LocalBroadcastManager机制分析

前言:

在Android中，Broadcast是一种广泛运用的在应用程序之间传输信息的机制。BroadcastReceiver设计的

初衷是从全局考虑可以方便应用程序和系统、应用程序之间、应用程序内的通信。但是有时候只是给进程

内部发送广播也需要绕一大圈,有没有简单的方法呢? LocalBroadcastManager。

LocalBroadcastManager是Android Support包提供了一个工具，用于在同一个应用内的不同组件间发送

Broadcast。LocalBroadcastManager也称为局部通知管理器，这种通知的好处是安全性高，效率也高，

适合局部通信，可以用来代替Handler更新UI。

LocalBroadcastManager使用也很简单,四个步骤如下,

1,获取LocalBroadcastManager对象,

LocalBroadcastManager mlocalBroadcastManager = LocalBroadcastManager.getInstance( this ) ;

2, 注册广播接收器

mlocalBroadcastManager.registerReceiver( broadcastReceiver , intentFilter );

3, 发送广播

mlocalBroadcastManager.sendBroadcast( intent ) ;

4, 取消注册广播接收器

mlocalBroadcastManager.unregisterReceiver( broadcastReceiver );

LocalBroadcastManager的全局变量如下,

private final Context mAppContext; //上下文对象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();//同步锁private static LocalBroadcastManager mInstance;//单例

mReceivers：记录注册的BroadcastReceiver及其IntentFilter的数组。

mActions：记录IntentFilter中的action对应的BroadcastReceiver数组。虽然这里写的是ReceiverRecord类型，

但它实际上是一个内部类，主要保存了BroadcastReceiver及其对应的IntentFilter。

mPendingBroadcasts：在发送广播时，会根据Intent的action，找到与之相对应的BroadcastReceiver。action

是可以对应多个BroadcastReceiver，所以这里是数组。

1, getInstance分析

LocalBroadcastManager是一个单例,代码如下,

public static LocalBroadcastManager getInstance(Context context) {        synchronized (mLock) {            if (mInstance == null) {                mInstance = new LocalBroadcastManager(context.getApplicationContext());            }            return mInstance;        }    }

其构造方法如下,

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);                }            }        };    }

构造了一个Handler对象,并且该mHandler仅处理一个消息。

2, registerReceiver分析

LocalBroadcastManager的registerReceiver方法如下,

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);            }        }    }

将BroadcastReceiver和IntentFilter建立一对多的对应关系。可以通过BroadcastReceiver找到其对应的IntentFilter，

也可以通过IntentFilter中的action找到所对应的BroadcastReceiver。

3, unregisterReceiver分析

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);                        }                    }                }            }        }    }

和注册的过程完全相反, 将BroadcastReceiver从mReceivers和mActions中移除掉。由于BroadcastReceiver是mReceivers的键，

所以移除掉比较简单。而mActions就稍微复杂一些，需要根据BroadcastReceiver中的IntentFilter数组，从mActions中移除掉。

4 sendBroadcast分析

sendBroadcast主要逻辑如下,

1,首先获取intent的参数信息,

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();

2, 从mActions中取出intent的action所对应的ReceiverRecord,逐个匹配

ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());•••ArrayList<ReceiverRecord> receivers = null;for (int i=0; i<entries.size(); i++) {•••    //进行匹配   int match = receiver.filter.match(action, type, scheme, data, categories, "LocalBroadcastManager");   receivers.add(receiver);//如果匹配上,添加到receivers中   receiver.broadcasting = true;//将匹配上的状态置为发送中

3,最后将匹配的receivers添加到mPendingBroadcasts中,然后发送MSG_EXEC_PENDING_BROADCASTS消息，

实际上是切换到主线程执行。

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; }

mHandler收到MSG_EXEC_PENDING_BROADCASTS消息后,调用executePendingBroadcasts方法进行处理,

executePendingBroadcasts()方法就很简单了，就是取出mPendingBroadcasts数组中的BroadcastReceiver

(在ReceiverRecord中保存其对象)，调用其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];                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);                }            }        }    }

小结:

1,BroadcastReceiver的通信是走 Binder 机制，所以可以跨进程通信；但是LocalBroadcastManager是handler机制,

所以只能进行进程内部通信。并且, LocalBroadcastManager都是动态注册的, 如果广播仅经过LocalBroadcastManager注册,

也只能收到LocalBroadcastManager sendBroadcast的广播。BroadcastReceiver和LocalBroadcastManager不要混为一团。

2, LocalBroadcastManager仅适用于进程内的广播。

3,优点: 发送的广播只会在当前APP中传播，不会泄露给其它APP，确保了数据传输的安全。其它APP的广播无法发送到

本APP中，不用担心安全漏洞被其它APP所利用。比系统全局广播更加高效。