFactory: null
}