[Video] MediaSession API event handlers can resume background video.

third_party/WebKit/Source/core/html/HTMLMediaElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp b/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
index a577e6bcd7236ca592683384c57bd4a7ce84b39c..ad75aa483de9c01d28ed6296317b6f9d7941aa23 100644
--- a/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
@@ -26,6 +26,7 @@
 
 #include "core/html/HTMLMediaElement.h"
 
+#include <limits>
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/Microtask.h"
 #include "bindings/core/v8/ScriptController.h"
@@ -35,6 +36,7 @@
 #include "core/css/MediaList.h"
 #include "core/dom/Attribute.h"
 #include "core/dom/DOMException.h"
+#include "core/dom/DocumentUserGestureToken.h"
 #include "core/dom/ElementTraversal.h"
 #include "core/dom/ElementVisibilityObserver.h"
 #include "core/dom/Fullscreen.h"
@@ -97,7 +99,6 @@
 #include "wtf/MathExtras.h"
 #include "wtf/PtrUtil.h"
 #include "wtf/text/CString.h"
-#include <limits>
 
 #ifndef BLINK_MEDIA_LOG
 #define BLINK_MEDIA_LOG DVLOG(3)
@@ -2315,10 +2316,15 @@ void HTMLMediaElement::pause() {
 
   // Only buffer aggressively on a user-initiated pause. Other types of pauses
   // (which go directly to pauseInternal()) should not cause this behavior.
-  if (webMediaPlayer() && UserGestureIndicator::utilizeUserGesture())
+  if (webMediaPlayer() && UserGestureIndicator::utilizeUserGesture()) {
     webMediaPlayer()->setBufferingStrategy(
         WebMediaPlayer::BufferingStrategy::Aggressive);
 
+    // Don't allow webpages to override user-initiated pause.
+    if (hasVideo() && computeLockedPendingUserGesture(document()))
+      m_lockedPendingUserGesture = true;
+  }
+
   if (m_autoplayVisibilityObserver) {
     m_autoplayVisibilityObserver->stop();
     m_autoplayVisibilityObserver = nullptr;
@@ -3120,10 +3126,13 @@ void HTMLMediaElement::playbackStateChanged() {
   if (!webMediaPlayer())
     return;
 
+  // playbackStateChanged() is always a result of a user gesture.
+  UserGestureIndicator gestureIndicator(
+      DocumentUserGestureToken::create(&document()));

[mlamouri (slow - plz ping), 2017/02/17 12:31:31]

You can create the user gesture token from the callers. Why having it here?

[whywhat, 2017/02/17 16:30:24]

So the callers don't know a user gesture. Ideally we'd implement the default
notification in the same way the MediaSession API is implemented - so it would
just call play()/pause() on the HTMLMediaElement with the user gesture token.
However, that change is for another CL.

   if (webMediaPlayer()->paused())
-    pauseInternal();
+    pause();
   else
-    playInternal();
+    play();

[mlamouri (slow - plz ping), 2017/02/17 12:31:31]

I'm not sure I follow the reason why this is no longer calling the internal
methods.

[whywhat, 2017/02/17 16:30:24]

So that we respect the user gesture token and unlock the element back.

 }
 
 void HTMLMediaElement::requestSeek(double time) {
@@ -3185,6 +3194,11 @@ void HTMLMediaElement::remotePlaybackStarted() {
     remotePlaybackClient()->stateChanged(WebRemotePlaybackState::Connected);
 }
 
+void HTMLMediaElement::wasBackgrounded() {
+  if (hasVideo() && computeLockedPendingUserGesture(document()))
+    m_lockedPendingUserGesture = true;

[mlamouri (slow - plz ping), 2017/02/17 12:31:31]

We should probably discuss this more but I think this is going to break web
pages. For example, what would happen in the case of YouTube that auto-pauses
when in the background: will they be able to auto-resume when visible again? In
general, we never reset the lock boolean to true so I'm a bit worried about
this.

[whywhat, 2017/02/17 16:30:25]

I want to reuse the user gesture lock to enforce the following:
- user needs to resume playback for the background video to play
- if user pauses the video, website can't override it

I could add another flag to indicate with set the lock to true here or in
pause() while in the background so we reset it to false when foregrounded to
unlock the element back.

The current CL indicates that if the user paused the video in any state by
pressing pause or hiding it, the page won't be able to resume playback without a
gesture.

I should at least check if the video is not paused here, perhaps. If YouTube
pauses the video before this call, then we'd not lock it.

+}
+
 bool HTMLMediaElement::hasSelectedVideoTrack() {
   DCHECK(RuntimeEnabledFeatures::backgroundVideoTrackOptimizationEnabled());
 
@@ -3209,6 +3223,10 @@ bool HTMLMediaElement::isAutoplayingMuted() {
   return !paused() && muted() && isLockedPendingUserGesture();
 }
 
+bool HTMLMediaElement::isBackgroundVideoPlaybackUnlocked() {
+  return !isLockedPendingUserGesture();
+}
+
 void HTMLMediaElement::requestReload(const WebURL& newUrl) {
   DCHECK(webMediaPlayer());
   DCHECK(!m_srcObject);
@@ -3853,7 +3871,7 @@ void HTMLMediaElement::unlockUserGesture() {
 }
 
 bool HTMLMediaElement::isGestureNeededForPlayback() const {
-  if (!m_lockedPendingUserGesture)
+  if (!isLockedPendingUserGesture())
     return false;
 
   return isGestureNeededForPlaybackIfPendingUserGestureIsLocked();