分析广播工作原理，我们大致分为两个模块，一个是LocalBroadcastManager控制的、只能在进程内通信的广播机制，另外一个还是通过ActivityManagerService作为中转的、可以在进程间通信的广播机制。
LocalBroadcastManager控制的应用内广播机制
本地广播整体逻辑比较简单，关键代码基本都在LocalBroadcastManager类中，基本上简略浏览一遍源码就能搞清楚里面的实现逻辑，此处先给出LocalBroadcastManager的源码：
public final class LocalBroadcastManager {
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;
}
}
private static final String TAG = "LocalBroadcastManager";
private static final boolean DEBUG = false;
private final Context mAppContext;
private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers
= new HashMap<>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();
private final ArrayList<BroadcastRecord> mPendingBroadcasts = new ArrayList<>();
static final int MSG_EXEC_PENDING_BROADCASTS = 1;
private final Handler mHandler;
private static final Object mLock = new Object();
private static LocalBroadcastManager mInstance;
public static LocalBroadcastManager getInstance(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);
}
}
};
}
/**
* 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<ReceiverRecord> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);
}
filters.add(entry);
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);
}
}
}
/**
* Unregister a previously registered BroadcastReceiver. <em>All</em>
* 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) {
final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
for (int i=filters.size()-1; i>=0; i--) {
final ReceiverRecord filter = filters.get(i);
filter.dead = true;
for (int j=0; j<filter.filter.countActions(); j++) {
final String action = filter.filter.getAction(j);
final ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=receivers.size()-1; k>=0; k--) {
final ReceiverRecord rec = receivers.get(k);
if (rec.receiver == receiver) {
rec.dead = true;
receivers.remove(k);
}
}
if (receivers.size() <= 0) {
mActions.remove(action);
}
}
}
}
}
}
/**
* 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
*
* @return Returns true if the intent has been scheduled for delivery to one or more
* broadcast receivers. (Note tha delivery may not ultimately take place if one of those
* receivers is unregistered before it is dispatched.)
*/
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();
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);
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++) {
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;
}
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>();
}
receivers.add(receiver);
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) {
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;
}
/**
* Like {@link #sendBroadcast(Intent)}, but if there are any receivers for
* the Intent this function will block and immediately dispatch them before
* returning.
*/
public void sendBroadcastSync(Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}
private 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<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);
}
}
}
}
}
}
LocalBroadcastManager 采用单例模式，从源头上统一了广播的发送和接收。从上面源码可以看出，LocalBroadcastManager 创建了两个内部类ReceiverRecord 和BroadcastRecord 来记录广播接收者和广播Intent对应的实体，并且初始化了三个HashMap对象mReceivers、mActions和mPendingBroadcasts来分别保存BroadcastReceiver与ReceiverRecord 的关系、Action与ReceiverRecord 的关系以及待发送Intent内容对应的BroadcastRecord ；其中mReceivers在发送广播逻辑中作用并不是特别大，主要作为同步锁和保存、移除数据用。
其实当我们看到上面的HashMap时基本上就可以自己在大脑中有个基本的实现逻辑了，无非就是通过对mActions、mPendingBroadcasts集合进行增删改查，最终实现对应的注册、发送以及反注册等操作，事实上里面的逻辑也就是这些操作组合而成的。
接下来我们就按平时的接口调用顺序来一一查看源码：
registerReceiver方法
一般我们使用本地广播首先就是先注册，而注册方法registerReceiver的逻辑其实也很简单，无非就是往mReceivers和mActions添加预置关系，方便之后发送广播时快速查找对应的广播接收者：
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<>(1);
mReceivers.put(receiver, filters);
}
filters.add(entry);
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);
}
}
}
unregisterReceiver方法
有注册就有反注册，通常反注册就是为了释放相关对象的关联，减少相应时间和内存开销，避免产生内存泄漏的问题。而此处的反注册也不例外，主要用来清除掉不再使用的广播接收者相关信息，
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
for (int i=filters.size()-1; i>=0; i--) {
final ReceiverRecord filter = filters.get(i);
filter.dead = true;
for (int j=0; j<filter.filter.countActions(); j++) {
final String action = filter.filter.getAction(j);
final ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=receivers.size()-1; k>=0; k--) {
final ReceiverRecord rec = receivers.get(k);
if (rec.receiver == receiver) {
rec.dead = true;
receivers.remove(k);
}
}
if (receivers.size() <= 0) {
mActions.remove(action);
}
}
}
}
}
}
sendBroadcast方法
整个流程中，最重要的方法就是sendBroadcast，里面的流程是本地广播的核心机制，但其实里面的逻辑却也很简单！无非就是对mActions的“查”以及对mPendingBroadcasts的“增”和“查”。
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();
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);
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++) {
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;
}
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>();
}
receivers.add(receiver);
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) {
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;
}
阅读源码我们可以发现，整个流程中首先从Intent拿到Action，然后从mActions中找到key为Action的广播接收者列表，接着拿列表中的各个广播接收者与intent中携带的相关信息进行匹配，匹配成功则添加进新的广播接收者列表receivers 中，最后用receivers生成BroadcastRecord对象并添加进mPendingBroadcasts中，mHandler发送消息MSG_EXEC_PENDING_BROADCASTS触发主线程执行。
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) {
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<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);
}
}
}
}
}
mHandler 收到消息后最终执行到了executePendingBroadcasts方法；然后你会很惊讶的发现，这居然用了一个死循环，当然也添加了打断条件（mPendingBroadcasts.size()为0的时候）。在同步锁的情况下用mPendingBroadcasts生成了一个BroadcastRecord[]数组brs，然后清空了mPendingBroadcasts的数据，这样做的是为了避免直接操作mPendingBroadcasts的数据进而造成数据混乱，最后双重遍历brs找到receiver并回调onReceive方法。
到此本地广播的源码基本上已经看完了，你会发现LocalBroadcastManager没有涉及到静态广播的东西，所以静态注册的广播接收者是接收不到本地广播的；同时mHandler 的使用也证明了onReceive是跑在主线程的；本地广播的onReceive是一个个串行跑的，不存在并行的情况.
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