BroadcastReceiver
BroadcastReceiver是Android四大组件之一，我们都比较熟悉，称它为“广播”
Android中的广播就类似于电台广播，一个广播可以有N个接收者。这是一个典型的发布——订阅模式，也可以叫做观察者模式。
广播机制最大的特点就是发送发并不关心接收方是否接收到数据，也不关心接收方是如何处理数据的，通过这样的形式达到接收双方的完全解耦。
BroadcastReceiver是一个全局的（可跨进程）的广播，也就是说所有应用都可收到你发送的BroadcastReceiver。这可能会带来安全隐患，比如其他应用恶意模拟发送广播、获取隐私数据等等。
在BroadcastReceiver源码类注释中有如下文字：
If you don't need to send broadcasts across applications, consider using
this class with LocalBroadcastManager instead of the more general facilities described below. This will give you a much more efficient implementation (no cross-process communication needed) and allow you to avoid thinking about any security issues related to other applications being able to receive or send your broadcasts.
意思大概是如果你不需要跨进程发送广播，请考虑使用LocalBroadcastManager替代BroadcastReceiver，这样我们就可以避免其他应用程序能够接收或发送你的应用程序的BroadcastReceiver所带来的安全问题
LocalBroadcastManager的基本使用
- 注册广播
LocalBroadcastManager.getInstance(context).registerReceiver(receiver,intentFilter);
- 反注册广播
LocalBroadcastManager.getInstance(context).unregisterReceiver(receiver);
- 发送广播
LocalBroadcastManager.getInstance(context).sendBroadcast(newIntent());
LocalBroadcastManager 源码解析
代码量比较少，我先用大白话说一遍，有个印象之后，看代码就好理解了
大体框架就是两个内部类、三个集合、一个Handler、注册广播的方法、反注册广播的方法、发送广播的方法、通知接收者的方法。
- 两个内部类用于承载一些广播数据，如IntentFilter、BroadcastReceiver等
- 三个集合就是将承载着数据的内部类存储起来，功能：存储注册的接收者、方便在需要的时候获取对象，比如获取接受者并回调onReceive方法等
- Handler用于在主线程发送并接收消息，调用executePendingBroadcasts()方法为接受者回调onReceive方法
- 注册广播的方法：用内部类封装数据，并存入集合
- 反注册广播的方法：从集合中删除对应的数据
- 发送广播的方法：获取到Intent中携带的数据，与集合中保存的数据进行匹配，将匹配的保存到集合中，通过handler发送消息回调executePendingBroadcasts()方法为接受者回调onReceive方法
- 通知接收者的方法：从集合中获取注册的BroadcastReceiver对象，调用其onReceive方法
public class LocalBroadcastManager {
/**
* 内部类ReceiverRecord 用于保存IntentFilter和BroadcastReceive
*/
private static class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
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);
builder.append("}");
return builder.toString();
}
}
/**
* 内部类BroadcastRecord 承载着ReceiverRecord集合，用于发送广播时获取BroadcastReceiver对象
*/
private static 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;
//以BroadcastReceiver对象为key，IntentFilter集合为value 在注册广播接收器时添加 用于以后方便取消注册广播接收器
private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
= new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();
//以Action字符串为key,以装载着IntentFilter和BroadcastReceive的内部类ReceiverRecord为value，用于发送广播后得到广播接收器
private final HashMap<String, ArrayList<ReceiverRecord>> mActions
= new HashMap<String, ArrayList<ReceiverRecord>>();
//存储装载着ReceiverRecord集合的内部类BroadcastRecord的集合 用于发送广播时回调BroadcastReceiver的onReceive方法
private final ArrayList<BroadcastRecord> mPendingBroadcasts
= new ArrayList<BroadcastRecord>();
static final int MSG_EXEC_PENDING_BROADCASTS = 1;
private final Handler mHandler;
private static final Object mLock = new Object();
private static LocalBroadcastManager mInstance;
//单例模式创建
public static LocalBroadcastManager getInstance(Context context) {
synchronized (mLock) {
if (mInstance == null) {
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
private LocalBroadcastManager(Context context) {
mAppContext = context;
//核心 使用一个主线程Looper的Handler 用于完成广播的消息处理
//也就是说 onReceive接收广播的时候 是处于主线程中
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);
}
}
};
}
/**
* 注册广播接收器
*
*/
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//承载IntentFilter和BroadcastReceiver的内部类，会将其存储于mActions集合，方便根据action查找广播接收器
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//将BroadcastReceiver和IntentFilter存入mReceivers集合中，方便取消广播监听器
ArrayList<IntentFilter> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<IntentFilter>(1);
mReceivers.put(receiver, filters);
}
filters.add(filter);
//将action和ReceiverRecord存入mActions集合
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);
}
}
}
/**
* 取消广播接收器
*/
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
//移除掉mActions集合中的该BroadcastReceiver对象
ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
//根据mActions存储的IntentFilter集合将mActions集合中相关action对应的ReceiverRecord也删掉
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);
}
}
}
}
}
}
/**
* 发送广播
* 1.获取到要发送的Intent包含的信息
* 2.从注册的接收器中找到与之匹配的接收器，存储到集合中
* 3.通过Handler在主线程回调注册的接收器的onReceive方法
*/
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
// 获取Intent中携带的相关信息
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);
// 根据action从mActions集合中获取到ReceiverRecord集合
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);
//broadcasting是内部类ReceiverRecord的一个属性，用于防止重复加入receiver到receivers集合
//被加入receivers集合时赋值为true,遍历结束后将改属性全部赋值为false
if (receiver.broadcasting) {
if (debug) {
Log.v(TAG, " Filter's target already added");
}
continue;
}
//用断定filter是否匹配
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
if (match >= 0) { //匹配
if (debug) Log.v(TAG, " Filter matched! match=0x" +
Integer.toHexString(match));
if (receivers == null) {
receivers = new ArrayList<ReceiverRecord>();
}
//将匹配的receiver加入receivers集合
receivers.add(receiver);
receiver.broadcasting = true;
} else { //不匹配
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) {
//添加结束，将所有receiver对象的broadcasting属性赋值为false
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
//封装进BroadcastRecord中，并添加到mPendingBroadcasts集合
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
//若主线程队列中没有MSG_EXEC_PENDING_BROADCASTS，则发送该消息进行onReceive回调
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
/**
* 发送同步广播
* 指的是在一次广播的发送完成之前，不会同时发送其他广播
*/
public void sendBroadcastSync(Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}
/**
* 回调注册的BroadcastReceiver的onReceive方法
*/
private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
//将所有匹配的集合这保存到BroadcastRecord数组中
mPendingBroadcasts.toArray(brs);
//将该集合清除 如果本次循环结束前未添加新的对象到该集合，那么下次循环将被return
mPendingBroadcasts.clear();
}
//遍历回调匹配的BroadcastReceiver的onReceive方法
for (int i=0; i<brs.length; i++) {
BroadcastRecord br = brs[i];
for (int j=0; j<br.receivers.size(); j++) {
br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
}
}
}
}
}
看完源码，我们会发现，BroadcastReceiver的跨进程通信是依赖Binder机制来完成的，而LocalBroadcastManager这个不跨进程的广播并没有那么复杂，说白了就是单例模式的LocalBroadcastManager就是普通观察这模式，将广播接收者（即观察者）存储到集合中，发送广播时去集合找到对应的接受者（即当可观察者发生变更时），配合了Handler的使用回调onReceive对象罢了（即通知观察者做出相应的处理），此模式可以看看Observable类的源码，加深理解。
利与弊
它的好处是：
- 仅在本应用内广播，不用担心数据泄露与其他应用模拟发送广播，比BroadcastReceiver更安全
- 效率比使用Binder的BroadcastReceiver高
相对而言，缺点就是：
- 不能跨进程通信。这就是它的双刃剑啊
- 不能用于系统Action，例如电量、Wifi状态等
总而言之，就是根据自己的实际情况去选择适合的工具，这样才能让它们都“工作”在自己真正应该在的岗位上，才能发挥出最大的价值！
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