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 ==

补充：移动开发 , Android ,