Send screen on and off state as a11y alert

ash/wm/power_button_controller.cc

Index: ash/wm/power_button_controller.cc
diff --git a/ash/wm/power_button_controller.cc b/ash/wm/power_button_controller.cc
index 9ea85af9688d4384cadcfba8d1538db73de06d38..0b126de45e9781a9d431d77882770dbf6ce36b73 100644
--- a/ash/wm/power_button_controller.cc
+++ b/ash/wm/power_button_controller.cc
@@ -5,6 +5,7 @@
 #include "ash/wm/power_button_controller.h"
 
 #include "ash/common/accelerators/accelerator_controller.h"
+#include "ash/common/accessibility_delegate.h"
 #include "ash/common/ash_switches.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/system/tray/system_tray.h"
@@ -65,7 +66,16 @@ PowerButtonController::~PowerButtonController() {
 }
 
 void PowerButtonController::OnScreenBrightnessChanged(double percent) {
-  brightness_is_zero_ = percent <= 0.001;
+  bool new_brightness_is_zero = percent <= 0.001;

[Daniel Erat, 2016/11/16 22:08:46]

is PowerButtonController the best place for this to live, or should it be
somewhere else (e.g. the code that displays the brightness bubble, or
display-configuration code, or ...)?

i ask because the backlight brightness can be changed by many things unrelated
to the power button (the user pressing the brightness keys, the user being
inactive, etc.). this isn't the first place i'd look for code that handles that.

also, does this get triggered when the brightness is set to 0 due to e.g. the
lid being closed? i'm not sure that i'd expect an a11y alert in that case (and
it might actually get cut off due to the system suspending, right?).

[Qiang(Joe) Xu, 2016/11/17 04:09:20]

I did the investigation more on this and found the case was more complex than I
thought.
About code location, I agree we might want to put in TrayBrightness or even
PowerEventObserver.

I missed the test for closelid this morning. It turns out that there is problem
with it. Closelid will announce nothing, but open lid will announce both "screen
off" and "screen on" sequentially. BrightnessChanged is sent first to trigger
a11y alert and then SuspendImminent is sent. I want to have a discussion with
dmazzoni@ to see if we can solve it on a11y side.

Also, we may only alert when BrightnessChanged is not user_initiated, since
chromevox will alert "xxx percent" when user presses brightness down/up keys.
(SetBacklightsForcedOff is not user_initiated for BrightnessChanged, which I
messed up in TabletButtonControllerTest. I will update it later.)

+
+  // Send an a11y alert when there is a switch between screen on and off.
+  AccessibilityDelegate* delegate = WmShell::Get()->accessibility_delegate();
+  if (brightness_is_zero_ && !new_brightness_is_zero)
+    delegate->TriggerAccessibilityAlert(A11Y_ALERT_SCEEEN_ON);
+  else if (!brightness_is_zero_ && new_brightness_is_zero)
+    delegate->TriggerAccessibilityAlert(A11Y_ALERT_SCREEN_OFF);
+
+  brightness_is_zero_ = new_brightness_is_zero;
 }
 
 void PowerButtonController::OnPowerButtonEvent(