Android歌词秀设计思路（4）通用的音乐播放服务(下)

11-09-13 来源：[db:作者]

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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.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) {
        //MusicPlaybackService service = (MusicPlaybackService)context;
         if (Intent.ACTION_MEDIA_BUTTON.equals(intent.getAction())) {
             KeyEvent key = (KeyEvent)intent.getParcelableExtra(Intent.EXTRA_KEY_EVENT);
             if(key.getAction() == KeyEvent.ACTION_DOWN){
                 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

作者“来自大连”

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