android实现一个简单的播放器，要求功能有： 1. 播放、暂停，停止，退出功能； 2. 后台播放功能； 3. 进度条显示播放进度、拖动进度条改变进度功能； 4. 播放时图片旋转，显示当前播放时间功能。

实现一个简单的Android播放器，可以按照以下步骤进行：

1. 创建一个新的Android项目，并在布局文件中添加一个MediaPlayer和一个SeekBar。

2. 在Activity中，初始化MediaPlayer并设置其数据源，设置播放器的监听器以便可以监听播放结束等事件。同时设置SeekBar的监听器以便可以监听进度条变化。

3. 在播放时，使用MediaPlayer的start()方法来启动播放器，并更新SeekBar的进度，同时显示当前播放时间。

4. 如果需要支持后台播放，可以通过将播放器绑定到service并启动service来实现。这样即使用户退出应用程序，播放器仍然可以继续播放。

5. 添加停止播放和退出功能，可以在Activity中添加按钮，并在按钮点击事件中调用相应的方法来实现这些功能。

6. 如果需要在播放时显示旋转图像，可以使用ImageView并在播放器监听器中更新其旋转角度。

下面是示例代码：

MainActivity.java

public class MainActivity extends AppCompatActivity implements SeekBar.OnSeekBarChangeListener {

    private MediaPlayer mediaPlayer;
    private SeekBar seekBar;
    private ImageView imageView;
    private boolean isPlaying = false;
    private boolean isStop = true;
    private int currentPosition = 0;
    private Timer timer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mediaPlayer = MediaPlayer.create(this, R.raw.music);
        seekBar = findViewById(R.id.seekBar);
        imageView = findViewById(R.id.imageView);

        seekBar.setOnSeekBarChangeListener(this);
        mediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
            @Override
            public void onCompletion(MediaPlayer mp) {
                stopPlaying();
            }
        });

        timer = new Timer();
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                if (mediaPlayer != null && isPlaying && !isStop) {
                    currentPosition = mediaPlayer.getCurrentPosition();
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            seekBar.setProgress(currentPosition);
                            updateImageView();
                        }
                    });
                }
            }
        }, 0, 100);
    }

    private void updateImageView() {
        imageView.setRotation(imageView.getRotation() + 5);
    }

    private void startPlaying() {
        mediaPlayer.start();
        isPlaying = true;
        isStop = false;
        seekBar.setMax(mediaPlayer.getDuration());
    }

    private void pausePlaying() {
        mediaPlayer.pause();
        isPlaying = false;
    }

    private void stopPlaying() {
        mediaPlayer.stop();
        mediaPlayer.reset();
        isPlaying = false;
        isStop = true;
        seekBar.setProgress(0);
        updateImageView();
    }

    private void releasePlayer() {
        mediaPlayer.release();
        mediaPlayer = null;
    }

    private void bindService() {
        Intent intent = new Intent(this, BackgroundMusicService.class);
        startService(intent);
        bindService(intent, serviceConnection, BIND_AUTO_CREATE);
    }

    private void unbindService() {
        if (backgroundMusicService != null) {
            unbindService(serviceConnection);
            backgroundMusicService = null;
        }
    }

    @Override
    public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
        if (fromUser) {
            mediaPlayer.seekTo(progress);
        }
    }

    @Override
    public void onStartTrackingTouch(SeekBar seekBar) {
        pausePlaying();
    }

    @Override
    public void onStopTrackingTouch(SeekBar seekBar) {
        startPlaying();
    }

    public void onClickPlay(View view) {
        if (mediaPlayer == null) {
            mediaPlayer = MediaPlayer.create(this, R.raw.music);
        }
        if (!isPlaying) {
            startPlaying();
        }
    }

    public void onClickPause(View view) {
        if (isPlaying) {
            pausePlaying();
        }
    }

    public void onClickStop(View view) {
        if (!isStop) {
            stopPlaying();
        }
    }

    public void onClickExit(View view) {
        stopPlaying();
        releasePlayer();
        unbindService();
        finish();
    }

    private BackgroundMusicService backgroundMusicService;

    private ServiceConnection serviceConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            BackgroundMusicService.LocalBinder binder = (BackgroundMusicService.LocalBinder) service;
            backgroundMusicService = binder.getService();
            backgroundMusicService.playMusic();
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            backgroundMusicService = null;
        }
    };

    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (mediaPlayer != null) {
            mediaPlayer.release();
            mediaPlayer = null;
        }
        if (timer != null) {
            timer.cancel();
            timer = null;
        }
    }
}

activity_main.xml

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <SeekBar
        android:id="@+id/seekBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:max="100"
        android:layout_marginTop="20dp"
        android:layout_marginLeft="20dp"
        android:layout_marginRight="20dp"
        android:layout_alignParentTop="true"/>

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_launcher_foreground"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_marginTop="20dp"
        android:layout_below="@id/seekBar"
        android:gravity="center_horizontal">

        <Button
            android:id="@+id/button_play"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="播放"
            android:onClick="onClickPlay"/>

        <Button
            android:id="@+id/button_pause"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="暂停"
            android:onClick="onClickPause"/>

        <Button
            android:id="@+id/button_stop"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="停止"
            android:onClick="onClickStop"/>

        <Button
            android:id="@+id/button_exit"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="退出"
            android:onClick="onClickExit"/>
    </LinearLayout>

</RelativeLayout>

BackgroundMusicService.java

public class BackgroundMusicService extends Service implements MediaPlayer.OnCompletionListener {

    private MediaPlayer mediaPlayer;

    public BackgroundMusicService() {
    }

    @Override
    public void onCreate() {
        super.onCreate();
        mediaPlayer = MediaPlayer.create(this, R.raw.music);
        mediaPlayer.setOnCompletionListener(this);
    }

    public void playMusic() {
        if (mediaPlayer != null && !mediaPlayer.isPlaying()) {
            mediaPlayer.start();
        }
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        if (mediaPlayer != null) {
            mediaPlayer.stop();
            mediaPlayer.release();
            mediaPlayer = null;
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
        return new LocalBinder();
    }

    @Override
    public void onCompletion(MediaPlayer mp) {
        stopSelf();
    }

    public class LocalBinder extends Binder {
        public BackgroundMusicService getService() {
            return BackgroundMusicService.this;
        }
    }
}