LocalBroadcastManager是Android Support v4包提供的一个工具,用来在同一个应用内的不同组件间发送Broadcast的。
相对 BroadcastReceiver,它只能用于应用内通信,安全性更好,同时拥有更高的运行效率。
注意,这是同一个应用,也就是其他应用就算和你共进程,也是无法互相发送广播的。因此可以 确保隐私数据不会泄露。
注意,这是同一个应用,也就是其他应用就算和你共进程,也是无法互相发送广播的。因此可以 确保隐私数据不会泄露。
那既然是同一个应用,为什么还要多此一举使用LocalBroadcast呢,直接调用不可以吗?
使用本地广播可以让代码组织性更好,耦合性更低。
我们知道BroadcastReceiver
的通信是通过 Binder 机制的,而 LocalBroadcastManager 因为叫
LocalBroadcast
,可能让人产生一种它也是以 Binder 通讯方式为底层实现的错觉。查看源码,我们会发现这个更安全高效的实现机制原来如此简单。
1.LocalBroadcastManager 的使用
LocalBroadcastManager 的使用跟一般 BroadcastReceiver 差别不大。
具体使用可看示例:
public class LocalBroadcastActivity extends Activity { private Button btSend; private TextView tvBroadcast; private Context mContext; private LocalBroadcastReceiver localReceiver; private static final String ACTION_LOCAL_SEND = "action_local_send"; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_localbroadcast); btSend = (Button) findViewById(R.id.btSend); tvBroadcast = (TextView) findViewById(R.id.tvBroadcast); mContext = this; btSend.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //发送广播(一般与广播接收者写在应用的不同组件中使用) Intent intent = new Intent(ACTION_LOCAL_SEND); intent.putExtra("msg", "I am a broadcast message."); LocalBroadcastManager.getInstance(mContext).sendBroadcast(intent); } }); //注册广播接收者 localReceiver = new LocalBroadcastReceiver(); LocalBroadcastManager.getInstance(mContext).registerReceiver(localReceiver, new IntentFilter(ACTION_LOCAL_SEND)); } @Override protected void onDestroy() { super.onDestroy(); //取消注册 LocalBroadcastManager.getInstance(mContext).unregisterReceiver(localReceiver); } //定义广播接收者(与一般的BroadcastReceiver写法一样) public class LocalBroadcastReceiver extends BroadcastReceiver { @Override public void onReceive(Context context, Intent intent) { if(intent.getAction().equals(ACTION_LOCAL_SEND)){ String msg = intent.getStringExtra("msg"); tvBroadcast.setText(msg); } } } }
可以看出,
LocalBroadcastManager
与
BroadcastReceiver 的使用步骤基本一致,具体使用方式在代码中进行了详细的注释。比较简单吧。
2.LocalBroadcastManager 源码分析
还是先整体看一下源码吧:
package android.support.v4.content; import java.util.ArrayList; import java.util.HashMap; import java.util.Set; import android.content.BroadcastReceiver; import android.content.Context; import android.content.Intent; import android.content.IntentFilter; import android.net.Uri; import android.os.Handler; import android.os.Message; import android.util.Log; /** * Helper to register for and send broadcasts of Intents to local objects * within your process. This has a number of advantages over sending * global broadcasts with {@link android.content.Context#sendBroadcast}: * <ul> * <li> You know that the data you are broadcasting won't leave your app, so * don't need to worry about leaking private data. * <li> It is not possible for other applications to send these broadcasts to * your app, so you don't need to worry about having security holes they can * exploit. * <li> It is more efficient than sending a global broadcast through the * system. * </ul> */ public final class LocalBroadcastManager { 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(); } } 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; 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; 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<IntentFilter> filters = mReceivers.get(receiver); if (filters == null) { filters = new ArrayList<IntentFilter>(1); mReceivers.put(receiver, filters); } 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); } 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) { 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); } } } } } } /** * 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(); 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) { 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); } } } } }
LocalBroadcastManager 的实现代码并不多,只有不到300行,但具体是怎么实现的呢?
(1) 构造方法
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); } } }; }
由单例实现,因而构造方法设为私有。静态的mInstance实例保证变量生命周期和应用程序进程一样,然后getInstance()方法里面使用同步方法,保证多线程环境下,单例模式不会出现问题。
值得
注意的是
在构造方法中
基于主线程的 Looper 新建了一个 Handler,保证消息的发送和处理都是在主线程执行的,在handleMessage
中会通过
executePendingBroadcasts()
调用接收器对广播的消息进行处理,具体可见后面对
executePendingBroadcasts()的
介绍。
(2) 注册接收者
private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers = new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>(); private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<String, ArrayList<ReceiverRecord>>();
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); } 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); } entries.add(entry); } } }
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()中
取消注册时找到相关的内容,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收它的
BroadcastReceiver
。
(3) 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(); 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); 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); receiver.broadcasting = true; } } 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; }
先根据
Action
从
mActions
中取出
ReceiverRecord
列表,循环每个
ReceiverRecord
判断 filter 和 intent 中的 action、type、scheme、data、categoried 是否匹配,是的话则保存到
receivers
列表中,然后发送 what 为
MSG_EXEC_PENDING_BROADCASTS
的消息,通过 Handler 去处理。
(4) 消息处理
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); } } } }
该方法在Handler的handleMessage()方法中调用,用来处理接收到的消息。将
mPendingBroadcasts
转换为数组
BroadcastRecord
,循环每个
receiver
,然后调用其
onReceive
函数,这样便完成了广播的核心逻辑。
(5) 取消注册
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); } } } } } }
从
mReceivers和
mActions
中找到并移除相应元素。
总结:
(1) LocalBroadcastManager 的核心实现实际上是借助 Handler实现的,只是利用到了 IntentFilter 的 match 功能。如果 BroadcastReceiver 换成其他接口也无所谓,这里只是顺便利用了现成的类和概念而已。
(2)因为是使用 Handler 实现的,只能用于应用内通信,因此安全性更好,效率更高。