Android的BroadcastReceiver

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 {

    @Override
    protected 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 {
        @Override
        public 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>() {
                 @Override
                 protected 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.java

public abstract void sendBroadcast(@RequiresPermission Intent intent);

调到实现类ContextImpl

// ContextImpl.java(extents Context.java)
    @Override
    public 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();
        }
    }