Move more code from mobile to common

common/lib/src/widgets/play_pause_button.dart

@@ -0,0 +1,59 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+
+import '../backend.dart';
+
+class PlayPauseButton extends StatefulWidget {
+  @override
+  _PlayPauseButtonState createState() => _PlayPauseButtonState();
+}
+
+class _PlayPauseButtonState extends State<PlayPauseButton>
+    with SingleTickerProviderStateMixin {
+  AnimationController playPauseAnimation;
+  MusicusBackendState backend;
+  StreamSubscription<bool> playingSubscription;
+
+  @override
+  void initState() {
+    super.initState();
+
+    playPauseAnimation = AnimationController(
+      vsync: this,
+      duration: Duration(milliseconds: 300),
+    );
+  }
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+
+    backend = MusicusBackend.of(context);
+    playPauseAnimation.value = backend.playback.playing.value ? 1.0 : 0.0;
+
+    if (playingSubscription != null) {
+      playingSubscription.cancel();
+    }
+
+    playingSubscription = backend.playback.playing.listen((playing) =>
+        playing ? playPauseAnimation.forward() : playPauseAnimation.reverse());
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return IconButton(
+      icon: AnimatedIcon(
+        icon: AnimatedIcons.play_pause,
+        progress: playPauseAnimation,
+      ),
+      onPressed: backend.playback.playPause,
+    );
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    playingSubscription.cancel();
+  }
+}