feat(web): add setBalance (#58)

feature_parity_table.md

@@ -59,7 +59,7 @@ Note: LLM means Low Latency Mode.
         <tr><td>stay awake</td><td>yes (except LLM)</td><td>yes</td><td>no</td><td>no</td><td>no</td><td>no</td></tr>
         <tr><td>recording active</td><td>not yet</td><td>yes</td><td>no</td><td>no</td><td>no</td><td>no</td></tr>
         <tr><td>playing route</td><td>yes (except LLM)</td><td>yes</td><td>no</td><td>no</td><td>no</td><td>no</td></tr>
-        <tr><td>balance</td><td>no</td><td>no</td><td>no</td><td>no</td><td>yes</td><td>yes</td></tr>
+        <tr><td>balance</td><td>no</td><td>no</td><td>no</td><td>yes</td><td>yes</td><td>yes</td></tr>
         <tr><td colspan="7"><strong>Streams</strong></td></tr>
         <tr><td>duration event</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td></tr>
         <tr><td>position event</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td></tr>

packages/audioplayers_web/lib/web_audio_js.dart

@@ -0,0 +1,57 @@
+import 'dart:html';
+
+import 'package:js/js.dart';
+
+@JS('AudioContext')
+@staticInterop
+class JsAudioContext {
+  external JsAudioContext();
+}
+
+extension JsAudioContextExtension on JsAudioContext {
+  external MediaElementAudioSourceNode createMediaElementSource(
+      AudioElement element,);
+
+  external StereoPannerNode createStereoPanner();
+
+  external AudioNode get destination;
+}
+
+@JS()
+@staticInterop
+abstract class AudioNode {
+  external AudioNode();
+
+}
+
+extension AudioNodeExtension on AudioNode {
+  external AudioNode connect(AudioNode audioNode);
+}
+
+@JS()
+@staticInterop
+class AudioParam {
+  external AudioParam();
+}
+
+extension AudioParamExtension on AudioParam {
+  external num value;
+}
+
+@JS()
+@staticInterop
+class StereoPannerNode
+    implements AudioNode {
+  external StereoPannerNode();
+}
+
+extension StereoPannerNodeExtension on StereoPannerNode {
+  external AudioParam get pan;
+}
+
+@JS()
+@staticInterop
+class MediaElementAudioSourceNode
+    implements AudioNode {
+  external MediaElementAudioSourceNode();
+}

packages/audioplayers_web/lib/wrapped_player.dart

@@ -4,6 +4,7 @@ import 'dart:html';
 import 'package:audioplayers_platform_interface/api/release_mode.dart';
 import 'package:audioplayers_platform_interface/streams_interface.dart';
 import 'package:audioplayers_web/num_extension.dart';
+import 'package:audioplayers_web/web_audio_js.dart';
 
 class WrappedPlayer {
   final String playerId;
@@ -17,6 +18,7 @@ class WrappedPlayer {
   bool isPlaying = false;
 
   AudioElement? player;
+  StereoPannerNode? stereoPanner;
   StreamSubscription? playerTimeUpdateSubscription;
   StreamSubscription? playerEndedSubscription;
   StreamSubscription? playerLoadedDataSubscription;
@@ -44,7 +46,7 @@ class WrappedPlayer {
   }
 
   void setBalance(double balance) {
-    throw UnimplementedError('setBalance is not currently implemented on Web');
+    stereoPanner?.pan.value = balance;
   }
 
   void setPlaybackRate(double rate) {
@@ -58,9 +60,18 @@ class WrappedPlayer {
     }
 
     final p = player = AudioElement(currentUrl);
+    p.crossOrigin = 'anonymous'; // need for stereo panning to work
     p.loop = shouldLoop();
     p.volume = currentVolume;
     p.playbackRate = currentPlaybackRate;
+
+    // setup stereo panning
+    final audioContext = JsAudioContext();
+    final source = audioContext.createMediaElementSource(player!);
+    stereoPanner = audioContext.createStereoPanner();
+    source.connect(stereoPanner!);
+    stereoPanner?.connect(audioContext.destination);
+
     playerPlaySubscription = p.onPlay.listen((_) {
       streamsInterface.emitDuration(
         playerId,
@@ -99,6 +110,7 @@ class WrappedPlayer {
   void release() {
     _cancel();
     player = null;
+    stereoPanner = null;
 
     playerLoadedDataSubscription?.cancel();
     playerLoadedDataSubscription = null;