android监听usb拔插事件

public class UsbActivity extends Activity {
/** Called when the activity is first created. */

private BroadcastReceiver receiver;

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

receiver=new BroadcastReceiver(){

@Override
public void onReceive(Context context, Intent intent) {
// TODO Auto-generated method stub
Log.d("+++++++++++","action="+intent.getAction());
Toast.makeText(UsbActivity.this, "action-----"+intent.getAction(), Toast.LENGTH_SHORT).show();
}

};

IntentFilter filter=new IntentFilter(Intent.ACTION_MEDIA_MOUNTED);
filter.addAction(Intent.ACTION_MEDIA_CHECKING);
filter.addAction(Intent.ACTION_MEDIA_EJECT);
filter.addAction(Intent.ACTION_MEDIA_REMOVED);
filter.addAction(Intent.ACTION_MEDIA_SHARED);
filter.addAction(Intent.ACTION_MEDIA_UNMOUNTED);
filter.addDataScheme("file");

registerReceiver(receiver,filter);

}


public void onDestroy(){
super.onDestroy();
unregisterReceiver(receiver);
}

}

这是我监听USB拔插事件的程序，在平板上运行的时候，没有任何toast出现，请问问题出在哪？action正确吗？
另外usb的action只在3.1才开始有，现在我要用的是3.0及其之前的东西。