Android游戲開發之旅16 異步音樂播放

  在Android游戲開發中我們必須考慮背景音樂播放問題，在Android平台中提供了MediaPlayer類可以播放聲音，但是游戲除了播放音樂外還需要考慮畫面的流暢性，以及多種音效同時播放，所以必須用到Android多線程機制和異步音效播放。Android SDK從1.0開始就提供了AsyncPlayer類，這裡我們為了根據我們自己的需要可以派生或修改出更靈活的播放類。

import android.content.Context;
import android.net.Uri;
import android.os.PowerManager;
import android.os.SystemClock;
import android.util.Log;

import java.io.IOException;
import java.lang.IllegalStateException;
import java.util.LinkedList;

 
public class AsyncPlayer {
    private static final int PLAY = 1;
    private static final int STOP = 2;
    private static final boolean mDebug = false;

    private static final class Command {
        int code;
        Context context;
        Uri uri;
        boolean looping;
        int stream;
        long requestTime;

        public String toString() {
            return "{ code=" + code + " looping=" + looping + " stream=" + stream
                    + " uri=" + uri + " }";
        }
    }

    private LinkedList<Command> mCmdQueue = new LinkedList();  //用一個鏈表保存播放參數隊列

    private void startSound(Command cmd) {
 
        try {
            MediaPlayer player = new MediaPlayer();
            player.setAudioStreamType(cmd.stream);
            player.setDataSource(cmd.context, cmd.uri);  //設置媒體源，這裡Android123提示大家本類的public void play (Context context, Uri uri, boolean looping, int stream) 類第二個參數Uri為媒體位置。
            player.setLooping(cmd.looping);
            player.prepare();
            player.start();
            if (mPlayer != null) {
                mPlayer.release();
            }
            mPlayer = player;
          }
        catch (IOException e) {
            Log.w(mTag, "error loading sound for " + cmd.uri, e);
        } catch (IllegalStateException e) {
            Log.w(mTag, "IllegalStateException (content provider died?) " + cmd.uri, e);
        }
    }

    private final class Thread extends java.lang.Thread {   //通過多線程方式不阻塞調用者
        Thread() {
            super("AsyncPlayer-" + mTag);
        }

        public void run() {
            while (true) {
                Command cmd = null;

                synchronized (mCmdQueue) {   //同步方式執行

                        cmd = mCmdQueue.removeFirst();
                }

                switch (cmd.code) {
                case PLAY:
                                 startSound(cmd);
                    break;
                case STOP:
 
                    if (mPlayer != null) {
                                         mPlayer.stop();
                        mPlayer.release();
                        mPlayer = null;
                    } else {
                        Log.w(mTag, "STOP command without a player");
                    }
                    break;
                }

                synchronized (mCmdQueue) {
                    if (mCmdQueue.size() == 0) {
           
                        mThread = null;
                        releaseWakeLock();
                        return;
                    }
                }
            }
        }
    }

    private String mTag;
    private Thread mThread;
    private MediaPlayer mPlayer;
    private PowerManager.WakeLock mWakeLock;

 
    private int mState = STOP;
 
    public AsyncPlayer(String tag) {
        if (tag != null) {
            mTag = tag;
        } else {
            mTag = "AsyncPlayer";
        }
    }

 
    public void play(Context context, Uri uri, boolean looping, int stream) {
        Command cmd = new Command();
        cmd.requestTime = SystemClock.uptimeMillis(); //這裡為了測試性能，傳遞了開始執行前的系統tickcount計時器值
        cmd.code = PLAY;
        cmd.context = context;
        cmd.uri = uri;
        cmd.looping = looping;
        cmd.stream = stream;
        synchronized (mCmdQueue) {
            enqueueLocked(cmd);
            mState = PLAY;
        }
    }
   
 
    public void stop() {
        synchronized (mCmdQueue) {
                 if (mState != STOP) {
                Command cmd = new Command();
                cmd.requestTime = SystemClock.uptimeMillis();
                cmd.code = STOP;
                enqueueLocked(cmd);
                mState = STOP;
            }
        }
    }

    private void enqueueLocked(Command cmd) {
        mCmdQueue.add(cmd);
        if (mThread == null) {
            acquireWakeLock();
            mThread = new Thread();
            mThread.start();
        }
    }

  一般對於Android游戲而言下面的代碼不用考慮，一般用戶都在交互操作，不會出現屏幕鎖問題

    public void setUsesWakeLock(Context context) {  //電源管理wakelock處理
        if (mWakeLock != null || mThread != null) {
                      throw new RuntimeException("assertion failed mWakeLock=" + mWakeLock
                    + " mThread=" + mThread);
        }
        PowerManager pm = (PowerManager)context.getSystemService(Context.POWER_SERVICE);
        mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, mTag);
    }

    private void acquireWakeLock() {   //加鎖
        if (mWakeLock != null) {
            mWakeLock.acquire();
        }
    }

    private void releaseWakeLock() { //解鎖
        if (mWakeLock != null) {
            mWakeLock.release();
        }
    }
}