Android向通知栏发送通知并且不让被清理掉，一直占领通知栏

import android.app.Activity;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class MainActivity extends Activity {

	private NotificationManager nm;
	private Button btnCancel;

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		btnCancel = (Button) findViewById(R.id.cancel);
		String service = Context.NOTIFICATION_SERVICE;
		nm = (NotificationManager) getSystemService(service); // get system
		Context context = MainActivity.this;
		Notification n = new Notification();
		n.icon = R.drawable.ic_launcher;
		n.tickerText = "test";
		n.when = System.currentTimeMillis();
		
		//如果不想被清理加上下面这两行代码的任何一行都可以
		n.flags |= Notification.FLAG_NO_CLEAR;
		// n.flags=Notification.FLAG_ONGOING_EVENT;
		
		
		Intent intent = new Intent(context, MainActivity.class);
		PendingIntent pi = PendingIntent.getActivity(context, 0, intent, 0);
		n.setLatestEventInfo(context, "test_title", "test_content", pi);
		nm.notify(1, n);

		btnCancel.setOnClickListener(new OnClickListener() {

			public void onClick(View v) {
				nm.cancel(1);
			}
		});
	}

}