Android基于Service的音乐播放器

本文开发一个基于service的音乐播放器，音乐由后台运行的service负责播放，当后台的播放状态发生变化时，程序将会通过发送广播通知前台activity更新界面；当点击activity的界面按钮时，系统将通过发送广播通知后台service来改变播放状态。

前台activity界面有两个按钮，分别用于控制播放/暂停、停止，另外还有两个文本框，用于显示正在播放的歌曲名、歌手名。前台activity的代码如下：

public class mainactivity extends appcompatactivity implements view.onclicklistener{

  private imagebutton mstart;
  private imagebutton mstop;
  private textview mmusicname;
  private textview msongername;
  private activityreceiver mactivityreceiver;
  public static final string ctl_action = "com.trampcr.action.ctl_action";
  public static final string update_action = "com.trampcr.action.update_action";

  //定义音乐播放状态，0x11代表没有播放，0x12代表正在播放，0x13代表暂停
  int status = 0x11;
  string[] musicnames = new string[]{"完美生活", "那一年", "故乡"};
  string[] songernames = new string[]{"许巍", "许巍", "许巍"};


  @override
  protected void oncreate(bundle savedinstancestate) {
    super.oncreate(savedinstancestate);
    setcontentview(r.layout.activity_main);

    mstart = (imagebutton) findviewbyid(r.id.start);
    mstop = (imagebutton) findviewbyid(r.id.stop);
    mmusicname = (textview) findviewbyid(r.id.music_name);
    msongername = (textview) findviewbyid(r.id.songer_name);

    mstart.setonclicklistener(this);
    mstop.setonclicklistener(this);

    mactivityreceiver = new activityreceiver();
    //创建intentfilter
    intentfilter filter = new intentfilter();
    //指定broadcastreceiver监听的action
    filter.addaction(update_action);
    //注册broadcastreceiver
    registerreceiver(mactivityreceiver, filter);

    intent intent = new intent(mainactivity.this, musicservice.class);
    //启动后台service
    startservice(intent);
  }

  public class activityreceiver extends broadcastreceiver {
    @override
    public void onreceive(context context, intent intent) {
      //获取intent中的update消息，update代表播放状态
      int update = intent.getintextra("update", -1);
      //获取intent中的current消息，current代表当前正在播放的歌曲
      int current = intent.getintextra("current", -1);
      if (current >= 0){
        mmusicname.settext(musicnames[current]);
        msongername.settext(songernames[current]);
      }
      switch (update){
        case 0x11:
          mstart.setbackgroundresource(r.drawable.play);
          status = 0x11;
          break;
        //控制系统进入播放状态
        case 0x12:
          //在播放状态下设置使用暂停图标
          mstart.setbackgroundresource(r.drawable.pause);
          status = 0x12;
          break;
        case 0x13:
          //在暂停状态下设置使用播放图标
          mstart.setbackgroundresource(r.drawable.play);
          status = 0x13;
          break;
      }
    }
  }

  @override
  public void onclick(view v) {
    intent intent = new intent(ctl_action);
    switch (v.getid()){
      case r.id.start:
        intent.putextra("control", 1);
        break;
      case r.id.stop:
        intent.putextra("control", 2);
        break;
    }
    //发送广播，将被service中的broadcastreceiver接收到
    sendbroadcast(intent);
  }
}

activityreceiver()用于响应后台service所发出的广播，该程序将会根据广播intent里的消息来改变播放状态，并更新程序界面中按钮的图标。

onclick中根据点击的按钮发送广播，发送广播时会把所按下的按钮标识发送出来。

接下来是后台service，会在播放状态发生改变时对外发送广播。代码如下：

public class musicservice extends service {

  myreceiver servicereceiver;
  assetmanager massetmanager;
  string[] musics = new string[]{"prefectlife.mp3", "thatyear.mp3", "country.mp3"};
  mediaplayer mmediaplayer;
  int status = 0x11;
  int current = 0; // 记录当前正在播放的音乐

  @nullable
  @override
  public ibinder onbind(intent intent) {
    return null;
  }

  @override
  public void oncreate() {
    super.oncreate();
    massetmanager = getassets();
    servicereceiver = new myreceiver();
    //创建intentfilter
    intentfilter filter = new intentfilter();
    filter.addaction(mainactivity.ctl_action);
    registerreceiver(servicereceiver, filter);
    //创建mediaplayer
    mmediaplayer = new mediaplayer();
    //为mediaplayer播放完成事件绑定监听器
    mmediaplayer.setoncompletionlistener(new mediaplayer.oncompletionlistener() {
      @override
      public void oncompletion(mediaplayer mp) {
        current++;
        if (current >= 3) {
          current = 0;
        }
        //发送广播通知activity更改文本框
        intent sendintent = new intent(mainactivity.update_action);
        sendintent.putextra("current", current);
        //发送广播，将被activity中的broadcastreceiver接收到
        sendbroadcast(sendintent);
        //准备并播放音乐
        prepareandplay(musics[current]);
      }
    });
  }

  public class myreceiver extends broadcastreceiver {
    @override
    public void onreceive(context context, intent intent) {
      int control = intent.getintextra("control", -1);
      switch (control){
        case 1: // 播放或暂停
          //原来处于没有播放状态
          if (status ==0x11){
            //准备播放音乐
            prepareandplay(musics[current]);
            status = 0x12;
          }
          //原来处于播放状态
          else if (status == 0x12){
            //暂停
            mmediaplayer.pause();
            status = 0x13; // 改变为暂停状态
          }
          //原来处于暂停状态
          else if (status == 0x13){
            //播放
            mmediaplayer.start();
            status = 0x12; // 改变状态
          }
          break;
        //停止声音
        case 2:
          //如果原来正在播放或暂停
          if (status == 0x12 || status == 0x13){
            //停止播放
            mmediaplayer.stop();
            status = 0x11;
          }
      }
      //广播通知activity更改图标、文本框
      intent sendintent = new intent(mainactivity.update_action);
      sendintent.putextra("update", status);
      sendintent.putextra("current", current);
      //发送广播，将被activity中的broadcastreceiver接收到
      sendbroadcast(sendintent);
    }
  }

  private void prepareandplay(string music) {
    try {
      //打开指定的音乐文件
      assetfiledescriptor assetfiledescriptor = massetmanager.openfd(music);
      mmediaplayer.reset();
      //使用mediaplayer加载指定的声音文件
      mmediaplayer.setdatasource(assetfiledescriptor.getfiledescriptor(), assetfiledescriptor.getstartoffset(), assetfiledescriptor.getlength());
      mmediaplayer.prepare(); // 准备声音
      mmediaplayer.start(); // 播放
    } catch (ioexception e) {
      e.printstacktrace();
    }
  }
}

myreceiver用于接收前台activity所发出的广播，并根据广播的消息内容改变service的播放状态，当播放状态改变时，该service对外发送一条广播，广播消息将会被前台activity接收，前台activity将会根据广播消息更新界面。

为了让该音乐播放器能按顺序依次播放歌曲，程序为mediaplayer增加了oncompletionlistener监听器，当mediaplayer播放完成后将自动播放下一首歌曲。

运行程序，效果图如下：

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