Only update system power indicators on change

ash/system/power/tray_power.cc

Index: ash/system/power/tray_power.cc
diff --git a/ash/system/power/tray_power.cc b/ash/system/power/tray_power.cc
index b2a7aa1d9a78f539d5f5282e7d698d692a0eec26..32b88a036bcff60d5fc41ce4cc7c4fe130f8c52d 100644
--- a/ash/system/power/tray_power.cc
+++ b/ash/system/power/tray_power.cc
@@ -63,7 +63,7 @@ namespace tray {
 // This view is used only for the tray.
 class PowerTrayView : public views::ImageView {
  public:
-  PowerTrayView() {
+  PowerTrayView() : battery_icon_index_(-1) {
     UpdateImage();
   }
 
@@ -82,33 +82,54 @@ class PowerTrayView : public views::ImageView {
 
  private:
   void UpdateImage() {
-    SetImage(TrayPower::GetBatteryImage(supply_status_, ICON_LIGHT,
-                                        GetImage()));
+    int index = TrayPower::GetBatteryImageIndex(supply_status_);
+    if (battery_icon_index_ != index) {
+      battery_icon_index_ = index;
+      if (battery_icon_index_ == -1)
+        SetImage(GetImage());
+      else
+        SetImage(TrayPower::GetBatteryImage(battery_icon_index_, ICON_LIGHT));
+    }
   }
 
   PowerSupplyStatus supply_status_;
 
+  // Index of the current icon in the icon array image, or -1 if unknown.
+  int battery_icon_index_;
+
   DISALLOW_COPY_AND_ASSIGN(PowerTrayView);
 };
 
 class PowerNotificationView : public TrayNotificationView {
  public:
   explicit PowerNotificationView(TrayPower* tray)
-      : TrayNotificationView(tray, 0) {
+      : TrayNotificationView(tray, 0),
+        battery_icon_index_(-1) {
     power_status_view_ =
         new PowerStatusView(PowerStatusView::VIEW_NOTIFICATION, true);
     InitView(power_status_view_);
   }
 
   void UpdatePowerStatus(const PowerSupplyStatus& status) {
-    SetIconImage(TrayPower::GetBatteryImage(status, ICON_DARK,
-                                            GetIconImage()));
+    int index = TrayPower::GetBatteryImageIndex(status);
+    if (battery_icon_index_ != index) {
+      battery_icon_index_ = index;
+      if (battery_icon_index_ == -1) {
+        SetIconImage(GetIconImage());
+      } else {
+        SetIconImage(TrayPower::GetBatteryImage(battery_icon_index_,
+                                                ICON_DARK));
+      }
+    }
     power_status_view_->UpdatePowerStatus(status);
   }
 
  private:
   PowerStatusView* power_status_view_;
 
+  // Index of the current icon in the icon array image, or -1 if unknown.
+  int battery_icon_index_;
+
   DISALLOW_COPY_AND_ASSIGN(PowerNotificationView);
 };
 
@@ -126,14 +147,7 @@ TrayPower::~TrayPower() {
 }
 
 // static
-gfx::ImageSkia TrayPower::GetBatteryImage(
-    const PowerSupplyStatus& supply_status,
-    IconSet icon_set,
-    const gfx::ImageSkia& default_image) {
-  gfx::Image all = ui::ResourceBundle::GetSharedInstance().GetImageNamed(
-      icon_set == ICON_DARK ?
-      IDR_AURA_UBER_TRAY_POWER_SMALL_DARK : IDR_AURA_UBER_TRAY_POWER_SMALL);
-
+int TrayPower::GetBatteryImageIndex(const PowerSupplyStatus& supply_status) {
   int image_index = 0;
   if (supply_status.battery_percentage >= 100) {
     image_index = kNumPowerImages - 1;
@@ -141,19 +155,27 @@ gfx::ImageSkia TrayPower::GetBatteryImage(
     image_index = kNumPowerImages;
   } else {
     // If power supply is calculating battery time, the battery percentage
-    // is uncertain, just return |default_image|.
+    // is uncertain, just return -1.
     if (supply_status.is_calculating_battery_time)
-      return default_image;
+      return -1;
     image_index = static_cast<int>(supply_status.battery_percentage /
                   100.0 * (kNumPowerImages - 1));
     image_index = std::max(std::min(image_index, kNumPowerImages - 2), 0);
   }
+  return (image_index << 1) | (supply_status.line_power_on ? 1 : 0);
+}
+
+// static
+gfx::ImageSkia TrayPower::GetBatteryImage(int image_index, IconSet icon_set) {
+  gfx::Image all = ui::ResourceBundle::GetSharedInstance().GetImageNamed(
+      icon_set == ICON_DARK ?
+      IDR_AURA_UBER_TRAY_POWER_SMALL_DARK : IDR_AURA_UBER_TRAY_POWER_SMALL);
 
   // TODO(mbolohan): Remove the 2px offset when the assets are centered. See
   // crbug.com/119832.
   gfx::Rect region(
-      (supply_status.line_power_on ? kBatteryImageWidth : 0) + 2,
-      image_index * kBatteryImageHeight,
+      ((image_index & 0x1) ? kBatteryImageWidth : 0) + 2,
+      (image_index >> 1) * kBatteryImageHeight,
       kBatteryImageWidth - 2, kBatteryImageHeight);
   return gfx::ImageSkiaOperations::ExtractSubset(*all.ToImageSkia(), region);
 }