Android歌词秀设计思路（4）通用的音乐播放服务(下)

这篇文章中我们将要说明在MediaPlayerService中用到的几个辅助功能。

1.AudioFocus相关处理

2.监视来电状态

3.监视耳机插头拔出

4.监视线控器按钮

5.Notification表示

AudioFocus相关处理

AudioFocus相关的处理已经被封装在AudioFocusHelper类中。这个类的直接目的虽然是为MediaPlayerService服务的，但是同时又独立与MediaPlayerService，可以独立使用。

功能

1.根据从AudioManager接收到的AudioFocus变化通知，管理内部的Focus状态。

2.结合内部状态和将通知转发给真正需要管理AudioFocus的类（在这里是MediaPlayerService类）

3.提供请求和释放AudioFocus的方法。

4.处理版本问题（AudioFocus只在Android2.2及以后的版本中可用）。

类图





我们在类图中

用蓝线标出了AudioFocus变化时通知的渠道（不是很严格）。当AudioManager发生AudioFocus的变化时，就会通知OnAudioFocusChangeListener，而这时的OnAudioFocusChangeListener实际上是由AudioFocusHelper提供的具象类的实例，在这个具象类中将通知处理后，又通知给作为MusicFocusable的具象类的MediaPlayerService。

用红线标出的AudioFocus请求和放弃的渠道：MediaPlayerServcie->AudioFocusHelper->AudioManager

以下AudioFocusHelper的源代码。

package LyricPlayer.xwg;

import android.content.Context;
import android.media.AudioManager;

public class AudioFocusHelper {
AudioManager mAM;
MusicFocusable mFocusable;

// do we have audio focus?
public static final int NoFocusNoDuck = 0;    // we don't have audio focus, and can't duck
public static final int NoFocusCanDuck = 1;   // we don't have focus, but can play at a low volume ("ducking")
public static final int Focused = 2;           // we have full audio focus

private int mAudioFocus = NoFocusNoDuck;
private AudioManager.OnAudioFocusChangeListener mListener = null;

public AudioFocusHelper(Context ctx, MusicFocusable focusable) {
if (android.os.Build.VERSION.SDK_INT >= 8){
mAM = (AudioManager) ctx.getSystemService(Context.AUDIO_SERVICE);
mListener = new AudioManager.OnAudioFocusChangeListener(){
/**
* Called by AudioManager on audio focus changes. We implement this by calling our
* MusicFocusable appropriately to relay the message.
*/
@Override
public void onAudioFocusChange(int focusChange) {
if (mFocusable == null) return;
switch (focusChange) {
case AudioManager.AUDIOFOCUS_GAIN:
mAudioFocus = Focused;
mFocusable.onGainedAudioFocus();
break;
case AudioManager.AUDIOFOCUS_LOSS:
case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
mAudioFocus = NoFocusNoDuck;
mFocusable.onLostAudioFocus();
break;
case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
mAudioFocus = NoFocusCanDuck;
mFocusable.onLostAudioFocus();
break;
default:
}
}

};
mFocusable = focusable;
}else{
mAudioFocus = Focused; // no focus feature, so we always "have" audio focus
}
}

/** Requests audio focus. Returns whether request was successful or not. */
public boolean requestFocus() {
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED ==
mAM.requestAudioFocus(mListener, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);
}

/** Abandons audio focus. Returns whether request was successful or not. */
public boolean abandonFocus() {
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED == mAM.abandonAudioFocus(mListener);
}

public void giveUpAudioFocus() {
if (mAudioFocus == Focused
&& android.os.Build.VERSION.SDK_INT >= 8
&& abandonFocus())
mAudioFocus = NoFocusNoDuck;
}

public void tryToGetAudioFocus() {
if (mAudioFocus != Focused
&& android.os.Build.VERSION.SDK_INT >= 8
&& requestFocus())
mAudioFocus = Focused;
}

int getAudioFocus(){
return mAudioFocus;
}
}

监视来电状态

AudioFocus是Android2.2以后才有的功能，对于比2.2低得版本，用的是另一种方法，就是监听电话的状态。最起码在电话打进来是能够暂停音乐的播放。

实现这一功能的第一步是在AndroidManifest.xml中声明用于接收PHONE_STATE通知的receiver

<receiver android:name=".PhoneStateReceiver">
<intent-filter>
<action android:name="android.intent.action.PHONE_STATE"/>
</intent-filter>
</receiver>

第二步是定义一个对应的PhoneStateReceiver，代码如下

package LyricPlayer.xwg;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.telephony.TelephonyManager;

public class PhoneStateReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
//if android.os.Build.VERSION.SDK_INT >= 8 we use audio focus.
if (android.os.Build.VERSION.SDK_INT < 8){
TelephonyManager tm = (TelephonyManager)context.getSystemService(Context.TELEPHONY_SERVICE);
if(tm.getCallState() != TelephonyManager.CALL_STATE_IDLE){
context.startService(new Intent(MediaPlayerService.ACTION_PAUSE));
}
}
}
}

这就够了。

监视耳机插头拔出

如果在音乐播放过程中拔出耳机，音乐就会通过扬声器播放出来。为了避免这种尴尬局面，我们会监视耳机拔出状态，并在耳机拔出时暂停播放。

首先是在AndroidManifest.xml中声明用于接收AUDIO_BECOMING_NOISY通知的receiver

<receiver android:name=".MusicIntentReceiver">
<intent-filter>
<action android:name="android.media.AUDIO_BECOMING_NOISY" />
</intent-filter>
</receiver>

然后就是定义用于处理通知的receiver，类名要和AndroidManifest.xml中声明的一样。

package LyricPlayer.xwg;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class MusicIntentReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context ctx, Intent intent) {
if (intent.getAction().equals(android.media.AudioManager.ACTION_AUDIO_BECOMING_NOISY)) {
ctx.startService(new Intent(LyricPlayerService.ACTION_PAUSE));
}
}
}

MEDIA_BUTTON处理

在讨论处理方法之前，必须先明确：那些键属于MEDIA_BUTTON？根据我的试验，MEDIA_BUTTON好像就是线控上面的上个按钮。网上也有用同样的方法取得音量键动作的内容，但是我没有试出来。

继续我们的话题，为了检测MEDIA_BUTTON需要一些准备工作。

首先是在AndroidManifest.xml中声明用于接收MEDIA_BUTTON通知的receiver

<receiver android:name="MediaButtonReceiver">
<intent-filter>
<action android:name="android.intent.action.MEDIA_BUTTON" />
</intent-filter>
</receiver>

当然需要定义真正的receiver，名字要和AndroidManifest.xml中的一样。

package LyricPlayer.xwg;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.telephony.TelephonyManager;
import android.util.Log;
import android.view.KeyEvent;

public class MediaButtonReceiver extends BroadcastReceiver {
private static final String TAG = new String("LyricVolumeKeyReceiver");
@Override
public void onReceive(Context context, Intent intent) {
if (Intent.ACTION_MEDIA_BUTTON.equals(intent.getAction())) {
KeyEvent key = (KeyEvent)intent.getParcelableExtra(Intent.EXTRA_KEY_EVENT);
if(key.getAction() == KeyEvent.ACTION_DOWN){
TelephonyManager tm = (TelephonyManager)context.getSystemService(Context.TELEPHONY_SERVICE);
if(tm.getCallState() == TelephonyManager.CALL_STATE_IDLE){
Log.i(TAG, "OnReceive, getKeyCode = " + key.getKeyCode());
switch(key.getKeyCode()){
case KeyEvent.KEYCODE_HEADSETHOOK :
context.startService(new Intent(MediaPlayerService.ACTION_PLAY_PAUSE));
break;
case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
context.startService(new Intent(MediaPlayerService.ACTION_PREVIOUS));
break;
case KeyEvent.KEYCODE_MEDIA_NEXT:
context.startService(new Intent(MediaPlayerService.ACTION_NEXT));
break;
}
}
}
}
}
}

比较特别的是中间的键的键值不是KEYCODE_PLAY_PAUSE而是KEYCODE_HEADSETHOOK。想想也是，接电话也用这个键。

准备工作的最后一步就是要把通过MediaButtonReceiver来接受MEDIA_BUTTON这件事报告给AudioMenager，由于这也是Android2.2及以后版本才有的功能，也需要做版本判断。

if (android.os.Build.VERSION.SDK_INT >= 8){
mReceiverName = new ComponentName(getPackageName(),MediaButtonReceiver.class.getName());
mAudioManager = (AudioManager)getSystemService(Context.AUDIO_SERVICE);
mAudioManager.registerMediaButtonEventReceiver(mReceiverName);
}

当然在结束的时候我们也会保持取消登录的良好习惯。

if(mAudioManager != null && mReceiverName != null){
mAudioManager.unregisterMediaButtonEventReceiver(mReceiverName);
}

Notification表示

Notification表示首先取得NotificationManager

mNotificationManager = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);

在需要表示的时候调用showNotification()方法。和showNotification()方法有关的代码：

public interface NotificationProvider{
public Notification createNotification(Context context);
}

NotificationProvider mNotificationProvider = null;

public void setNotificationProvider(NotificationProvider provider){
mNotificationProvider = provider;
}

/** * Show a notification while this service is running.     */
private void showNotification() {
if(mNotificationProvider != null){
// Send the notification.
mNotificationManager.notify(NOTIFICATION, mNotificationProvider.createNotification(this));
}
}

已经用了N次的办法了。不用再解释了吧。当然，看看实现侧的做法还有必要的。

mProxy.setNotificationProvider(new MediaPlayerService.NotificationProvider(){
@Override
public Notification createNotification(Context context) {
Notification notification = new Notification(R.drawable.button_blue_play, mProxy.getTitle(), System.currentTimeMillis());
// The PendingIntent to launch our activity if the user selects this notification
PendingIntent contentIntent = PendingIntent.getActivity(context, 0, new Intent(context, LyricMain.class), 0);
// Set the info for the views that show in the notification panel.
notification.setLatestEventInfo(context, getText(R.string.media_player_label), mProxy.getTitle(), contentIntent);
return notification;
}
});

代码本身没有什么，都是程式化的东西。

最后就是在不再需要表示Notification的时候，执行以下代码

mNotificationManager.cancel(NOTIFICATION);

完整的代码请参照以下博文的附件。

软件功能说明：原创：Android应用开发-Andorid歌词秀,含源码

工程，源码下载：Android歌词秀源码，工程文件2011/9/11版