Implement autoplay gesture override experiment.

Source/core/html/AutoplayExperimentHelper.cpp

+/*
+ * Copyright (C) 2015 Google Inc. All rights reserved.
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+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "core/html/AutoplayExperimentHelper.h"
+
+#include "core/dom/Document.h"
+#include "core/frame/Settings.h"
+#include "core/html/HTMLMediaElement.h"
+#include "platform/UserGestureIndicator.h"
+#include "platform/geometry/FloatRect.h"
+
+namespace blink {
+
+using namespace HTMLNames;
+
+// How long do we wait after a scroll event before deciding that no more
+// scroll events are going to arrive?
+static const double viewportTimerPollDelay = 0.5;
+
+// Event listener that just informs the AutoplayExperimentHelper that a
+// scroll has happened.
+class AutoplayExperimentHelper::ScrollListener : public EventListener  {
+    public:
+        ScrollListener(AutoplayExperimentHelper* helper) : EventListener(CPPEventListenerType), m_helper(helper) { }
+        virtual bool operator==(const EventListener& them)
+        {
+            return &them == this;
+        }
+
+        void handleEvent(ExecutionContext*, Event*) override
+        {
+            if (m_helper)
+                m_helper->notifyScrolled();
+        }
+
+    private:
+        AutoplayExperimentHelper* m_helper;

[philipj_slow, 2015/08/13 10:15:40]

It looks like the pointer is always passed and never cleared, so could this be a
reference so that the null check above goes away?

[liberato (no reviews please), 2015/09/01 06:54:19]

indeed, but the listener has since been removed.

+};
+
+
+AutoplayExperimentHelper::AutoplayExperimentHelper(HTMLMediaElement& element)
+    : m_element(element)
+    , m_mode(ExperimentOff)
+    , m_playPending(false)
+    , m_viewportTimer(this, &AutoplayExperimentHelper::viewportTimerFired)
+{
+    if (document().settings()) {
+        const String& autoplayMode = document().settings()->autoplayExperimentMode();

[ojan, 2015/08/11 02:45:21]

Did you also want to measure the case Philip suggested of basing it off the
tab's visibility instead of the video element's?

[ojan, 2015/08/11 02:49:34]

Also, I don't see anything for tab visibility. So, as it is, a background tab
could start playing a video?

[liberato (no reviews please), 2015/09/01 06:54:19]

added a check for page()->visibilityState() to isInViewport().

[philipj_slow, 2015/09/02 09:31:47]

What about the experiment to only require page visibility? That's one of the
better candidates for actually shipping, so I think we should measure it.

[philipj_slow, 2015/09/04 08:44:08]

I haven't seen the other CL yet, so just a gentle reminder to carry this issue
over to it.

+        if (autoplayMode.contains("enabled")) {
+            // Autoplay with no gesture requirement.
+            m_mode |= ExperimentEnabled;
+        }
+        if (autoplayMode.contains("-ifviewport")) {
+            // Override gesture requirement only if the player is within the
+            // current viewport.
+            m_mode |= ExperimentIfViewport;
+        }
+        if (autoplayMode.contains("-ifmuted")) {
+            // Override gesture requirement only if the media is muted or has
+            // no audio track.
+            m_mode |= ExperimentIfMuted;
+        }
+        if (autoplayMode.contains("-ifmobile")) {
+            // Override gesture requirement only if the page is optimized
+            // for mobile.
+            m_mode |= ExperimentIfMobile;
+        }
+        if (autoplayMode.contains("-playmuted")) {

[ojan, 2015/08/11 02:45:21]

For good measure, add a comment explaining this one too?

[liberato (no reviews please), 2015/09/01 06:54:19]

done, though the docs are with the enums in AutoplayExperimentConfig.h, rather
than in the parser.

+            m_mode |= ExperimentPlayMuted;
+        }
+
+        if (m_mode != ExperimentOff) {
+            WTF_LOG(Media, "HTMLMediaElement: autoplay experiment set to '%s' (%d)",
+                autoplayMode.ascii().data(), m_mode);
+        }
+    }
+}
+
+AutoplayExperimentHelper::~AutoplayExperimentHelper()
+{
+    clearEventListenerIfNeeded();
+}
+
+void AutoplayExperimentHelper::onReadyToPlay()
+{
+    // Assuming that we're eligible to override the user gesture requirement,
+    // either play if we meet the visibility checks, or install a listener
+    // to wait for them to pass.
+    if (isEligible()) {
+        if (isInViewportIfNeeded())
+            prepareToPlay(GesturelessPlaybackStartedByLoad);
+        else
+            installEventListenerIfNeeded();
+    }
+}
+
+void AutoplayExperimentHelper::onPlayMethodCalled()
+{
+    // Set the pending state, even if the play isn't going to be pending.
+    // Eligibility can change if, for example, the mute status changes.
+    // Having this set is okay.
+    m_playPending = true;
+
+    if (!UserGestureIndicator::processingUserGesture()) {
+
+        if (isEligible()) {
+            // Remember that userGestureRequiredForPlay is required for
+            // us to be eligible for the experiment.
+            // If we are able to override the gesture requirement now, then
+            // do so.  Otherwise, install an event listener if we need one.
+            if (isInViewportIfNeeded()) {
+                // Override the gesture and play.
+                prepareToPlay(GesturelessPlaybackStartedByPlayMethod);
+            } else {
+                // Wait for viewport visibility.
+                installEventListenerIfNeeded();
+            }
+        }
+
+    } else if (m_element.isUserGestureRequiredForPlay()) {
+        clearEventListenerIfNeeded();
+    }
+}
+
+void AutoplayExperimentHelper::onPauseMethodCalled()
+{
+    // Don't try to autoplay, if we would have.
+    m_playPending = false;
+    clearEventListenerIfNeeded();
+}
+
+void AutoplayExperimentHelper::onMuteChanged()
+{
+    // If we are no longer eligible for the autoplay experiment, then also
+    // quit listening for events.  If we are eligible, and if we should be
+    // playing, then start playing.  In other words, start playing if
+    // we just needed 'mute' to autoplay.
+    if (!isEligible()) {
+        clearEventListenerIfNeeded();
+    } else {
+        // Try to play.  If we can't, then install a listener.
+        if (!maybeStartPlaying())
+            installEventListenerIfNeeded();
+    }
+}
+
+void AutoplayExperimentHelper::installEventListenerIfNeeded()
+{
+    // If we don't require that the player is in the viewport, then we don't
+    // need the listener.
+    if (!(m_mode & ExperimentIfViewport))
+        return;
+
+    if (document().domWindow() && !m_scrollListener)  {
+        m_scrollListener = adoptRef(new ScrollListener(this));
+        document().domWindow()->addEventListener("scroll", m_scrollListener, false);
+    }
+}
+
+void AutoplayExperimentHelper::clearEventListenerIfNeeded()
+{
+    if (m_scrollListener) {
+        LocalDOMWindow* domWindow = document().domWindow();
+        if (domWindow) {
+            domWindow->removeEventListener("scroll", m_scrollListener, false);
+        }
+        // Either way, clear our ref.
+        m_scrollListener.clear();
+    }
+}
+
+void AutoplayExperimentHelper::notifyScrolled()
+{
+    // Reset the timer to indicate that scrolling has happened
+    // recently, and might still be ongoing.
+    m_viewportTimer.startOneShot(viewportTimerPollDelay, FROM_HERE);
+}
+
+void AutoplayExperimentHelper::viewportTimerFired(Timer<AutoplayExperimentHelper>*)
+{
+    // Sufficient time has passed since the last scroll that we'll
+    // treat it as the end of scroll.  Autoplay if we should.
+    maybeStartPlaying();
+}
+
+bool AutoplayExperimentHelper::isInViewportIfNeeded()
+{
+    // We could check for eligibility here, but we skip it.  Some of our
+    // callers need to do it separately, and we don't want to check more
+    // than we need to.
+    // Also remember that page visibility is assumed for clank.
+
+    // If viewport visibility isn't required, then it's visible enough.
+    if (!(m_mode & ExperimentIfViewport))
+        return true;
+
+    // Check if we're in the viewport.
+    const LocalDOMWindow* domWindow = document().domWindow();
+    if (!domWindow)
+        return false;
+
+    FloatRect us(m_element.offsetLeft(), m_element.offsetTop(), m_element.clientWidth(), m_element.clientHeight());
+    FloatRect screen(domWindow->scrollX(), domWindow->scrollY(), domWindow->innerWidth(), domWindow->innerHeight());
+
+    return screen.contains(us);

[ojan, 2015/08/11 02:45:21]

What if the video is larger than the screen or part of the video overflows off
the right-side?

Let me propose a different and simpler solution. How about we make it so that we
play when the screen contains the play button? That way we know the user can
pause it if they want to, it avoids all these edge cases and, in the common
case, it will still mean that the video will only play when the whole video is
visible.

[liberato (no reviews please), 2015/09/01 06:54:19]

for those videos that don't show controls, or use custom controls, we'd be out
of luck.

i'm going to leave it as-is for now, but some change does need to be made for
the "video-larger-than-viewport" case at least.

+}
+
+bool AutoplayExperimentHelper::maybeStartPlaying()
+{
+    // See if we're allowed to autoplay now.
+    if (!isEligible()
+        || !isInViewportIfNeeded()) {
+        return false;
+    }
+
+    // Start playing!
+    prepareToPlay(GesturelessPlaybackStartedByScroll);
+    m_element.playInternal();
+
+    return true;
+}
+
+bool AutoplayExperimentHelper::isEligible() const
+{
+    // If no user gesture is required, then the experiment doesn't apply.
+    // This is what prevents us from starting playback more than once.
+    // Since this flag is never set to true once it's cleared, it will block
+    // the autoplay experiment forever.
+    if (!m_element.isUserGestureRequiredForPlay())
+        return false;
+
+    if (m_mode == ExperimentOff)
+        return false;
+
+    // If nobody has requested playback, either by the autoplay attribute or
+    // a play() call, then do nothing.
+    if (!m_playPending && !m_element.autoplay())
+        return false;
+
+    // If the video is already playing, then do nothing.  Note that there
+    // is not a path where a user gesture is required but the video is
+    // playing.  However, we check for completeness.
+    if (!m_element.paused())
+        return false;
+
+    // Note that the viewport test always returns false on desktop, which is
+    // why video-autoplay-experiment.html doesn't check -ifmobile .
+    if ((m_mode & ExperimentIfMobile)
+        && !document().viewportDescription().isLegacyViewportType())
+        return false;
+
+    if (m_mode & ExperimentIfMuted) {
+        // If media is muted, then autoplay when it comes into view.
+        return m_element.fastHasAttribute(mutedAttr) || m_element.muted();
+    }
+
+    // Autoplay when it comes into view (if needed), maybe muted.
+    return true;
+}
+
+void AutoplayExperimentHelper::muteIfNeeded()
+{
+    if (m_mode & ExperimentPlayMuted && !m_element.muted()) {
+        // This will call onMuteChanged(), which we really don't want
+        // to do anything, since we're called when trying to play.  If
+        // the element is still marked as eligible, then we'll probably

[philipj_slow, 2015/08/13 10:15:40]

setMuted returns early if there's no change, so I don't think there could be
infinite recursion. Given the same fact, I think this could be simplified to
just if (m_mode & ExperimentPlayMuted) m_element.setMuted(true);

[liberato (no reviews please), 2015/09/01 06:54:20]

infinite recursion: true, though it's not obvious that setMuted() should set
m_muted before calling onMuteChanged.  i was duplicating the check here because
it's cheap.  although, since !isEligible, it won't loop anyway, so i removed the
muted check.

simplified: indeed, but i kept the assert.

+        // recurse indefinitely.
+        ASSERT(!isEligible());
+        m_element.setMuted(true);
+    }
+}
+
+void AutoplayExperimentHelper::prepareToPlay(AutoplayMetrics metric)
+{
+    m_element.recordAutoplayMetric(metric);
+
+    // This also causes !isEligible, so that we don't alow autoplay more than
+    // once.  Be sure to do this before muteIfNeeded().
+    m_element.removeUserGestureRequirement();
+
+    clearEventListenerIfNeeded();
+    muteIfNeeded();
+
+    // Record that this autoplayed without a user gesture.  This is normally
+    // set when we discover an autoplay attribute, but we include all cases
+    // where playback started without a user gesture, e.g., play().
+    m_element.setInitialPlayWithoutUserGestures(true);
+
+    // Do not actually start playback here.
+}
+
+Document& AutoplayExperimentHelper::document() const
+{
+    return m_element.document();
+}
+
+}