change overlay scrollbar hover show to hover fade in

cc/input/scrollbar_animation_controller.cc

Index: cc/input/scrollbar_animation_controller.cc
diff --git a/cc/input/scrollbar_animation_controller.cc b/cc/input/scrollbar_animation_controller.cc
index c018016c8d7f93be8c052b088abf5e6fef919cfd..896ab02abe8a45619ad6bbe0c2bf99e9165027f4 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -30,11 +30,13 @@ ScrollbarAnimationController::CreateScrollbarAnimationControllerAuraOverlay(
     base::TimeDelta show_delay,
     base::TimeDelta fade_out_delay,
     base::TimeDelta fade_out_resize_delay,
+    base::TimeDelta fade_in_duration,
     base::TimeDelta fade_out_duration,
     base::TimeDelta thinning_duration) {
   return base::WrapUnique(new ScrollbarAnimationController(
       scroll_layer_id, client, show_delay, fade_out_delay,
-      fade_out_resize_delay, fade_out_duration, thinning_duration));
+      fade_out_resize_delay, fade_in_duration, fade_out_duration,
+      thinning_duration));
 }
 
 ScrollbarAnimationController::ScrollbarAnimationController(
@@ -46,13 +48,14 @@ ScrollbarAnimationController::ScrollbarAnimationController(
     : client_(client),
       fade_out_delay_(fade_out_delay),
       fade_out_resize_delay_(fade_out_resize_delay),
+      fade_out_duration_(fade_out_duration),
       need_trigger_scrollbar_show_(false),
       is_animating_(false),
+      animation_change_(NONE),
       scroll_layer_id_(scroll_layer_id),
       currently_scrolling_(false),
       show_in_fast_scroll_(false),
       opacity_(0.0f),
-      fade_out_duration_(fade_out_duration),
       show_scrollbars_on_scroll_gesture_(false),
       need_thinning_animation_(false),
       weak_factory_(this) {
@@ -65,19 +68,22 @@ ScrollbarAnimationController::ScrollbarAnimationController(
     base::TimeDelta show_delay,
     base::TimeDelta fade_out_delay,
     base::TimeDelta fade_out_resize_delay,
+    base::TimeDelta fade_in_duration,
     base::TimeDelta fade_out_duration,
     base::TimeDelta thinning_duration)
     : client_(client),
       show_delay_(show_delay),
       fade_out_delay_(fade_out_delay),
       fade_out_resize_delay_(fade_out_resize_delay),
+      fade_in_duration_(fade_in_duration),
+      fade_out_duration_(fade_out_duration),
       need_trigger_scrollbar_show_(false),
       is_animating_(false),
+      animation_change_(NONE),
       scroll_layer_id_(scroll_layer_id),
       currently_scrolling_(false),
       show_in_fast_scroll_(false),
       opacity_(0.0f),
-      fade_out_duration_(fade_out_duration),
       show_scrollbars_on_scroll_gesture_(true),
       need_thinning_animation_(true),
       weak_factory_(this) {
@@ -118,19 +124,23 @@ void ScrollbarAnimationController::StopAnimation() {
   delayed_scrollbar_show_.Cancel();
   delayed_scrollbar_fade_out_.Cancel();
   is_animating_ = false;
+  animation_change_ = NONE;
 }
 
-void ScrollbarAnimationController::PostDelayedShow() {
+void ScrollbarAnimationController::PostDelayedFadeIn() {
   DCHECK(delayed_scrollbar_fade_out_.IsCancelled());
+  animation_change_ = FADE_IN;
   delayed_scrollbar_show_.Cancel();
-  delayed_scrollbar_show_.Reset(base::Bind(&ScrollbarAnimationController::Show,
-                                           weak_factory_.GetWeakPtr()));
+  delayed_scrollbar_show_.Reset(

[bokan, 2017/04/13 14:38:14]

Rename delayed_scrollbar_show too.

[chaopeng, 2017/04/13 15:15:37]

Done.

+      base::Bind(&ScrollbarAnimationController::StartAnimation,
+                 weak_factory_.GetWeakPtr()));
   client_->PostDelayedScrollbarAnimationTask(delayed_scrollbar_show_.callback(),
                                              show_delay_);
 }
 
 void ScrollbarAnimationController::PostDelayedFadeOut(bool on_resize) {
   DCHECK(delayed_scrollbar_show_.IsCancelled());
+  animation_change_ = FADE_OUT;
   base::TimeDelta delay = on_resize ? fade_out_resize_delay_ : fade_out_delay_;
   delayed_scrollbar_fade_out_.Cancel();
   delayed_scrollbar_fade_out_.Reset(
@@ -143,7 +153,7 @@ void ScrollbarAnimationController::PostDelayedFadeOut(bool on_resize) {
 bool ScrollbarAnimationController::Animate(base::TimeTicks now) {
   bool animated = false;
 
-  if (is_animating_) {
+  if (is_animating_ && animation_change_ != NONE) {
     if (last_awaken_time_.is_null())
       last_awaken_time_ = now;
 
@@ -166,12 +176,22 @@ bool ScrollbarAnimationController::Animate(base::TimeTicks now) {
 float ScrollbarAnimationController::AnimationProgressAtTime(
     base::TimeTicks now) {
   base::TimeDelta delta = now - last_awaken_time_;
-  float progress = delta.InSecondsF() / fade_out_duration_.InSecondsF();
+  base::TimeDelta duration =
+      animation_change_ == FADE_IN ? fade_in_duration_ : fade_out_duration_;
+  float progress = delta.InSecondsF() / duration.InSecondsF();
   return std::max(std::min(progress, 1.f), 0.f);
 }
 
 void ScrollbarAnimationController::RunAnimationFrame(float progress) {
-  ApplyOpacityToScrollbars(1.f - progress);
+  float opacity;
+
+  if (animation_change_ == FADE_IN) {
+    opacity = std::max(progress, opacity_);
+  } else {

[bokan, 2017/04/13 14:38:14]

DCHECK above that animation_change_ isn't NONE

[chaopeng, 2017/04/13 15:15:37]

Done.

+    opacity = std::min(1.f - progress, opacity_);
+  }
+
+  ApplyOpacityToScrollbars(opacity);
   if (progress == 1.f)
     StopAnimation();
 }
@@ -296,7 +316,7 @@ void ScrollbarAnimationController::DidMouseMoveNear(
   if (ScrollbarsHidden()) {
     if (need_trigger_scrollbar_show_before != need_trigger_scrollbar_show_) {
       if (need_trigger_scrollbar_show_) {
-        PostDelayedShow();
+        PostDelayedFadeIn();
       } else {
         delayed_scrollbar_show_.Cancel();
       }
@@ -324,7 +344,7 @@ bool ScrollbarAnimationController::CalcNeedTriggerScrollbarShow(
     if (scrollbar->orientation() != orientation)
       continue;
 
-    if (distance < kMouseMoveDistanceToTriggerShow)
+    if (distance < kMouseMoveDistanceToTriggerFadeIn)
       return true;
   }
 
@@ -359,7 +379,6 @@ bool ScrollbarAnimationController::Captured() const {
 }
 
 void ScrollbarAnimationController::Show() {
-  delayed_scrollbar_show_.Cancel();

[bokan, 2017/04/13 14:38:14]

Why don't we want to cancel this now? If we scroll while the mouse is nearby,
this will cause the scrollbars to show instantly - at that point, we don't need
the "fade in due to hover" animation anymore, right?

[chaopeng, 2017/04/13 15:15:37]

Yes, we should keep this.

   ApplyOpacityToScrollbars(1.0f);
 }