sendBroadcast和sendStickyBroadcast的区别

我们平时最经常使用的是sendBroadcast,就是把一个Intent广播出去。今天我在看wifi的时候，还发现了sendStickyBroadcast。官方文档是这样写的：


public abstract void sendStickyBroadcast (Intent intent)

Since: API Level 1
Perform a sendBroadcast(Intent) that is "sticky," meaning the Intent you are sending stays around after the broadcast is complete, so that others can quickly retrieve that data through the return value of registerReceiver(BroadcastReceiver, IntentFilter). In all other ways, this behaves the same as sendBroadcast(Intent).
You must hold the BROADCAST_STICKY permission in order to use this API. If you do not hold that permission, SecurityException will be thrown.
Parameters

intent The Intent to broadcast; all receivers matching this Intent will receive the broadcast, and the Intent will be held to be re-broadcast to future receivers.

光从字面的意思是很难理解的。只有你写例子才会明白的。

package com.android.testbroadcast;

import android.app.Activity;
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 {
	Button btnSendi;
	Button btnSends;
	Button btnStart;
	Context mContext;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btnSendi=(Button) findViewById(R.id.sendi);
        btnSends=(Button) findViewById(R.id.sends);
        btnStart=(Button) findViewById(R.id.start);
        mContext=getApplicationContext();
        btnSendi.setOnClickListener(new OnClickListener(){

			@Override
			public void onClick(View v) {
				// TODO Auto-generated method stub
				Intent intent = new Intent();
			    intent.setAction("com.android.my.action");
			    intent.setFlags(1);
			    mContext.sendBroadcast(intent);
			}
        	
        });
        
        btnStart.setOnClickListener(new OnClickListener(){

			@Override
			public void onClick(View v) {
				// TODO Auto-generated method stub
				Intent intent = new Intent(MainActivity.this,ReceiverActivity.class);
			   
			    startActivity(intent);
			}
        	
        });
        
        btnSends.setOnClickListener(new OnClickListener(){

			@Override
			public void onClick(View v) {
				// TODO Auto-generated method stub
				Intent intent = new Intent();
			    intent.setAction("com.android.my.action.sticky");
			    intent.setFlags(2);
			    mContext.sendStickyBroadcast(intent);
			}
        	
        });
    }
}

package com.android.testbroadcast;

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.net.wifi.WifiManager;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class ReceiverActivity extends Activity {
	 private IntentFilter mIntentFilter;
	
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        mIntentFilter = new IntentFilter();
        mIntentFilter.addAction("com.android.my.action");
        mIntentFilter.addAction("com.android.my.action.sticky");

			
    }
    private BroadcastReceiver mReceiver = new BroadcastReceiver() {

        @Override
        public void onReceive(Context context, Intent intent) {
            final String action = intent.getAction();
            System.out.println("action"+action);
            
        }
    };
    
    @Override
    protected void onResume() {
    	// TODO Auto-generated method stub
    	super.onResume();
    	registerReceiver(mReceiver, mIntentFilter);
    }
    
    @Override
    protected void onPause() {
    	// TODO Auto-generated method stub
    	super.onPause();
    	unregisterReceiver(mReceiver);
    }
    
    
}

在MainActivity里面会有sendBroadcast和sendStickyBroacat.在ReceverActivity里面通过BroadcastReceiver来接收这两个消息，在ReceiverActivity里是通过代码来注册Recevier而不是在Manifest里面注册的。所以通过sendBroadcast中发出的intent在ReceverActivity不处于onResume状态是无法接受到的，即使后面再次使其处于该状态也无法接受到。而sendStickyBroadcast发出的Intent当ReceverActivity重新处于onResume状态之后就能重新接受到其Intent.这就是the Intent will be held to be re-broadcast to future receivers这句话的表现。就是说sendStickyBroadcast发出的最后一个Intent会被保留，下次当Recevier处于活跃的时候，又会接受到它。