Add ManagedUserSettingsService and a SupervisedUserPrefStore using it.

chrome/browser/managed_mode/managed_user_settings_service.h

+// Copyright 2013 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.
+
+#ifndef CHROME_BROWSER_MANAGED_MODE_MANAGED_USER_SETTINGS_SERVICE_H_
+#define CHROME_BROWSER_MANAGED_MODE_MANAGED_USER_SETTINGS_SERVICE_H_
+
+#include <string>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/values.h"
+#include "chrome/browser/managed_mode/managed_users.h"
+
+// Defines an abstract base class for a service that stores managed user
+// settings, which are key-value pairs configured by a different user to
+// control certain browser behaviors for managed users.
+// Setting names are defined in managed_user_settings.h.
+class ManagedUserSettingsService {
+ public:
+  // A callback with all managed user settings. If the dictionary is NULL, it

[Pam (message me for reviews), 2013/08/30 13:35:53]

What does "with" mean here?

[Bernhard Bauer, 2013/08/30 14:54:27]

It's the first parameter. "A callback whose first parameter is..." seemed
awkward to me.

[Pam (message me for reviews), 2013/09/02 09:38:41]

A little bulky, but it would be clearer.

[Bernhard Bauer, 2013/09/02 11:01:07]

Done.

+  // means that the service is inactive, i.e. the user is not managed.
+  typedef base::Callback<void(const base::DictionaryValue*)> SettingsCallback;
+
+  // Adds a callback to be called when managed user settings are initially
+  // available, or when they change.
+  void Subscribe(const SettingsCallback& callback);

[Pam (message me for reviews), 2013/08/30 13:35:53]

Do we really need Yet Another Settings Notifier? And if we do, should it use Add
rather than Subscribe, to match PrefChangeRegistrar? And the notification model,
rather than a callback?

[Bernhard Bauer, 2013/08/30 14:54:27]

You mean in general, do we need a mechanism for the pref store to be notified
about changes to the managed user settings? I think so; separating the actual
syncing part from the part that makes preferences out of the settings was one of
the goals of this refactoring.
I used Subscribe because it also fires the first time settings are available,
not only when they change. [...]
Well, notifications in the NotificationService sense are deprecated, and using a
callback seemed easier to me than defining a single-method interface. It also
means we can use weak pointers instead of having to be notified when the
ManagedUserSettingsService goes away.

[Pam (message me for reviews), 2013/09/02 09:38:41]

Will anything besides the PrefStore be subscribing to changes? Will there be
more than one subclass of this? If not, I don't see it as worthwhile to create
an abstract interface for only one implementer and one subscriber.

[Bernhard Bauer, 2013/09/02 11:01:07]

The only other implementation is in tests. Testing is in fact the reason I added
the abstract interface here.

[Bernhard Bauer, 2013/09/04 09:29:05]

Removed the abstract interface. The syncing code is not in this CL yet, to keep
it smaller.

+
+  // Activates the service. This happens when the user is managed.
+  void Activate();
+
+ protected:
+  ManagedUserSettingsService();
+  virtual ~ManagedUserSettingsService();
+
+  // Sends the settings to all subscribers.

[Pam (message me for reviews), 2013/08/30 13:35:53]

What triggers this? Sync?

[Bernhard Bauer, 2013/08/30 14:54:27]

Yes. From the point of view of this class, the (not yet landed) subclass will. I
expanded the comment.

+  void NotifySettingsChanged();

[Pam (message me for reviews), 2013/08/30 13:35:53]

If it does keep using callbacks, this should be renamed; it's misleading now.
(As is the comment.)

[Bernhard Bauer, 2013/08/30 14:54:27]

Done.

[Pam (message me for reviews), 2013/09/02 09:38:41]

I meant to avoid the word "notify", with all its NotificationService
implications. Maybe the painfully literal "CallSubscriberCallbacks"? Or
InformSubscribers, if that's too awkward or too specific.

[Bernhard Bauer, 2013/09/02 11:01:07]

I like InformSubscribers! Done.

[Pam (message me for reviews), 2013/09/03 10:45:32]

CallSubscribersBack :)
Sorry, that's the cold meds talking.

+
+ private:
+  // Whether managed user settings are available.

[Pam (message me for reviews), 2013/08/30 13:35:53]

What makes them available?

[Bernhard Bauer, 2013/08/30 14:54:27]

The subclass has to load them from a Json file on disk.

+  virtual bool IsReady() const = 0;
+
+  // Returns a dictionary with all managed user settings. This method will
+  // only be called once IsReady() returns true.
+  virtual scoped_ptr<base::DictionaryValue> GetSettings() const = 0;
+
+  scoped_ptr<base::DictionaryValue> GetSettingsIfActive();

[Pam (message me for reviews), 2013/08/30 13:35:53]

Comment please. (Mainly that it returns an empty dict if not active.)

[Bernhard Bauer, 2013/08/30 14:54:27]

Done.

+
+  bool active_;
+
+  std::vector<SettingsCallback> subscribers_;
+
+  DISALLOW_COPY_AND_ASSIGN(ManagedUserSettingsService);
+};
+
+#endif  // CHROME_BROWSER_MANAGED_MODE_MANAGED_USER_SETTINGS_SERVICE_H_