好用的假廣播 LocalBroadcastManager

簡介 從名字來看，這貨就是個廣播管理類，其實否則，它套用了廣播的空殼，卻實現了線程間通訊；遠看是廣播，近看是線程間通訊；

其使用方式也和廣播同樣；讓咱們看看源碼

源碼分析

其相關的有兩個內部類： ReceiverRecord、BroadcastRecord；先從軟柿子開始

接收者信息 ReceiverRecord

final IntentFilter filter;
        final BroadcastReceiver receiver;
        boolean broadcasting;
        boolean dead;
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有如下信息：

1. 消息處理對象
2. 可處理消息對象
3. 輔助是否添加處處理集合（詳情見發送消息代碼）
4. 是否能夠處理相關事件（詳情看，解注、消息處理流程）

消息全部信息 BroadcastRecord

final Intent intent;
        final ArrayList<ReceiverRecord> receivers;
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包括了消息intent、須要處理此消息的接收者信息集合

LocalBroadcastManager 構造器以及成員變量

private final Context mAppContext;

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

    private final ArrayList<BroadcastRecord> mPendingBroadcasts = new ArrayList<>();

    static final int MSG_EXEC_PENDING_BROADCASTS = 1;

    private final Handler mHandler;

    private static final Object mLock = new Object();
    private static LocalBroadcastManager mInstance;

    @NonNull
    public static LocalBroadcastManager getInstance(@NonNull 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);
                }
            }
        };
    }
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1. 採用了單例模式
2. 使用handler進行通訊
3. 以廣播接收者爲key， 以廣播接收者信息做爲value存儲全部的 廣播接收者信息；來進行快速存儲、取消接收者信息
4. 以廣播動做爲key，以廣播接收者信息做爲value存儲全部的 廣播接收者信息，快速定位消息處理者集合
5. 消息處理信息集合

爲何有兩個集合呢？ 查找消息可能處理者時，mActions比較快速；mReceivers，因爲鎖的重入性，刪除時，避免當前線程其它的action處理，也就是保證雖然解注，但仍然有可能在處理者信息集合中，經過這種方式快速阻止繼續執行（經過dead參數）

不過我我的任務仍是有點勉強說通，也有可能有其它緣由

註冊信息接收

public void registerReceiver(@NonNull BroadcastReceiver receiver,
            @NonNull IntentFilter filter) {
        synchronized (mReceivers) {
            ReceiverRecord entry = new ReceiverRecord(filter, receiver);
            ArrayList<ReceiverRecord> filters = mReceivers.get(receiver);
            if (filters == null) {
                filters = new ArrayList<>(1);
                mReceivers.put(receiver, filters);
            }
            filters.add(entry);
            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);
            }
        }
    }
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mActions、mReceivers添加註冊者信息

解注消息接收

public void unregisterReceiver(@NonNull BroadcastReceiver receiver) {
        synchronized (mReceivers) {
            final ArrayList<ReceiverRecord> filters = mReceivers.remove(receiver);
            if (filters == null) {
                return;
            }
            for (int i=filters.size()-1; i>=0; i--) {
                final ReceiverRecord filter = filters.get(i);
                filter.dead = true;
                for (int j=0; j<filter.filter.countActions(); j++) {
                    final String action = filter.filter.getAction(j);
                    final ArrayList<ReceiverRecord> receivers = mActions.get(action);
                    if (receivers != null) {
                        for (int k=receivers.size()-1; k>=0; k--) {
                            final ReceiverRecord rec = receivers.get(k);
                            if (rec.receiver == receiver) {
                                rec.dead = true;
                                receivers.remove(k);
                            }
                        }
                        if (receivers.size() <= 0) {
                            mActions.remove(action);
                        }
                    }
                }
            }
        }
    }
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mActions、mReceivers刪除信息；同時刪除空數據集合

消息處理

void executePendingBroadcasts() {
        while (true) {
            final BroadcastRecord[] brs;
            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++) {
                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);
                    }
                }
            }
        }
    }
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mPendingBroadcasts 中存儲了處理的消息intent、處理者集合；根據處理者解注狀態來肯定是否須要處理

消息發送

public boolean sendBroadcast(@NonNull 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;
    }
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異步處理，利用Handler發送消息，在主線程中處理；代碼流程：

1. 經過intent的action查找可能的處理者
2. 經過intent的其它信息，肯定須要處理的處理者信息，而且避免重複（broadcasting添加後爲true，未添加爲false）
3. mPendingBroadcasts 中加入消息和須要處理者信息集合
4. 發送消息處理

public void sendBroadcastSync(@NonNull Intent intent) {
        if (sendBroadcast(intent)) {
            executePendingBroadcasts();
        }
    }
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發送和處理在同一個線程；雖然發送了消息，可是當前線程馬上就執行了；中間方法調用棧未枷加鎖，也就是有極小几率，不能保證同步執行

原理總結

1. 利用了廣播的使用方式，利於開發者使用
2. 利用Intent能夠在進程間傳遞，而用作消息
3. 使用Handler機制，使消息能夠在主線程回調處理
4. 使用synchronized 進行同步
5. 使用了單例模式，保證同一個類加載器內惟一
6. 同步執行消息調用，並不必定能保證同步

總體來講利用了廣播使用方式，和類的一些實現，外加Handler來實現線程間通訊

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