1.基本概念：BroadCast用于进程间或者线程间通信
本质上是用Binder方法，以AMS为订阅中心，完成注册，发布，监听的操作。
2.简单实现的例子
package com.android.car.myapplication;import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.widget.Toast;import androidx.appcompat.app.AppCompatActivity;public class MainActivityBroadCast extends AppCompatActivity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_carsetting);//系统广播IntentFilter intentFilter = new IntentFilter();intentFilter.addAction(Intent.ACTION_BATTERY_CHANGED);BatteryReceiver batteryReceiver = new BatteryReceiver();registerReceiver(batteryReceiver,intentFilter);}public class BatteryReceiver extends BroadcastReceiver {@Overridepublic void onReceive(Context context, Intent intent) {Bundle bundle = intent.getExtras();int current = bundle.getInt("level");int total = bundle.getInt("scale");if(current * 1.0 / total ==1){System.out.println("当前手机电量为"+(current / total)*100+"%");}}}
}
用法很简单不多赘述。
3.接收端registerReceiver
ContextImp：Context的最终实现类。class ContextImpl extends Context
- ContextImpl.registerReceiver方法：
。。。。。。//注册binder应用和action到系统
final Intent intent = ActivityManager.getService().registerReceiver(mMainThread.getApplicationThread(), mBasePackageName, rd,filter,...);
- ActivityManager中getService()实际上是一个Binder
在ActivityManager.java中可看到：
public static IActivityManager getService() {return IActivityManagerSingleton.get();}private static final Singleton<IActivityManager> IActivityManagerSingleton =new Singleton<IActivityManager>() {@Overrideprotected IActivityManager create() {final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);final IActivityManager am = IActivityManager.Stub.asInterface(b);return am;}};}
- 所以registerReceiver最后是一个向ActivityManagerService的Binder调用，只不过设计上加上了的发布订阅模式
4.发送端 sendBroadcast
- 首先调用context的sendBroadCast方法
// Context.javapublic abstract void sendBroadcast(@RequiresPermission Intent intent);
- 调到实现类ContextImpl
// ContextImpl.java(extents Context.java)@Overridepublic void sendBroadcast(Intent intent) {warnIfCallingFromSystemProcess();String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());try {intent.prepareToLeaveProcess(this);ActivityManager.getService().broadcastIntent(mMainThread.getApplicationThread(), intent, resolvedType, null,Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, null, false, false,getUserId());} catch (RemoteException e) {throw e.rethrowFromSystemServer();}}
- 同理调用到ActivityManagerService的broadcastIntent方法。
（AMS中关于发布订阅的实现，不在本文范围之内，后续遇到问题再继续研究）
5.总结
如果了解发布订阅的设计模式
了解发布，订阅，订阅中心的模型，那么BroadCastReceiver的基本实现原理就清楚了。
- 接收端首先订阅，即调用Binder方法registerReceiver。
- AMS会记下对这个Intent感兴趣的进程
- 发送端通过Binder方法sendBroadCast发送一个Intent。
- AMS会收到Intent消息
- AMS找到所有对Intent消息感兴趣的接收端，将这个消息发到接收端
- 接收端进入实现的onReceive方法。
参考图：https://www.cnblogs.com/andy-songwei/p/10400259.html
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