LocalBroadcastManager源码解析

LocalBroadcastManager源码解析

1.简介

LocalBroadcastManager是Android v4兼容包提供的应用内广播发送与接收的工具类。BroadcastReceiver的通信是基于Binder机制，而LocalBroadcastManager的核心是基于Handler机制。

相比BroadcastReceiver的广播，LocalBroadcastManager有以下几点优点。

广播数据只在本应用内传播，不用担心数据泄露，

广播数据不用担心别的应用伪造广播，更加安全。

因为只在应用内广播，所以更加的高效。

2.基本使用方法

2.1 自定义 BroadcastReceiver 子类

public class LocalBroadcastReceiver extends BroadcastReceiver {

@Override
public void onReceive(Context context, Intent intent) {
//处理广播信息
}
}

2.2 注册广播

LocalBroadcastReceiver localReceiver = new LocalBroadcastReceiver();
IntentFilter filter = new IntentFilter(ACTION_LOCAL_SEND);
LocalBroadcastManager.getInstance(context).registerReceiver(localReceiver, filter);

2.3 发送广播

LocalBroadcastManager.getInstance(context).sendBroadcast(new Intent(ACTION_LOCAL_SEND));

2.4 取消广播注册

LocalBroadcastManager.getInstance(context).unregisterReceiver(localReceiver);

3.源码解析

3.1 LocalBroadcastManager原理概要

LocalBroadcastManager使用单例模式对象，初始化时会在内部初始化一个Handler对象用来接受广播。注册广播时，会将自定义的BroadcastReceiver对象和IntentFilter对象保存到HashMap中。发送广播时，则根据IntentFilter的Action值从已保存的HashMap找到对应接受者，并发送Handler消息去执行receiver的onReceive方法。

LocalBroadcastManager核心代码为以下四个函数。

registerReceiver(BroadcastReceiver receiver, IntentFilter filter) //注册广播函数

unregisterReceiver(BroadcastReceiver receiver) //取消注册函数

sendBroadcast(Intent intent) //发送广播

executePendingBroadcasts() //处理接受到的广播

3.2 LocalBroadcastManager基本数据结构

LocalBroadcastManager需要保存三样东西，一个是 mReceivers, 用来保存已注册的自定义的receiver和intentFilter。一个是 mActions 键值对，保存action和ReceiverRecord列表的键值对。一个是 mPendingBroadcasts , 用来保存待通知的receiver对象。

//注册广播Record类
private static class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;

ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
filter = _filter;
receiver = _receiver;
}
...
}

private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
= new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();

private final HashMap<String, ArrayList<ReceiverRecord>> mActions
= new HashMap<String, ArrayList<ReceiverRecord>>();

private final ArrayList<BroadcastRecord> mPendingBroadcasts
= new ArrayList<BroadcastRecord>();

//待广播的Record类
private static class BroadcastRecord {
final Intent intent;
final ArrayList<ReceiverRecord> receivers;

BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
intent = _intent;
receivers = _receivers;
}
}

3.3 注册广播

将需要注册的receiver对象和该receiver需要监听的filter保存到 mReceivers 和 mPendingBroadcasts 中。

/**
* Register a receive for any local broadcasts that match the given IntentFilter.
*
* @param receiver The BroadcastReceiver to handle the broadcast.
* @param filter Selects the Intent broadcasts to be received.
*
* @see #unregisterReceiver
*/
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
ArrayList<IntentFilter> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<IntentFilter>(1);
mReceivers.put(receiver, filters);    //保存receiver和filter到List
}
filters.add(filter);
for (int i=0; i<filter.countActions(); i++) {
String action = filter.getAction(i);
ArrayList<ReceiverRecord> entries = mActions.get(action);
if (entries == null) {
entries = new ArrayList<ReceiverRecord>(1);
mActions.put(action, entries);   //保存到action和ReceiverRecord到HashMap
}
entries.add(entry);
}
}
}

3.4 取消广播注册

根据receiver对象移除 mReceivers 和 mPendingBroadcasts 中对应的对象。

/**
* Unregister a previously registered BroadcastReceiver.  All
* filters that have been registered for this BroadcastReceiver will be
* removed.
*
* @param receiver The BroadcastReceiver to unregister.
*
* @see #registerReceiver
*/
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
//从mReceivers中移除
ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
for (int i=0; i<filters.size(); i++) {
IntentFilter filter = filters.get(i);
for (int j=0; j<filter.countActions(); j++) {
String action = filter.getAction(j);
ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=0; k<receivers.size(); k++) {
if (receivers.get(k).receiver == receiver) {
receivers.remove(k);
k--;
}
}
if (receivers.size() <= 0) {
mActions.remove(action);  //从mActions中移除
}
}
}
}
}
}

3.5 通过Handler发送广播

发送广播时，先根据intent中的action到mActions中找到对应的记录，然后再完整匹配filter里面的各个字段，若匹配成功，则将对应的receiver添加的mPendingBroadcasts列表中，等待handler对象的handleMessage()方法处理。

/**
* Broadcast the given intent to all interested BroadcastReceivers.  This
* call is asynchronous; it returns immediately, and you will continue
* executing while the receivers are run.
*
* @param intent The Intent to broadcast; all receivers matching this
*     Intent will receive the broadcast.
*
* @see #registerReceiver
*/
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
final String action = intent.getAction();
final String type = intent.resolveTypeIfNeeded(
mAppContext.getContentResolver());
final Uri data = intent.getData();
final String scheme = intent.getScheme();
final Set<String> categories = intent.getCategories();

...

//根据intent的action寻找ReceverRecord
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());

if (entries != null) {

ArrayList<ReceiverRecord> receivers = null;
for (int i=0; i<entries.size(); i++) {

ReceiverRecord receiver = entries.get(i);
//相同的receiver,只添加一次
if (receiver.broadcasting) {
continue;
}

int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");

if (match >= 0) {
if (receivers == null) {
receivers = new ArrayList<ReceiverRecord>();
}
receivers.add(receiver);
//标记为已添加，待广播状态
receiver.broadcasting = true;
} else {
...
}
}

if (receivers != null) {

//receivers添加完成后，将broadcasting状态回归
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}

//添加到待广播列表
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));

//若无正在处理的消息，则handler发送广播消息
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}

return true;
}
}
}
return false;

3.6 Handler接受和消费广播

在handler对象的handleMessage()方法中遍历 mPendingBroadcasts 列表, 依次循环调用其中的onReceive()方法，并将intent中的数据传入，从而消费广播信息。

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);
}
}
};
}

private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
//拷贝数据到brs数组
brs = new BroadcastRecord
;
mPendingBroadcasts.toArray(brs);
mPendingBroadcasts.clear();
}

for (int i=0; i<brs.length; i++) {
BroadcastRecord br = brs[i];
for (int j=0; j<br.receivers.size(); j++) {
//循环数组里的内容，调用其onReceive方法，消费广播内容
br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
}
}
}

4.总结

LocalBroadcastManager在应用内使用起来比较简单高效，但是其也是有一些缺点的。比如LocalBroadcastManager并不支持静态注册广播，也不支持有序广播的一些功能。不过如果仅仅是普通广播通信也是够用了。

github连接:https://github.com/czhzero/AndroidSdkSourceAnalysis/blob/master/article/LocalBroadcastManager%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90.md