Implement autoplay gesture override experiment.

Source/core/html/AutoplayExperimentHelper.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef AutoplayExperimentHelper_h
+#define AutoplayExperimentHelper_h
+
+#include "core/html/AutoplayExperimentConfig.h"
+#include "core/page/Page.h"
+#include "platform/Timer.h"
+#include "platform/geometry/IntRect.h"
+
+namespace blink {
+class Document;
+class HTMLMediaElement;
+class EventListener;
+
+// These values are used for a histogram. Do not reorder.
+enum AutoplayMetrics {
+    // Media element with autoplay seen.
+    AutoplayMediaFound = 0,
+    // Autoplay enabled and user stopped media play at any point.
+    AutoplayPaused = 1,
+    // Autoplay enabled but user bailed out on media play early.
+    AutoplayBailout = 2,
+    // Autoplay disabled but user manually started media.
+    AutoplayManualStart = 3,
+    // Autoplay was (re)enabled through a user-gesture triggered load()
+    AutoplayEnabledThroughLoad = 4,
+    // Autoplay disabled by sandbox flags.
+    AutoplayDisabledBySandbox = 5,
+
+    // These metrics indicate "no gesture detected, but the gesture
+    // requirement was overridden by experiment".  They do not include cases
+    // where no user gesture is required (!m_userGestureRequiredForPlay).
+
+    // User gesture requirement was bypassed, and playback started, during
+    // initial load via the autoplay flag.  If playback was deferred due to
+    // visibility requirements, then this does not apply.
+    GesturelessPlaybackStartedByAutoplayFlagImmediately = 6,
+
+    // User gesture requirement was bypassed, and playback started, when
+    // the play() method was called.  If playback is deferred due to
+    // visibility requirements, then this does not apply.
+    GesturelessPlaybackStartedByPlayMethodImmediately = 7,
+
+    // User gesture requirement was bypassed, and playback started, after
+    // an element with the autoplay flag moved into the viewport.  Playback
+    // was deferred earlier due to visibility requirements.
+    GesturelessPlaybackStartedByAutoplayFlagAfterScroll = 8,
+
+    // User gesture requirement was bypassed, and playback started, after
+    // an element on which the play() method was called was moved into the
+    // viewport.  Playback had been deferred due to visibility requirements.
+    GesturelessPlaybackStartedByPlayMethodAfterScroll = 9,
+
+    // play() failed to play due to gesture requirement.
+    PlayMethodFailed = 10,
+
+    // Some play, whether user initiated or not, started.
+    AnyPlaybackStarted = 11,
+    // Some play, whether user initiated or not, paused.
+    AnyPlaybackPaused = 12,
+    // Some playback, whether user initiated or not, bailed out early.
+    AnyPlaybackBailout = 13,
+
+    // This enum value must be last.
+    NumberOfAutoplayMetrics,
+};
+
+class AutoplayExperimentHelper {
+public:
+    AutoplayExperimentHelper(HTMLMediaElement&);
+    ~AutoplayExperimentHelper();
+
+    void becameReadyToPlay();
+    void playMethodCalled();
+    void pauseMethodCalled();
+    void mutedChanged();
+    void positionChanged();
+
+    // For testing.
+    void triggerAutoplayViewportCheck();
+
+private:
+    // The location and size of our element, and the viewport.  Also supports
+    // "not valid", in case some of the information isn't available.
+    class LocationState {
+    public:
+        LocationState() : m_valid(false) {}
+        LocationState(Element&);
+
+        bool valid() const { return m_valid; }
+        PageVisibilityState visibilityState() const { return m_visibilityState; }
+        const IntRect& element() const { return m_element; }
+        const IntRect& screen() const { return m_screen; }
+
+        bool operator==(const LocationState&) const;
+        bool operator!=(const LocationState&) const;
+
+        // Return true if and only if the player is visible.
+        bool isInViewport();
+
+    private:
+        bool m_valid;
+        PageVisibilityState m_visibilityState;
+        IntRect m_element;
+        IntRect m_screen;
+    };
+
+    // Install an event listener to check for changes in visibility.  If a
+    // listener is already installed, then this does nothing.
+    void installEventListenerIfNeeded();

[ojan, 2015/09/01 20:20:11]

Nit: They're not event listeners anymore now that you're not using scroll
events. FWIW, it sounds like the name we're going to ship to web authors will be
IntersectionObserver.

[liberato (no reviews please), 2015/09/04 06:49:46]

i renamed to "[un]registerForPositionUpdatesIfNeeded".  wanted to avoid anything
confusingly similar to IO.

+
+    // Remove any event listener.  It's okay to call this if one isn't
+    // installed already.
+    void clearEventListenerIfNeeded();
+
+    // Return true if any only if this player meets (most) of the eligibility
+    // requirements for the experiment to override the need for a user
+    // gesture.  This includes everything except the visibility test.
+    bool isEligible() const;
+
+    // Return false if and only if the player is not visible, and we care
+    // that it must be visible.
+    bool isInViewportIfNeeded();
+
+    // Set the muted flag on the media if we're in an experiment mode that
+    // requires it, else do nothing.
+    void muteIfNeeded();
+
+    // Maybe override the requirement for a user gesture, and start playing
+    // autoplay media.  Returns true if only if it starts playback.
+    bool maybeStartPlaying();
+
+    // Configure internal state to record that the autoplay experiment is
+    // going to start playback.  This doesn't actually start playback, since
+    // there are several different cases.
+    void prepareToPlay(AutoplayMetrics);
+
+    // Process a timer for checking visibility.
+    void viewportTimerFired(Timer<AutoplayExperimentHelper>*);
+
+    // Return our media element's document.
+    Document& document() const;
+
+    inline bool enabled(AutoplayExperimentConfig::Mode mode) const
+    {
+        return ((int)m_mode) & ((int)mode);
+    }
+
+private:
+    HTMLMediaElement& m_element;
+
+    AutoplayExperimentConfig::Mode m_mode;
+
+    // Autoplay experiment state.
+    // True if we've received a play() without a pause().
+    bool m_playPending : 1;
+
+    Timer<AutoplayExperimentHelper> m_viewportTimer;
+
+    bool m_registeredWithView;

[philipj_slow, 2015/09/02 09:24:12]

Put this together with the other bool to get them packed together.

[liberato (no reviews please), 2015/09/04 06:49:46]

Done.

+
+    LocationState m_lastLocation;
+
+    friend class Internals;
+};
+
+} // namespace blink
+
+#endif // AutoplayExperimentHelper_h