Media Controls: Create panel listener on demand

Create the 'transitionend' event listener on the panel on demand instead
of leaving it around when we do not need it.

BUG=832771

Change-Id: If722535b63052a0fc6383a7b1fbcc158a8fb284c
Reviewed-on: https://chromium-review.googlesource.com/1022317
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552769}

third_party/blink/renderer/modules/media_controls/elements/media_control_panel_element.cc

@@ -33,14 +33,24 @@ class MediaControlPanelElement::TransitionEventListener final
         element_(element) {
     DCHECK(callback_);
     DCHECK(element_);
+  }
+
+  void Attach() {
+    DCHECK(!attached_);
+    attached_ = true;
 
-    element->addEventListener(EventTypeNames::transitionend, this, false);
+    element_->addEventListener(EventTypeNames::transitionend, this, false);
   }
 
   void Detach() {
+    DCHECK(attached_);
+    attached_ = false;
+
     element_->removeEventListener(EventTypeNames::transitionend, this, false);
   }
 
+  bool IsAttached() const { return attached_; }
+
   bool operator==(const EventListener& other) const override {
     return this == &other;
   }
@@ -60,6 +70,8 @@ class MediaControlPanelElement::TransitionEventListener final
     NOTREACHED();
   }
 
+  bool attached_ = false;
+
   Callback callback_;
   Member<Element> element_;
 };
@@ -67,11 +79,7 @@ class MediaControlPanelElement::TransitionEventListener final
 MediaControlPanelElement::MediaControlPanelElement(
     MediaControlsImpl& media_controls)
     : MediaControlDivElement(media_controls, kMediaControlsPanel),
-      event_listener_(new MediaControlPanelElement::TransitionEventListener(
-          this,
-          WTF::BindRepeating(
-              &MediaControlPanelElement::HandleTransitionEndEvent,
-              WrapWeakPersistent(this)))) {
+      event_listener_(nullptr) {
   SetShadowPseudoId(AtomicString("-webkit-media-controls-panel"));
 }
 
@@ -95,6 +103,9 @@ void MediaControlPanelElement::MakeOpaque() {
   opaque_ = true;
 
   if (is_displayed_) {
+    // Make sure we are listening for the 'transitionend' event.
+    EnsureTransitionEventListener();
+
     SetIsWanted(true);
     removeAttribute("class");
     DidBecomeVisible();
@@ -105,13 +116,16 @@ void MediaControlPanelElement::MakeTransparent() {
   if (!opaque_)
     return;
 
+  // Make sure we are listening for the 'transitionend' event.
+  EnsureTransitionEventListener();
+
   setAttribute("class", kTransparentClassName);
 
   opaque_ = false;
 }
 
 void MediaControlPanelElement::RemovedFrom(ContainerNode*) {
-  event_listener_->Detach();
+  DetachTransitionEventListener();
 }
 
 void MediaControlPanelElement::Trace(blink::Visitor* visitor) {
@@ -123,6 +137,33 @@ void MediaControlPanelElement::SetKeepDisplayedForAccessibility(bool value) {
   keep_displayed_for_accessibility_ = value;
 }
 
+bool MediaControlPanelElement::EventListenerIsAttachedForTest() const {
+  return event_listener_->IsAttached();
+}
+
+void MediaControlPanelElement::EnsureTransitionEventListener() {
+  // Create the event listener if it doesn't exist.
+  if (!event_listener_) {
+    event_listener_ = new MediaControlPanelElement::TransitionEventListener(
+        this,
+        WTF::BindRepeating(&MediaControlPanelElement::HandleTransitionEndEvent,
+                           WrapWeakPersistent(this)));
+  }
+
+  // Attach the event listener if we are not attached.
+  if (!event_listener_->IsAttached())
+    event_listener_->Attach();
+}
+
+void MediaControlPanelElement::DetachTransitionEventListener() {
+  if (!event_listener_)
+    return;
+
+  // Detach the event listener if we are attached.
+  if (event_listener_->IsAttached())
+    event_listener_->Detach();
+}
+
 void MediaControlPanelElement::DefaultEventHandler(Event* event) {
   // Suppress the media element activation behavior (toggle play/pause) when
   // any part of the control panel is clicked.
@@ -146,6 +187,10 @@ void MediaControlPanelElement::HandleTransitionEndEvent() {
   // Hide the element in the DOM once we have finished the transition.
   if (!opaque_ && !keep_displayed_for_accessibility_)
     SetIsWanted(false);
+
+  // Now that we have received the 'transitionend' event we can dispose of
+  // the listener.
+  DetachTransitionEventListener();
 }
 
 }  // namespace blink

third_party/blink/renderer/modules/media_controls/elements/media_control_panel_element.h

@@ -33,9 +33,17 @@ class MODULES_EXPORT MediaControlPanelElement final
 
   virtual void Trace(blink::Visitor*);
 
+ protected:
+  friend class MediaControlPanelElementTest;
+
+  bool EventListenerIsAttachedForTest() const;
+
  private:
   class TransitionEventListener;
 
+  void EnsureTransitionEventListener();
+  void DetachTransitionEventListener();
+
   void DefaultEventHandler(Event*) override;
   bool KeepEventInNode(Event*) override;
 

third_party/blink/renderer/modules/media_controls/elements/media_control_panel_element_test.cc

@@ -38,6 +38,16 @@ class MediaControlPanelElementTest : public PageTestBase {
 
   void ExpectPanelIsNotDisplayed() { EXPECT_FALSE(GetPanel().IsWanted()); }
 
+  void EventListenerNotCreated() { EXPECT_FALSE(GetPanel().event_listener_); }
+
+  void EventListenerAttached() {
+    EXPECT_TRUE(GetPanel().EventListenerIsAttachedForTest());
+  }
+
+  void EventListenerDetached() {
+    EXPECT_FALSE(GetPanel().EventListenerIsAttachedForTest());
+  }
+
   MediaControlPanelElement& GetPanel() { return *panel_element_.Get(); }
 
  private:
@@ -52,16 +62,31 @@ class MediaControlPanelElementTest : public PageTestBase {
 };
 
 TEST_F(MediaControlPanelElementTest, StateTransitions) {
+  // Make sure we are displayed (we are already opaque).
   GetPanel().SetIsDisplayed(true);
-  GetPanel().MakeOpaque();
-
   ExpectPanelIsDisplayed();
-  SimulateTransitionEnd();
 
+  // Ensure the event listener has not been created and make the panel
+  // transparent.
+  EventListenerNotCreated();
   GetPanel().MakeTransparent();
 
+  // The event listener should now be attached so we should simulate the
+  // transition end and the panel will be hidden.
+  EventListenerAttached();
   SimulateTransitionEnd();
   ExpectPanelIsNotDisplayed();
+
+  // The event listener should be detached. We should now make the panel
+  // opaque again.
+  EventListenerDetached();
+  GetPanel().MakeOpaque();
+
+  // The event listener should now be attached so we should simulate the
+  // transition end event and the panel will be hidden.
+  EventListenerAttached();
+  SimulateTransitionEnd();
+  ExpectPanelIsDisplayed();
 }
 
 }  // namespace blink