Bug 1478208 - Implement HTMLMediaElement.allowedToPlay. r=alwu,bz

Various web authors have expressed desire to know in advance whether autoplay
will work.

They want this in order to avoid paying the price for downloading media that
won't play. Or they want to take other action such as showing a poster image
instead.

This is of particular interest to Firefox, as we're planning on showing a
prompt to ask the user whether they would like a site to play. If sites want to
determine whether they can autoplay but avoid the prompt showing, they won't be
able to just call play() in Firefox and see whether it works, as that would
likely show the prompt if the user doesn't already have a stored permission.

We've been working out a spec here:
whatwg/html#3617 (comment)

This implements what is the consensus to date there;
HTMLMediaElement.allowedToPlay, which returns true when a play() call would not
be blocked with NotAllowedError by autoplay blocking policies.

MozReview-Commit-ID: AkBu0G7uCJ0

UltraBlame original commit: f5f55e363d318effdfd504061c50f87e06301c6f

dom/html/HTMLMediaElement.cpp

@@ -10441,6 +10441,32 @@ LoadFromSourceChildren
 }
 }
 }
+bool
+HTMLMediaElement
+:
+:
+AllowedToPlay
+(
+)
+const
+{
+return
+AutoplayPolicy
+:
+:
+IsAllowedToPlay
+(
+*
+this
+)
+=
+=
+nsIAutoplay
+:
+:
+ALLOWED
+;
+}
 void
 HTMLMediaElement
 :

dom/html/HTMLMediaElement.h

@@ -1730,6 +1730,12 @@ mIsCasting
 aShow
 ;
 }
+bool
+AllowedToPlay
+(
+)
+const
+;
 already_AddRefed
 <
 MediaSource

dom/media/test/AutoplayTestUtils.js

@@ -58,6 +58,13 @@ appendChild
 element
 )
 ;
+let
+allowedToPlay
+=
+element
+.
+allowedToPlay
+;
 element
 .
 play
@@ -79,6 +86,7 @@ postMessage
 played
 :
 true
+allowedToPlay
 }
 "
 *
@@ -99,6 +107,7 @@ postMessage
 played
 :
 false
+allowedToPlay
 }
 "
 *

dom/media/test/test_autoplay_policy_activation.html

@@ -707,10 +707,37 @@
 .
 data
 .
+allowedToPlay
+test_case
+.
+should_play
+"
+allowed
+-
+"
++
+test_case
+.
+name
+)
+;
+SimpleTest
+.
+is
+(
+result
+.
+data
+.
 played
 test_case
 .
 should_play
+"
+played
+-
+"
++
 test_case
 .
 name

dom/media/test/test_autoplay_policy_permission.html

@@ -384,10 +384,35 @@
 .
 data
 .
+allowedToPlay
+testCase
+.
+shouldPlay
+"
+allowedToPlay
+-
+"
++
+testCase
+.
+message
+)
+;
+is
+(
+result
+.
+data
+.
 played
 testCase
 .
 shouldPlay
+"
+played
+-
+"
++
 testCase
 .
 message

dom/webidl/HTMLMediaElement.webidl

@@ -1483,3 +1483,69 @@ hasSuspendTaint
 ;
 }
 ;
+/
+*
+*
+API
+that
+exposes
+whether
+a
+call
+to
+HTMLMediaElement
+.
+play
+(
+)
+would
+be
+*
+blocked
+by
+autoplay
+policies
+;
+whether
+the
+promise
+returned
+by
+play
+(
+)
+*
+would
+be
+rejected
+with
+NotAllowedError
+.
+*
+/
+partial
+interface
+HTMLMediaElement
+{
+[
+Pref
+=
+"
+media
+.
+allowed
+-
+to
+-
+play
+.
+enabled
+"
+]
+readonly
+attribute
+boolean
+allowedToPlay
+;
+}
+;

modules/libpref/init/all.js

@@ -5259,6 +5259,45 @@ false
 ;
 #
 endif
+#
+ifdef
+NIGHTLY_BUILD
+pref
+(
+"
+media
+.
+allowed
+-
+to
+-
+play
+.
+enabled
+"
+true
+)
+;
+#
+else
+pref
+(
+"
+media
+.
+allowed
+-
+to
+-
+play
+.
+enabled
+"
+false
+)
+;
+#
+endif
 pref
 (
 "

testing/profiles/common/user.js

@@ -400,3 +400,19 @@ webaudio
 false
 )
 ;
+user_pref
+(
+"
+media
+.
+allowed
+-
+to
+-
+play
+.
+enabled
+"
+true
+)
+;