本地广播介绍

  LocalBroadcastManager是Android Support包提供了一个工具，是用来在同一个应用内的不同组件间发送Broadcast的。

使用LocalBroadcastManager有如下好处：

1. 发送的广播只会在自己App内传播，不会泄露给其他App，确保隐私数据不会泄露 2. 其他App也无法向你的App发送该广播，不用担心其他App会来搞破坏比系统全局广播更加高效

使用方式

先通过

LocalBroadcastManager lbm = LocalBroadcastManager.getInstance(this);

获取实例,然后通过函数 registerReceiver来注册监听器

lbm.registerReceiver(new BroadcastReceiver() { @Override public void onReceive(Context context, Intent intent) { // TODO Handle the received local broadcast } }, new IntentFilter(LOCAL_ACTION));

通过 sendBroadcast 函数来发送广播

lbm.sendBroadcast(new Intent(LOCAL_ACTION));

以下我们简单来做个Demo,实际体会一下。例子很简单，一个布局文件加一个主Activity。

布局文件

<?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent"> <Button android:id="@+id/sendLocal" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="发送本地广播" /> </RelativeLayout>

主Activity

public class MainActivity extends AppCompatActivity implements View.OnClickListener { private final String ACTION = "org.bridge.localreceiver.LOCAL_BROADCAST"; private LocalBroadcastManager manager; private LocalReceiver localReceiver; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); manager = LocalBroadcastManager.getInstance(this); //新建intentFilter IntentFilter filter = new IntentFilter(); filter.addAction(ACTION); localReceiver = new LocalReceiver(); manager.registerReceiver(localReceiver, filter); findViewById(R.id.sendLocal).setOnClickListener(this); } @Override public void onClick(View v) { switch (v.getId()) { case R.id.sendLocal: Intent intent = new Intent(ACTION); manager.sendBroadcast(intent); // manager.sendBroadcastSync(); break; } } @Override protected void onDestroy() { super.onDestroy(); manager.unregisterReceiver(localReceiver); } }

运行结果

点击发送按钮，我们的应用成功收到了广播，接下来我们分析一下，本地广播是如何实现只在当前应用内发送和接受的。

原理分析

以下是LocalBroadcastManager的源代码，整个类代码量不多，还是较为简单的。

public final class LocalBroadcastManager { //静态内部类，ReceiverRecord 封装了BroadcastReceiver和IntentFilter 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(); } } //静态内部类BroadcastRecord ，封装了Intent 和对应的ReceiverRecord 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; //BroadReceiver和对应IntentFilter映射 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>(); static final int MSG_EXEC_PENDING_BROADCASTS = 1; private final Handler mHandler; private static final Object mLock = new Object(); private static LocalBroadcastManager mInstance; //单例模式获取实例 public static LocalBroadcastManager getInstance(Context context) { synchronized (mLock) { if (mInstance == null) { mInstance = new LocalBroadcastManager(context.getApplicationContext()); } return mInstance; } } //私有构造方法，构造了一个Handler 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); } } }; } //注册广播 public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) { synchronized (mReceivers) { ReceiverRecord entry = new ReceiverRecord(filter, receiver); //从映射表取得receiver对应的IntentFilter ArrayList<IntentFilter> filters = mReceivers.get(receiver); //如果没有找到，就添加这个映射 if (filters == null) { filters = new ArrayList<IntentFilter>(1); mReceivers.put(receiver, filters); } filters.add(filter); //遍历filter,保存action到mActions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); } } } //注销广播 public void unregisterReceiver(BroadcastReceiver receiver) { synchronized (mReceivers) { 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); } } } } } } //发送广播 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; } //发送同步广播 public void sendBroadcastSync(Intent intent) { if (sendBroadcast(intent)) { executePendingBroadcasts(); } } private void executePendingBroadcasts() { while (true) { BroadcastRecord[] brs = null; 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++) { BroadcastRecord br = brs[i]; for (int j=0; j<br.receivers.size(); j++) { br.receivers.get(j).receiver.onReceive(mAppContext, br.intent); } } } } }

主要成员变量含义

mReceivers存储广播和过滤器信息，以BroadcastReceiver作为key,IntentFilter链表作为value.mReceivers是接收器和IntentFilter的对应表，主要作用是方便在unregisterReceiver()取消注册，同时作为对象锁限制注册接收器，发送广播，取消接收器注册等几个过程的并发访问。

mActions 以Action为key,注册这个Action的BroadCastReceiver链表为value.mActions的主要作用是方便在广播发送后快速得到可以接受它的BroadCastReceiver.

发送广播的过程

先根据Action从mActions 中取出ReceiverRecord列表，循环每个ReceiverRecord 判断filter和intent 中action，type,scheme,data,categoried 是否match,是的话则保存到receivers列表中，发送what为 MSG_EXEC_PENDING_BROADCASTS的消息，通过Handler处理。

消息处理过程

mPendingBroadcasts转换为数组BroadcastRecord,循环每个receiver,调用其onReceive函数，这样便完成了广播的核心逻辑。

取消注册过程

从mReceivers及mActions中移除相应元素。

总结

LocalBroadcastManager 的核心实现实际上还是Handler，只是利用到了IntentFilter的match功能，至于BroadcastReceiver换成其他接口也无所谓，顺便利用了现成的类和概念而已。

因为采用了Handler实现了应用内通信，自然安全性更好，效率更高。