Android中通过耳机按键控制音乐播放的实现

今天在研究Android中实现Android 4.2.2源码中的Music应用的源码，关于通过耳机按键控制音乐播放的实现，有点好奇，就仔细分析了一下源码，

主要由 MediaButtonIntentReceiver 这个类来实现。

在AndroidManifest.xml中有如下Receiver的注册：

        <receiver android:name="com.huawei.imax.music.MediaButtonIntentReceiver" >
            <intent-filter>
                <action android:name="android.intent.action.MEDIA_BUTTON" />
                <action android:name="android.media.AUDIO_BECOMING_NOISY" />
            </intent-filter>
        </receiver>

其实关键是对这两个ACTION的监控：

  android.intent.action.MEDIA_BUTTON的说明如下，从注释看，就是媒体按键被按下后，通过 Intent.EXTRA_KEY_EVENT 中带上触发此事件的具体按钮事件：

   /**
     * Broadcast Action:  The "Media Button" was pressed.  Includes a single
     * extra field, {@link #EXTRA_KEY_EVENT}, containing the key event that
     * caused the broadcast.
     */
    @SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
    public static final String ACTION_MEDIA_BUTTON = "android.intent.action.MEDIA_BUTTON";

  android.intent.action.AUDIO_BECOMING_NOISY的说明如下（自己翻译的，大概这个意思），

从注释看，当音频变得“吵闹”，比如耳机拔出，或者A2DP音频通道（比如通过蓝牙音箱播放音乐就是一种A2DP的应用场景）断开，

音频系统将会自动将音频转到自带的扬声器。收到此intent的控制音频的应用可以暂停，降低音量或其它操作，以防扬声器突然发出声音让用户受惊：

    /**
     * Broadcast intent, a hint for applications that audio is about to become
     * 'noisy' due to a change in audio outputs. For example, this intent may
     * be sent when a wired headset is unplugged, or when an A2DP audio
     * sink is disconnected, and the audio system is about to automatically
     * switch audio route to the speaker. Applications that are controlling
     * audio streams may consider pausing, reducing volume or some other action
     * on receipt of this intent so as not to surprise the user with audio
     * from the speaker.
     */
    @SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
    public static final String ACTION_AUDIO_BECOMING_NOISY = "android.media.AUDIO_BECOMING_NOISY";

MediaButtonIntentReceiver 的关键实现如下：

public class MediaButtonIntentReceiver extends BroadcastReceiver
{
    ...
    
    @Override
    public void onReceive(Context context, Intent intent)
    {
        String intentAction = intent.getAction();
        if (AudioManager.ACTION_AUDIO_BECOMING_NOISY.equals(intentAction))
        {
            Intent i = new Intent(context, MediaPlaybackService.class);
            i.setAction(MediaPlaybackService.SERVICECMD);
            i.putExtra(MediaPlaybackService.CMDNAME,
                    MediaPlaybackService.CMDPAUSE);
            context.startService(i);
        }
        else if (Intent.ACTION_MEDIA_BUTTON.equals(intentAction))
        {
            KeyEvent event = (KeyEvent) intent.getParcelableExtra(Intent.EXTRA_KEY_EVENT);
            
            if (event == null)
            {
                return;
            }
            
            int keycode = event.getKeyCode();
            int action = event.getAction();
            long eventtime = event.getEventTime();
            
            // single quick press: pause/resume. 
            // double press: next track
            // long press: start auto-shuffle mode.
            
            String command = null;
            switch (keycode)
            {
                case KeyEvent.KEYCODE_MEDIA_STOP:
                    command = MediaPlaybackService.CMDSTOP;
                    break;
                case KeyEvent.KEYCODE_HEADSETHOOK:
                case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:
                    command = MediaPlaybackService.CMDTOGGLEPAUSE;
                    break;
                case KeyEvent.KEYCODE_MEDIA_NEXT:
                    command = MediaPlaybackService.CMDNEXT;
                    break;
                case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
                    command = MediaPlaybackService.CMDPREVIOUS;
                    break;
                case KeyEvent.KEYCODE_MEDIA_PAUSE:
                    command = MediaPlaybackService.CMDPAUSE;
                    break;
                case KeyEvent.KEYCODE_MEDIA_PLAY:
                    command = MediaPlaybackService.CMDPLAY;
                    break;
            }

通过接收此事件的广播，想进行什么操作，完全看你的业务需求了。

一开始以为耳机插拔，是通过ACTION_HEADSET_PLUG这个Action来控制，但Music并没有监听此事件，

    /**
     * Broadcast Action: Wired Headset plugged in or unplugged.
     *
     * <p>The intent will have the following extra values:
     * <ul>
     *   <li><em>state</em> - 0 for unplugged, 1 for plugged. </li>
     *   <li><em>name</em> - Headset type, human readable string </li>
     *   <li><em>microphone</em> - 1 if headset has a microphone, 0 otherwise </li>
     * </ul>
     * </ul>
     */
    @SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
    public static final String ACTION_HEADSET_PLUG =
            "android.intent.action.HEADSET_PLUG";

自己测试了一下，在AndroidManifest.xml中静态注册，是不会生效的，为啥？

一个很简单的解释，如果你的应用还没有运行，这时插入耳机，系统如何把这个消息给这个应用？

因此，"android.intent.action.HEADSET_PLUG" 只能通过动态注册来接收此广播消息。

如果让 MediaButtonIntentReceiver 还接收"android.intent.action.HEADSET_PLUG"的广播消息，

则MediaButtonIntentReceiver会先收到 "android.media.AUDIO_BECOMING_NOISY" 这个消息，然后才会收到"android.intent.action.HEADSET_PLUG"这个消息？

为什么这样，代码层面还没有分析，后面抽空再研究下Android源码。