[Night Light] CL2: Ash and system tray work

ash/system/night_light/night_light_controller.h

+// Copyright 2017 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 ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_CONTROLLER_H_
+#define ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_CONTROLLER_H_
+
+#include "ash/ash_export.h"
+#include "ash/session/session_observer.h"
+#include "base/observer_list.h"
+
+class PrefRegistrySimple;
+
+namespace ash {
+
+class SessionController;
+
+// Controls the NightLight feature that adjusts the color temperature of the
+// screen.
+class ASH_EXPORT NightLightController : public SessionObserver {
+ public:
+  class Observer {
+   public:
+    // Emitted when the NightLight status is changed.
+    virtual void OnNightLightEnabledChanged(bool enabled) = 0;
+
+   protected:
+    virtual ~Observer() {}
+  };
+
+  explicit NightLightController(SessionController* session_controller);
+  ~NightLightController() override;
+
+  static void RegisterPrefs(PrefRegistrySimple* registry);
+
+  float color_temperature() const { return color_temperature_; }
+  bool enabled() const { return enabled_; }
+
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
+  void Toggle();
+
+  void SetEnabled(bool enabled);
+
+  // Set the screen color temperature. |temperature| should be a value from
+  // 0.0f (least warm) to 1.0f (most warm).
+  void SetColorTemperature(float temperature);
+
+  // ash::SessionObserver:
+  void OnActiveUserSessionChanged(const AccountId& account_id) override;
+  void OnSessionStateChanged(session_manager::SessionState state) override;
+
+ private:
+  void Refresh();
+
+  void InitFromUserPrefs();
+
+  void PersistUserPrefs();
+
+  void NotifyStatusChanged();
+
+  // The observed session controller instance from which we know when to
+  // initialize the NightLight settings from the user preferences.
+  SessionController* const session_controller_;
+
+  // The applied color temperature value when NightLight is turned ON. It's a
+  // value from 0.0f (least warm -- NightLight is OFF) to 1.0f (most warm).

[James Cook, 2017/05/08 16:09:07]

nit: Instead of "NightLight is OFF" (which sounds to me like the feature is off)
I would say something like "least warm, default display color" or layer or
something like that.

[afakhry, 2017/05/08 17:36:26]

Yes, much better! Thanks.
Done.

+  float color_temperature_ = 0.5f;
+
+  bool enabled_ = false;
+
+  base::ObserverList<Observer> observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(NightLightController);
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_CONTROLLER_H_