Android Service 再次 unbindSrvice 时的问题

public class MainActivity extends AppCompatActivity {

    private MyService.DownloadBinder downloadBinder;

    private boolean isBind = false; //是否绑定服务

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final Button startService = (Button)findViewById(R.id.start_service);
        final Button stopService = (Button)findViewById(R.id.stop_service);
        final Button bindService = (Button)findViewById(R.id.bind_service);
        final Button unbindService = (Button)findViewById(R.id.unbind_service);

        startService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent startIntent = new Intent(MainActivity.this, MyService.class);
                startService(startIntent);
            }
        });

        stopService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent stopIntent = new Intent(MainActivity.this, MyService.class);
                stopService(stopIntent);
            }
        });

        bindService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent bindIntent = new Intent(MainActivity.this, MyService.class);
                isBind = bindService(bindIntent, connection, BIND_AUTO_CREATE);   //绑定服务
            }
        });

        unbindService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(isBind){ //解除绑定前要判断服务是否是绑定状态,其实此处应该还要加入 connection 是否为null 的判断
                    unbindService(connection);  //解绑服务
                    isBind = false;
                }

            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    private ServiceConnection connection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            downloadBinder = (MyService.DownloadBinder)service;
            downloadBinder.startDownload();
            downloadBinder.getProgress();
        }

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

public class MyService extends Service {
    private DownloadBinder mBinder = new DownloadBinder();



    public MyService() {
    }

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

    @Override
    public void onCreate() {
        super.onCreate();
        Log.d("MyService", "onCreate execute");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.d("MyService", "onStartCommand execute");
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d("MyService", "onDestroy execute");
    }

    class DownloadBinder extends Binder{
        public void startDownload(){
            Log.d("MyService", "startDownload executed");
        }

        public int getProgress(){
            Log.d("MyService", "getProgress executed");
            return 0;
        }
    }
}