本地廣播（ LocalBroadcastManager）源碼解析

本地廣播（ LocalBroadcastManager）源碼解析

一、什麼是LocalBroadcastManager

LocalBroadcastManager是註冊和發送本地廣播的helper。本地廣播與全局廣播有如下優勢：

• 廣播攜帶的數據不會離開app，所以無需擔心數據泄露。
• 不允許其他app向你發送廣播所以不必擔心存在安全問題
• 相對於全局廣播，本地廣播效率更高

二、LocalBroadcastManager類

先看下其結構：
[image:536CD83A-7465-4287-916E-29490CC0F734-286-000C6069B3001F04/C9A82A22-9A65-4B6A-82E5-1FA647789D64.png]

我們看到其有兩個內部類ReceiverRecord和BroadcastRecord.然後提供了幾個方法，這幾個方法就是讓我們進行綁定、註冊、註銷本地廣播用的。

先來看下其中兩個內部類ReceiverRecord和BroadcastRecord。這兩個內部類的作用就是用來記錄廣播接收器和註冊的本地廣播。

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;
    }
}

可以看出這兩個類並未對Broadcast和Receiver做什麼具體的操作。

我們一般使用本地廣播都是通過下面代碼：

LocalBroadcastManager.getInstance(getContext()).sendBroadcast();
LocalBroadcastManager.getInstance(getContext()).sendBroadcastSync();
LocalBroadcastManager.getInstance(getContext()).registerReceiver();
LocalBroadcastManager.getInstance(getContext()).unregisterReceiver();

可以看到都是通過LocalBroadcastManager.getInstance來獲取一個LocalBroadcastManager實例，此處實際是一個單例。

private static LocalBroadcastManager mInstance;

public static LocalBroadcastManager getInstance(Context context) {
    //採用DCL模式的創建單例
    synchronized (mLock) {
        if (mInstance == null) {
            mInstance = new LocalBroadcastManager(context.getApplicationContext());
        }
        return mInstance;
    }
}
//LocalBroadcastManager唯一的構造函數。
private LocalBroadcastManager(Context context) {
    mAppContext = context;
//用傳入的context參數構造handler
    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);
            }
        }
    };
}

接下來我們來依次看下LocalBroadcastManager提供的幾個可調用函數。

registerReceiver

public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
    synchronized (mReceivers) {
        ReceiverRecord entry = new ReceiverRecord(filter, receiver);//構造一個ReceiverRecord實例
        ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);//1、查找當前receiver是否已有對應的filters
        if (filters == null) {
            filters = new ArrayList<>(1);
            mReceivers.put(receiver, filters);
        }
        filters.add(entry);
        //2遍歷filter的Action存入list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);
        }
    }
}

registerReceiver主要做了兩件事：把與Receiver對應的filters存到一個list中，把filter包含的action存到一個list中。這其中涉及了兩個變量mReceivers和mActions，他倆定義如下

private final HashMap<BroadcastReceiver, ArrayList<ReceiverRecord>> mReceivers
        = new HashMap<>();
private final HashMap<String, ArrayList<ReceiverRecord>> mActions = new HashMap<>();

可以看出mReceivers一個以BroadcastReceiver爲key以ReceiverRecord list爲value的HashMap。mActions是一個以String爲key以ReceiverRecord爲value的HashMap。
定義mReceivers的目的是可以根據Receiver找到其所有的filter，定義mActions的目的則是可以根據某個action找到所有的Receiver。

unregisterReceiver

public void unregisterReceiver(BroadcastReceiver receiver) {
    synchronized (mReceivers) {
        final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);//1 在mReceivers移除對應的receiver
        if (filters == null) {
            return;
        }
        for (int i=filters.size()-1; i>=0; i--) {//2 遍歷註釋1獲得的ReceiverRecord list，該list中存放的是要註銷的receiver所對應的filter。
            final ReceiverRecord filter = filters.get(i);
            filter.dead = true;//3 置dead爲true表示當前ReceiverRecord不在活躍
            for (int j=0; j<filter.filter.countActions(); j++) { // 4 遍歷當前ReceiverRecord的filter包含的action
                final String action = filter.filter.getAction(j);
                final ArrayList<ReceiverRecord> receivers = mActions.get(action);// 5 根據action查找對應的ReceiverRecord list
                if (receivers != null) {
                    for (int k=receivers.size()-1; k>=0; k--) {
                        final ReceiverRecord rec = receivers.get(k);
                        if (rec.receiver == receiver) { // 6 註釋5中查到的list中如果包含要註銷的receiver則把其dead置爲true
                            rec.dead = true;
                            receivers.remove(k);
                        }
                    }
                    if (receivers.size() <= 0) {//7 receivers.size()小於等於0表示沒有recriver會接收receivers對應的action
                        mActions.remove(action);
                    }
                }
            }
        }
    }
}

總結下LocalBroadcastManager註冊和註銷過程：
首先LocalBroadcastManager定義了兩個hashMap，mReceivers和mActions。mReceivers一個以BroadcastReceiver爲key以ReceiverRecord list爲value的HashMap。mActions是一個以String爲key以ReceiverRecord爲value的HashMap。
我們註冊時會傳入receiver、filter，receiver就是我們要註冊的廣播接收器而filter則表示我們的接收器要接收哪種類型的廣播（實際filter中的action，每個action表示一個廣播類型）。LocalBroadcastManager的註冊過程是先利用傳入的receiver、filter構造一個ReceiverRecord實例這樣這個ReceiverRecord實例就建立一個receiver和其filter的記錄塊。然後就會以receiver爲key把之前構造的ReceiverRecord實例存入mReceivers中。同樣用類似操作會以註冊時傳入的filter參數中的action爲key把之前構造的ReceiverRecord實例存入mActions中。
在註銷時會根據receiver把其對應的ReceiverRecord list從mReceivers中移除。因爲在註冊時構造的ReceiverRecord存了兩遍在mReceivers和mActions都存過一遍，所以此時還需要把mActions中要註銷的receiver之前存入的ReceiverRecord移除。

由上面可以看出註冊和註銷正好是相反的操作，註冊是在mReceivers和mActions存入對應的ReceiverRecord，註銷則是相反的要在mReceivers和mActions移除對應的ReceiverRecord。

sendBroadcast

public boolean sendBroadcast(Intent intent) {
    synchronized (mReceivers) {
        //1 獲取傳入的intent參數的相關屬性
        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);

        //2 根據intent的action獲取對應的ReceiverRecord list
        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++) {//3 遍歷註釋2處獲取的list
                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;
                }
                 //4 判斷當前的ReceiverRecord所記錄的filter是否跟傳入的intent相匹配
                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>();
                    }
                    //5 如果匹配則添加到receivers中，receivers表示接受此廣播的receiver list
                    receivers.add(receiver);
                    //6 broadcasting置爲true表示正在處理廣播
                    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) {//7 如果接收此廣播的receiver list不爲空則依次添加到mPendingBroadcasts中（mPendingBroadcasts表示需要處理的廣播），然後通過發送MSG_EXEC_PENDING_BROADCASTS消息
                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;
}

sendBroadcast是異步發送廣播，由上面註釋我們可以知道發送的流程是會根據intent中的action找到可以接收此廣播的receiver list（實際找到的是ReceiverRecord list但是ReceiverRecord中記錄了相關的Receiver和filter信息），然後會把receiver的filter和傳入的intent做匹配對比，如果匹配上那麼就把當前的信息塊（ReceiverRecord）添加到一個名爲receivers的list中，此處的receivers表示接可以接收此廣播的receiver list，之後會把receivers中的元素添加到mPendingBroadcasts中（mPendingBroadcasts表示需要處理的廣播，它是一個BroadcastRecord list，BroadcastRecord則是用來記錄廣播和其對應的receivers）最後會通過handler發送一個MSG_EXEC_PENDING_BROADCASTS消息。消息的處理在LocalBroadcastManager構造函數中我們已經看過了，當收到MSG_EXEC_PENDING_BROADCASTS消息時會調用executePendingBroadcasts()。

private void executePendingBroadcasts() {
    while (true) {
        final BroadcastRecord[] brs;
        //把mPendingBroadcasts中的元素轉換爲數組
        synchronized (mReceivers) {
            final int N = mPendingBroadcasts.size();
            if (N <= 0) {
                return;
            }
            brs = new BroadcastRecord[N];
            mPendingBroadcasts.toArray(brs);
            mPendingBroadcasts.clear();
        }
        //遍歷上邊的brs數組，取出BroadcastRecord中的ReceiverRecord list（此處的ReceiverRecord list可以看做是接收廣播的receiver list），如果ReceiverRecord的記錄還“有效”那麼調用其中存儲的Receiver的onReceive，至此廣播接收器就收到了要接收的廣播
        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);
                }
            }
        }
    }
}

executePendingBroadcasts主要完成的還是廣播的分發。我們可以看到本地廣播的異步分發其實是通過Handler完成的。

sendBroadcastSync

sendBroadcast是異步發送廣播，sendBroadcastSync則是同步發送廣播。

public void sendBroadcastSync(Intent intent) {
    if (sendBroadcast(intent)) {
        executePendingBroadcasts();
    }
}

可以看到它的內部是調用了sendBroadcast但是跟sendBroadcast不同的是廣播發送完後它直接調用了executePendingBroadcasts來對廣播進行分發。

總結：
我們通過對LocalBroadcastManager分析可知，本地廣播通過Handler來實現發送和分發的異步處理，我們還可以知道本地廣播之所以不能跨進程、效率高是因爲，其註冊、分發等都是通過本地變量完成的，而不像全局廣播那樣通過binder機制實現。