BroadcastReceiver onReceive被多次调用

Question

我正在编写一个检查外部存储状态的应用。为了监视状态，我使用BroadcastReceiver并且我注意到，onReceive被多次调用（例如：3x用于卸载，1x用于mount）用于一个事件（例如：拔掉usb）。

这是我的代码：

 public class MainActivity extends ActionBarActivity {

    final static String TAG = "BroadcastReceiverDemo";

    private IntentFilter filter;
    private BroadcastReceiver receiver;

    private void registerReceiver() {
        filter = new IntentFilter();
        filter.addAction(Intent.ACTION_MEDIA_MOUNTED);
        filter.addAction(Intent.ACTION_MEDIA_UNMOUNTED);
        filter.addAction(Intent.ACTION_MEDIA_REMOVED);
        filter.addAction(Intent.ACTION_MEDIA_SHARED);
        filter.addDataScheme("file");
        filter.setPriority(IntentFilter.SYSTEM_HIGH_PRIORITY);

        receiver = new BroadcastReceiver() {

            @Override
            public void onReceive(Context context, Intent intent) {
                String broadcast;

                String state = Environment.getExternalStorageState();

                Log.d(TAG, "State: " + state);

                if (Environment.MEDIA_MOUNTED.equals(state) && !Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
                    broadcast = "External storage aviable";
                } else {
                    broadcast = "External storage NOT aviable";
                }

                String msg = intent.getAction() + " received broadcast: " + broadcast;

                Log.d(TAG, msg);
            }
        };

        this.registerReceiver(receiver, filter);
    }

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

        registerReceiver();
    }

    @Override
    protected void finalize() throws Throwable {
        if (receiver != null) {
            this.unregisterReceiver(receiver);
        }
        super.finalize();

    }
}

问：为什么onReceive被调用了这么多次以及如何只调用一次？

感谢您的帮助。