Include viewport visibility checks for autoplay experiment.

Source/core/html/AutoplayExperimentHelper.h

Index: Source/core/html/AutoplayExperimentHelper.h
diff --git a/Source/core/html/AutoplayExperimentHelper.h b/Source/core/html/AutoplayExperimentHelper.h
index c9d015f062bce1126923f2ec3613ad7067c92219..8cb0b2358396325c28530e9692734f81f0d90ad1 100644
--- a/Source/core/html/AutoplayExperimentHelper.h
+++ b/Source/core/html/AutoplayExperimentHelper.h
@@ -79,12 +79,58 @@ public:
     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.

[philipj_slow, 2015/09/21 15:02:05]

I looks like PageVisibilityState is doesn't have an "invalid" state, so I guess
this is for m_element and m_screen. Can't those invalid states then simply be
expressed by one or both of those being the empty rect? Less stuff to keep in
sync.

[liberato (no reviews please), 2015/09/23 06:14:57]

i don't see a way to make it work automatically.  intersects() always returns
false for empty rectangles, but contains() does not.  we'd still have to check
explicitly for isEmpty().

anyway, it does remove the flag so i'll give it a try.  i suspect that it will
be less clear.

[philipj_slow, 2015/09/25 15:32:27]

OK, looks like it was successfully removed.

+    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.

[philipj_slow, 2015/09/21 15:02:05]

s/player/element/ and maybe rename to isElementVisible() as that's a nice pair
with isPageVisibile().

[liberato (no reviews please), 2015/09/23 06:14:57]

Done.

+        bool isInViewport() const;
+
+        // Return true if and only if the element's page is visible.
+        bool isPageVisible() const;
+
+    private:
+        bool m_valid;
+        PageVisibilityState m_visibilityState;
+        IntRect m_element;
+        IntRect m_screen;
+    };
+
+    // Register to receive position updates, if we haven't already.  If we
+    // have, then this does nothing.
+    void registerForPositionUpdatesIfNeeded();
+
+    // Un-register for position updates, if we are currently registered.
+    void unregisterForPositionUpdatesIfNeeded();
+
     // Return true if any only if this player meets (most) of the eligibility

[philipj_slow, 2015/09/25 15:32:27]

Another "player" here if you want to fix it.

     // 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 element described by LocationState is
+    // not visible, and we care that it must be visible.

[philipj_slow, 2015/09/21 15:02:05]

It's pretty hard to tell from this header when you should pass the LocationState
and when you don't need to. It looks like it has to do with comparing the
before/after state.

[liberato (no reviews please), 2015/09/23 06:14:57]

if yuo have LocationState, pass it in.  if not, then the second form will
construct one for you.  i'll update the docs to make that clearer.

[philipj_slow, 2015/09/25 15:32:27]

Acknowledged.

+    bool meetsVisibilityRequirements(const LocationState&) const;
+
+    // Return false if and only if the player is not visible, and we care

[philipj_slow, 2015/09/21 15:02:05]

s/player/element/?

[liberato (no reviews please), 2015/09/23 06:14:57]

Done.

+    // that it must be visible.
+    bool meetsVisibilityRequirements() const;
+
     // Set the muted flag on the media if we're in an experiment mode that
     // requires it, else do nothing.
     void muteIfNeeded();
@@ -98,6 +144,9 @@ private:
     // 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;
 
@@ -115,6 +164,20 @@ private:
     // True if we've received a play() without a pause().
     bool m_playPending : 1;
 
+    // Are we registered with the view for position updates?
+    bool m_registeredWithView : 1;
+
+    // According to our last position update, are we in the viewport?
+    bool m_wasInViewport : 1;
+
+    // According to our last position update, where was our element?
+    LocationState m_lastLocation;
+
+    // When was m_lastLocation set?
+    double m_lastLocationUpdateTime;
+
+    Timer<AutoplayExperimentHelper> m_viewportTimer;
+
     friend class Internals;
 };