本地广播用途
本地广播用于同一个app内通信(是否可以多进程通信？)，且其他app无法接收到本地广播的消息，可以大大提高安全性，且只能动态注册。
本地广播的使用和原理分析
本地广播使用到的几个类
1、public final class LocalBroadcastManager这个类是无法继承的，直接使用即可，后续我们深入分析这个类。
2、public abstract class BroadcastReceiver这个类是抽象类，在使用过程中一般需要继承，实现onReceive函数。后续我们会分析为什么需要继承这个类且为什么要实现onReceive函数。
3、public class IntentFilter implements Parcelable这个类是用于添加本地广播Inten过滤动作的，且我们后续会深入分析这个类。
本地广播实现源代码
1、新建一个BroadcastTest工程，创建一个空的活动即可，在MainActivity中增加若干私有成员，新建一个LocalReceiver类继承BroadcastReceiver并重写onReceive函数，以下是一些代码片段，后续我也会先贴出片段代码方便分析，最后再贴出完整代码。
public class MainActivity extends AppCompatActivity {
private IntentFilter intentFilter;
private static final String TAG="MainActivity";
private LocalReceiver localReceiver;
private LocalBroadcastManager localBroadcastManager;
private static final String localBroadcastAction = "com.example.broadcasttest.LOCAL_BROADCAST";
private class LocalReceiver extends BroadcastReceiver{
@Override
public void onReceive(Context context, Intent intent) {
Toast.makeText(context, "received a localbroadcast", Toast.LENGTH_SHORT).show();
}
}
我们跳进BroadcastReceiver找到onReceive函数，源代码如下：
*
* @param context The Context in which the receiver is running.
* @param intent The Intent being received.
*/
public abstract void onReceive(Context context, Intent intent);
从这里我们可以看出onReceive函数是抽象函数，所以我们需要重新实现这个函数。刚开始学习Android的时候通常我们会看到书上说我们要实现xxx函数，但是通常不会告诉我们为什么要重新实现这个函数，而且函数的参数是哪些也都不知道。这个时候我们可以点开源码，阅读源码知根知底。关于如何调用onReceive后面会详细分析。
2、创建一个按钮控件，方便后面测试，按下按钮使用本地广播发送一条消息，并用Toast显示。
3、在MainActivity的onCreate函数中进行初始化变量，注册按钮监听器。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
localBroadcastManager = LocalBroadcastManager.getInstance(this);
Button button = (Button)findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view){
Log.d(TAG, "button clicked");
Intent intent = new Intent(localBroadcastAction);
localBroadcastManager.sendBroadcast(intent);
//localBroadcastManager.sendBroadcastSync(intent);
}
});
IntentFilter intentFilter = new IntentFilter();
intentFilter.addAction(localBroadcastAction);
localReceiver = new LocalReceiver();
localBroadcastManager.registerReceiver(localReceiver, intentFilter);
进入LocalBroadcastManager源码我们发现该类的构造函数是私有的，也就是我们无法通过new创建LocalBroadcastManager的实例，而且还发现它有一个静态私有的LocalBroadcastManager成员变量，那么意思很明显了，只能通过一个公有的方法获取LocalBroadcastManager单例对象实例。
private static final Object mLock = new Object();
private static LocalBroadcastManager mInstance;
@NonNull
public static LocalBroadcastManager getInstance(@NonNull Context context) {
synchronized (mLock) {
if (mInstance == null) {
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
private LocalBroadcastManager(Context context) {
mAppContext = context;
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();
break;
default:
super.handleMessage(msg);
}
}
};
}
通过代码我们知道，基于懒汉式单例模式通过公有方法getInstance创建对象实例，所以我们只能通过调用getInstance函数获取LocalBroadcastManager对象实例。从构造函数我们也能大概猜出，初始化的过程中创建了一个线程处理发送的广播消息。由于本地广播可以发送多条消息，因此要想接收到指定的本地广播消息，则需要一个广播消息ID。IntentFilter则是通过存储本地广播消息ID，并和广播接收器BroadcastReceiver绑定在一起实现接收指定的广播数据。
* @param action Name of the action to match, such as Intent.ACTION_VIEW.
*/
public final void addAction(String action) {
if (!mActions.contains(action)) {
mActions.add(action.intern());
}
}
@UnsupportedAppUsage
private final ArrayList mActions;
* @see #unregisterReceiver
*/
public void registerReceiver(@NonNull BroadcastReceiver receiver,
@NonNull IntentFilter filter) {
synchronized (mReceivers) {
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
ArrayList filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);
}
filters.add(entry);
for (int i=0; i
String action = filter.getAction(i);
ArrayList entries = mActions.get(action);
if (entries == null) {
entries = new ArrayList(1);
mActions.put(action, entries);
}
entries.add(entry);
}
}
}
private static final class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
boolean dead;
ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
filter = _filter;
receiver = _receiver;
}IntentFilter将action存储在ArrayList中，LocalBroadcastManager的registerReceiver函数将传入的receiver和filter通过ReceiverRecord绑定在一起，实现了接收特定本地广播消息的功能。
4、调用sendBroadcast将本地广播消息发送出去(实际上只是把数据放在了一个消息队列里面，线程异步将消息发送出去)，我们直接点开sendBroadcast源码，以下是代码片段。
if (receivers != null) {
for (int i=0; i
receivers.get(i).broadcasting = false;
}
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
从源码我们知道，存储Intent并发送一条广播消息到消息队列里面，并且将receiver和BroadcastRecord联系在一起，且将BroadcastRecord存储在mPendingBroadcasts里面。我们进入LocalBroadcastManager的构造函数调用的executePendingBroadcasts函数源码。
void executePendingBroadcasts() {
while (true) {
final BroadcastRecord[] brs;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);
mPendingBroadcasts.clear();
}
for (int i=0; i
final BroadcastRecord br = brs[i];
final int nbr = br.receivers.size();
for (int j=0; j
final ReceiverRecord rec = br.receivers.get(j);
if (!rec.dead) {
rec.receiver.onReceive(mAppContext, br.intent);
}
}
}
}
从mPendingBroadcasts中取出BroadcastRecord对象实例，获取调用receiver，调用onReceive函数。这就是我们为什么可以在onReceive函数中接收到本地广播消息了。
总结
本地广播的基本原理和使用我们已经讲得差不多了，其实还有很多没有讲到。但是没办法，要是把全部实现源代码追踪下去，无穷无尽。所以点到为止，继续学习后续的知识，先应付工作先。
我们在学习Android的时候不仅仅学习表面如何使用，也要适当的追踪源码，学习底层实现原理。Android实在是知识太丰富了，有很多可以学习的地方，如果不是时间有限，真想把所有知识点追踪下去。
完整代码
package com.example.broadcasttest;
import androidx.appcompat.app.AppCompatActivity;
import androidx.localbroadcastmanager.content.LocalBroadcastManager;
import android.app.admin.NetworkEvent;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.net.ConnectivityManager;
import android.net.Network;
import android.net.NetworkCapabilities;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
private IntentFilter intentFilter;
private static final String TAG="MainActivity";
private LocalReceiver localReceiver;
private LocalBroadcastManager localBroadcastManager;
private static final String localBroadcastAction = "com.example.broadcasttest.LOCAL_BROADCAST";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
localBroadcastManager = LocalBroadcastManager.getInstance(this);
Button button = (Button)findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view){
Log.d(TAG, "button clicked");
Intent intent = new Intent(localBroadcastAction);
localBroadcastManager.sendBroadcast(intent);
//localBroadcastManager.sendBroadcastSync(intent);
}
});
IntentFilter intentFilter = new IntentFilter();
intentFilter.addAction(localBroadcastAction);
localReceiver = new LocalReceiver();
localBroadcastManager.registerReceiver(localReceiver, intentFilter);
}
private class LocalReceiver extends BroadcastReceiver{
@Override
public void onReceive(Context context, Intent intent) {
Toast.makeText(context, "received a localbroadcast", Toast.LENGTH_SHORT).show();
}
}
public void onDestroy(){
super.onDestroy();
localBroadcastManager.unregisterReceiver(localReceiver);
}
}
