From 2ae3eaafcae4330d8ccabf4a8c57f0df13f7fefa Mon Sep 17 00:00:00 2001
From: jbphet <john.blanco@colorado.edu>
Date: Fri, 4 Oct 2019 16:28:38 -0600
Subject: [PATCH] added more general handling of audio context resumption, see
 #39

---
 js/Sound.js | 69 ++++++++++++++++++++++++++---------------------------
 1 file changed, 34 insertions(+), 35 deletions(-)

diff --git a/js/Sound.js b/js/Sound.js
index 3a3fa45..e66259c 100644
--- a/js/Sound.js
+++ b/js/Sound.js
@@ -12,13 +12,9 @@ define( require => {
   // modules
   const Display = require( 'SCENERY/display/Display' );
   const inherit = require( 'PHET_CORE/inherit' );
-  const platform = require( 'PHET_CORE/platform' );
   const Property = require( 'AXON/Property' );
   const vibe = require( 'VIBE/vibe' );
 
-  // sounds
-  const empty = require( 'sound!VIBE/empty.mp3' );
-
   // global property that allows all audio to be turned on/off, see #11
   const audioEnabledProperty = new Property( true );
 
@@ -189,46 +185,49 @@ define( require => {
     audioEnabledProperty: audioEnabledProperty
   } );
 
-  // Below is some platform-specific code for handling a number of issues related to audio.  It may be possible to
-  // remove some or all of this as Web Audio becomes more consistently implemented.
+  // If an audio context was created, it means that we are using Web Audio.  Many browsers have adopted policies to
+  // prevent a web page from being able to play a sound before a user interacts with it, so the following code was
+  // necessary to essentially detect when the user starts interacting with the sim and enable the audio context, which
+  // in turn enables the ability to produce sound.
   if ( audioContext ) {
 
-    if ( !platform.mobileSafari ) {
+    // function to remove the listeners, used to avoid code duplication
+    const removeUserInteractionListeners = () => {
+      window.removeEventListener( 'touchstart', resumeAudioContext, false );
+      if ( Display.userGestureEmitter.hasListener( resumeAudioContext ) ) {
+        Display.userGestureEmitter.removeListener( resumeAudioContext );
+      }
+    };
+
+    // listener that resumes the audio context
+    const resumeAudioContext = () => {
 
-      // In some browsers the audio context is not allowed to run before the user interacts with the simulation.  The
-      // motivation for this is to prevent auto-play of sound (mostly videos) when users land on websites, but it ends
-      // up preventing PhET sims from being able to play sound.  To deal with this, we add a listener that can check the
-      // state of the audio context and "resume" it if necessary when the user starts interacting with the sim.  See
-      // https://github.com/phetsims/vibe/issues/32 for more information.
       if ( audioContext.state !== 'running' ) {
 
-        Display.userGestureEmitter.addListener( function resumeAudioContext() {
-          if ( audioContext.state !== 'running' ) {
+        // tell the audio context to resume
+        audioContext.resume()
+          .then( () => {
+            removeUserInteractionListeners();
+          } )
+          .catch( err => {
+            const errorMessage = 'error when trying to resume audio context, err = ' + err;
+            console.error( errorMessage );
+            assert && alert( errorMessage );
+          } );
+      }
+      else {
 
-            // the audio context isn't running, so tell it to resume
-            audioContext.resume().catch( function( err ) {
-              assert && assert( false, 'error when trying to resume audio context, err = ' + err );
-            } );
-          }
-          Display.userGestureEmitter.removeListener( resumeAudioContext ); // only do this once
-        } );
+        // audio context is already running, no need to listen anymore
+        removeUserInteractionListeners();
       }
-    }
-    else {
+    };
 
-      // There is a different issue for audio on iOS+Safari: On this platform, we must play an audio file from a thread
-      // initiated by a user event such as touchstart before any sounds will play.  This requires the user to touch the
-      // screen before audio can be played. See
-      // http://stackoverflow.com/questions/12517000/no-sound-on-ios-6-web-audio-api
-      const silence = new Sound( empty );
-      var playSilence = function() {
-        silence.play();
-        window.removeEventListener( 'touchstart', playSilence, false );
-      };
-      window.addEventListener( 'touchstart', playSilence, false );
-    }
+    // listen for a touchstart - this only works to resume the audio context on iOS devices (as of this writing)
+    window.addEventListener( 'touchstart', resumeAudioContext, false );
+
+    // listen for other user gesture events
+    Display.userGestureEmitter.addListener( resumeAudioContext );
   }
 
   return Sound;
-
 } );
\ No newline at end of file