[ash-md] Reduce thread hopping for cheap wallpaper color extraction.

components/wallpaper/wallpaper_color_calculator.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 COMPONENTS_WALLPAPER_WALLPAPER_COLOR_CALCULATOR_H_
 #define COMPONENTS_WALLPAPER_WALLPAPER_COLOR_CALCULATOR_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "base/time/time.h"
 #include "components/wallpaper/wallpaper_export.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/gfx/color_analysis.h"
 #include "ui/gfx/image/image_skia.h"
 
 namespace base {
 class TaskRunner;
 }
 
 namespace wallpaper {
 class WallpaperColorCalculatorObserver;
 
-// Asynchronously calculates colors based on a wallpaper image.
+// Calculates colors based on a wallpaper image.
 class WALLPAPER_EXPORT WallpaperColorCalculator {
  public:
   // |image|, |luma| and |saturation| are the input parameters to the color
   // calculation that is executed on the |task_runner|.
   WallpaperColorCalculator(const gfx::ImageSkia& image,
                            color_utils::LumaRange luma,
                            color_utils::SaturationRange saturation,
                            scoped_refptr<base::TaskRunner> task_runner);
   ~WallpaperColorCalculator();
 
   void AddObserver(WallpaperColorCalculatorObserver* observer);
 
   void RemoveObserver(WallpaperColorCalculatorObserver* observer);
 
   // Initiates the calculation and returns false if the calculation fails to be
-  // initiated.  Callers should be aware that this will make |image_| read-only.
+  // initiated. Observers may be notified synchronously or asynchronously.
+  // Callers should be aware that this will make |image_| read-only.
   bool StartCalculation() WARN_UNUSED_RESULT;
 
   SkColor prominent_color() const { return prominent_color_; }
 
   void set_prominent_color_for_test(SkColor prominent_color) {
     prominent_color_ = prominent_color;
   }
 
   // Explicitly sets the |task_runner_| for testing.
   void SetTaskRunnerForTest(scoped_refptr<base::TaskRunner> task_runner);
 
  private:
+  // Handles asynchronous calculation results. |async_start_time| is used to
+  // record duration metrics.
+  void OnAsyncCalculationComplete(base::TimeTicks async_start_time,
+                                  SkColor prominent_color);
+
   // Notifies observers that a color calulation has completed. Called on the
   // same thread that constructed |this|.
   void NotifyCalculationComplete(SkColor prominent_color);
 
   // The result of the color calculation.
   SkColor prominent_color_ = SK_ColorTRANSPARENT;
 
   // The image to calculate colors from.
   gfx::ImageSkia image_;
 
   // Input for the color calculation.
   color_utils::LumaRange luma_;
 
   // Input for the color calculation.
   color_utils::SaturationRange saturation_;
 
   // The task runner to run the calculation on.
   scoped_refptr<base::TaskRunner> task_runner_;
 
-  // The time that StartCalculation() was last called. Used for recording
-  // timing metrics.
-  base::Time start_calculation_time_;
-
   base::ObserverList<WallpaperColorCalculatorObserver> observers_;
 
   base::WeakPtrFactory<WallpaperColorCalculator> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WallpaperColorCalculator);
 };
 
 }  // namespace wallpaper
 
 #endif  // COMPONENTS_WALLPAPER_WALLPAPER_COLOR_CALCULATOR_H_