本地广播（ LocalBroadcastManager）源码解析
本地广播（ LocalBroadcastManager）源码解析
一、什么是LocalBroadcastManager
LocalBroadcastManager是注册和发送本地广播的helper。本地广播与全局广播有如下优势：
- 广播携带的数据不会离开app，所以无需担心数据泄露。
- 不允许其他app向你发送广播所以不必担心存在安全问题
- 相对于全局广播，本地广播效率更高
二、LocalBroadcastManager类
先看下其结构：
[image:536CD83A-7465-4287-916E-29490CC0F734-286-000C6069B3001F04/C9A82A22-9A65-4B6A-82E5-1FA647789D64.png]
我们看到其有两个内部类ReceiverRecord和BroadcastRecord.然后提供了几个方法，这几个方法就是让我们进行绑定、注册、注销本地广播用的。
先来看下其中两个内部类ReceiverRecord和BroadcastRecord。这两个内部类的作用就是用来记录广播接收器和注册的本地广播。
private static final class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
boolean dead;
ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
filter = _filter;
receiver = _receiver;
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder(128);
builder.append("Receiver{");
builder.append(receiver);
builder.append(" filter=");
builder.append(filter);
if (dead) {
builder.append(" DEAD");
}
builder.append("}");
return builder.toString();
}
}
private static final class BroadcastRecord {
final Intent intent;
final ArrayList<ReceiverRecord> receivers;
BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
intent = _intent;
receivers = _receivers;
}
}
可以看出这两个类并未对Broadcast和Receiver做什么具体的操作。
我们一般使用本地广播都是通过下面代码：
LocalBroadcastManager.getInstance(getContext()).sendBroadcast();
LocalBroadcastManager.getInstance(getContext()).sendBroadcastSync();
LocalBroadcastManager.getInstance(getContext()).registerReceiver();
LocalBroadcastManager.getInstance(getContext()).unregisterReceiver();
可以看到都是通过LocalBroadcastManager.getInstance来获取一个LocalBroadcastManager实例，此处实际是一个单例。
private static LocalBroadcastManager mInstance;
public static LocalBroadcastManager getInstance(Context context) {
//采用DCL模式的创建单例
synchronized (mLock) {
if (mInstance == null) {
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
//LocalBroadcastManager唯一的构造函数。
private LocalBroadcastManager(Context context) {
mAppContext = context;
//用传入的context参数构造handler
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);
}
}
};
}
接下来我们来依次看下LocalBroadcastManager提供的几个可调用函数。
registerReceiver
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
ReceiverRecord entry = new ReceiverRecord(filter, receiver);//构造一个ReceiverRecord实例
ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);//1、查找当前receiver是否已有对应的filters
if (filters == null) {
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);
}
filters.add(entry);
//2遍历filter的Action存入list中
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);
}
entries.add(entry);
}
}
}
registerReceiver主要做了两件事：把与Receiver对应的filters存到一个list中，把filter包含的action存到一个list中。这其中涉及了两个变量mReceivers和mActions，他俩定义如下
private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers
= new HashMap<>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();
可以看出mReceivers一个以BroadcastReceiver为key以ReceiverRecord list为value的HashMap。mActions是一个以String为key以ReceiverRecord为value的HashMap。
定义mReceivers的目的是可以根据Receiver找到其所有的filter，定义mActions的目的则是可以根据某个action找到所有的Receiver。
unregisterReceiver
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);//1 在mReceivers移除对应的receiver
if (filters == null) {
return;
}
for (int i=filters.size()-1; i>=0; i--) {//2 遍历注释1获得的ReceiverRecord list，该list中存放的是要注销的receiver所对应的filter。
final ReceiverRecord filter = filters.get(i);
filter.dead = true;//3 置dead为true表示当前ReceiverRecord不在活跃
for (int j=0; j<filter.filter.countActions(); j++) { // 4 遍历当前ReceiverRecord的filter包含的action
final String action = filter.filter.getAction(j);
final ArrayList<ReceiverRecord> receivers = mActions.get(action);// 5 根据action查找对应的ReceiverRecord list
if (receivers != null) {
for (int k=receivers.size()-1; k>=0; k--) {
final ReceiverRecord rec = receivers.get(k);
if (rec.receiver == receiver) { // 6 注释5中查到的list中如果包含要注销的receiver则把其dead置为true
rec.dead = true;
receivers.remove(k);
}
}
if (receivers.size() <= 0) {//7 receivers.size()小于等于0表示没有recriver会接收receivers对应的action
mActions.remove(action);
}
}
}
}
}
}
总结下LocalBroadcastManager注册和注销过程：
首先LocalBroadcastManager定义了两个hashMap，mReceivers和mActions。mReceivers一个以BroadcastReceiver为key以ReceiverRecord list为value的HashMap。mActions是一个以String为key以ReceiverRecord为value的HashMap。
我们注册时会传入receiver、filter，receiver就是我们要注册的广播接收器而filter则表示我们的接收器要接收哪种类型的广播（实际filter中的action，每个action表示一个广播类型）。LocalBroadcastManager的注册过程是先利用传入的receiver、filter构造一个ReceiverRecord实例这样这个ReceiverRecord实例就建立一个receiver和其filter的记录块。然后就会以receiver为key把之前构造的ReceiverRecord实例存入mReceivers中。同样用类似操作会以注册时传入的filter参数中的action为key把之前构造的ReceiverRecord实例存入mActions中。
在注销时会根据receiver把其对应的ReceiverRecord list从mReceivers中移除。因为在注册时构造的ReceiverRecord存了两遍在mReceivers和mActions都存过一遍，所以此时还需要把mActions中要注销的receiver之前存入的ReceiverRecord移除。
由上面可以看出注册和注销正好是相反的操作，注册是在mReceivers和mActions存入对应的ReceiverRecord，注销则是相反的要在mReceivers和mActions移除对应的ReceiverRecord。
sendBroadcast
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
//1 获取传入的intent参数的相关属性
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();
final boolean debug = DEBUG ||
((intent.getFlags() & Intent.FLAG_DEBUG_LOG_RESOLUTION) != 0);
if (debug) Log.v(
TAG, "Resolving type " + type + " scheme " + scheme
+ " of intent " + intent);
//2 根据intent的action获取对应的ReceiverRecord list
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
if (entries != null) {
if (debug) Log.v(TAG, "Action list: " + entries);
ArrayList<ReceiverRecord> receivers = null;
for (int i=0; i<entries.size(); i++) {//3 遍历注释2处获取的list
ReceiverRecord receiver = entries.get(i);
if (debug) Log.v(TAG, "Matching against filter " + receiver.filter);
if (receiver.broadcasting) {
if (debug) {
Log.v(TAG, " Filter's target already added");
}
continue;
}
//4 判断当前的ReceiverRecord所记录的filter是否跟传入的intent相匹配
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
if (match >= 0) {
if (debug) Log.v(TAG, " Filter matched! match=0x" +
Integer.toHexString(match));
if (receivers == null) {
receivers = new ArrayList<ReceiverRecord>();
}
//5 如果匹配则添加到receivers中，receivers表示接受此广播的receiver list
receivers.add(receiver);
//6 broadcasting置为true表示正在处理广播
receiver.broadcasting = true;
} else {
if (debug) {
String reason;
switch (match) {
case IntentFilter.NO_MATCH_ACTION: reason = "action"; break;
case IntentFilter.NO_MATCH_CATEGORY: reason = "category"; break;
case IntentFilter.NO_MATCH_DATA: reason = "data"; break;
case IntentFilter.NO_MATCH_TYPE: reason = "type"; break;
default: reason = "unknown reason"; break;
}
Log.v(TAG, " Filter did not match: " + reason);
}
}
}
if (receivers != null) {//7 如果接收此广播的receiver list不为空则依次添加到mPendingBroadcasts中（mPendingBroadcasts表示需要处理的广播），然后通过发送MSG_EXEC_PENDING_BROADCASTS消息
for (int i=0; i<receivers.size(); 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;
}
}
}
return false;
}
sendBroadcast是异步发送广播，由上面注释我们可以知道发送的流程是会根据intent中的action找到可以接收此广播的receiver list（实际找到的是ReceiverRecord list但是ReceiverRecord中记录了相关的Receiver和filter信息），然后会把receiver的filter和传入的intent做匹配对比，如果匹配上那么就把当前的信息块（ReceiverRecord）添加到一个名为receivers的list中，此处的receivers表示接可以接收此广播的receiver list，之后会把receivers中的元素添加到mPendingBroadcasts中（mPendingBroadcasts表示需要处理的广播，它是一个BroadcastRecord list，BroadcastRecord则是用来记录广播和其对应的receivers）最后会通过handler发送一个MSG_EXEC_PENDING_BROADCASTS消息。消息的处理在LocalBroadcastManager构造函数中我们已经看过了，当收到MSG_EXEC_PENDING_BROADCASTS消息时会调用executePendingBroadcasts()。
private void executePendingBroadcasts() {
while (true) {
final BroadcastRecord[] brs;
//把mPendingBroadcasts中的元素转换为数组
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);
mPendingBroadcasts.clear();
}
//遍历上边的brs数组，取出BroadcastRecord中的ReceiverRecord list（此处的ReceiverRecord list可以看做是接收广播的receiver list），如果ReceiverRecord的记录还“有效”那么调用其中存储的Receiver的onReceive，至此广播接收器就收到了要接收的广播
for (int i=0; i<brs.length; i++) {
final BroadcastRecord br = brs[i];
final int nbr = br.receivers.size();
for (int j=0; j<nbr; j++) {
final ReceiverRecord rec = br.receivers.get(j);
if (!rec.dead) {
rec.receiver.onReceive(mAppContext, br.intent);
}
}
}
}
}
executePendingBroadcasts主要完成的还是广播的分发。我们可以看到本地广播的异步分发其实是通过Handler完成的。
sendBroadcastSync
sendBroadcast是异步发送广播，sendBroadcastSync则是同步发送广播。
public void sendBroadcastSync(Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}
可以看到它的内部是调用了sendBroadcast但是跟sendBroadcast不同的是广播发送完后它直接调用了executePendingBroadcasts来对广播进行分发。
总结：
我们通过对LocalBroadcastManager分析可知，本地广播通过Handler来实现发送和分发的异步处理，我们还可以知道本地广播之所以不能跨进程、效率高是因为，其注册、分发等都是通过本地变量完成的，而不像全局广播那样通过binder机制实现。
