Switch Preferences to use Factory

services/preferences/public/interfaces/preferences.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module prefs.mojom;
 
 import "mojo/common/values.mojom";
 
 const string kServiceName = "preferences";
 
+// Used for the creation of a PreferencesManager and to ensure that the
+// PreferencesObserver is bound at creation time.
+interface PreferencesFactory {
+  // Creates a PreferencesManager bound to the provided |observer|.
+  Create(PreferencesObserver observer, PreferencesManager& manager);

[Ben Goodger (Google), 2017/01/19 18:56:54]

fwiw, now that you've enforced a more strict 1:1 mapping, I'd try and find a
name that used the term "Client" instead of observer. Also the prefix should
probably be the same. So, e.g.

PreferencesService
PreferencesServiceClient

This is just a suggestion & can be pursued in a subsequent CL.

[jonross, 2017/01/19 19:07:05]

I was planning for a rename in a follow up.

I'll make the switch:
  PreferencesManager  -> PreferencesService
  PreferencesObserver -> PreferencesServiceClient
  PreferencesFactory  -> PreferencesServiceFactory

+};
+
 // Used to subscribe to preference changes within PreferenceManager. After
 // requesting to observe, the current values for all requested keys are sent.
 interface PreferencesObserver {
   OnPreferencesChanged(mojo.common.mojom.DictionaryValue preferences);
 };
 
 // Manages actual read/write of preference data. Accepts observers who subscribe
 // to preferences, notifying them of changes.
 interface PreferencesManager {
-  AddObserver(PreferencesObserver client);
   SetPreferences(mojo.common.mojom.DictionaryValue preferences);
   Subscribe(array<string> preferences);
 };