LocalBroadcastManager源码分析

源码分析

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

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 is 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>
 */
// 同一进程内的广播注册和发送
// 本地广播是同一进程内的，不用担心私有数据的泄露
// 其他app也不能发送广播到本地广播接收器，不用担心安全问题
// 当然本地广播比全局广播更高效
public class LocalBroadcastManager {

	// ReceiverRecord记录了receiver和filter的对应关系，主要用于注册和取消注册时
    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();
        }
    }
	
	// BroadcastRecord记录了intent和对应的receivers，主要用于发送广播时
    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;

	// receiver和action的对应关系是多对多的。
	// mReceivers记录receiver和对应的action
	// mActions记录action和对应的receiver
    private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
            = new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();
    private final HashMap<String, ArrayList<ReceiverRecord>> mActions
            = new HashMap<String, ArrayList<ReceiverRecord>>();

	mPendingBroadcasts存放即将要广播的action和对应的receiver
    private final ArrayList<BroadcastRecord> mPendingBroadcasts
            = new ArrayList<BroadcastRecord>();

    static final int MSG_EXEC_PENDING_BROADCASTS = 1;

	// 在主线程执行receiver的onReceive
    private final Handler mHandler;

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

	// 单例，一个进程中的唯一实例
    public static LocalBroadcastManager getInstance(Context context) {
        synchronized (mLock) {
            if (mInstance == null) {
                mInstance = new LocalBroadcastManager(context.getApplicationContext());
            }
            return mInstance;
        }
    }

    private LocalBroadcastManager(Context context) {
        mAppContext = context;
		// handler是和mainLooper关联的
        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);
			// 上述几行代码保持了receiver和action的映射关系
			// 下面几行代码保持了action和receiver的映射关系
            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) {
			// 和注册广播时相比，取消注册就是取消action和receiver的映射
			// 先从mReceivers里remove对应的receiver，返回的actions用于后续操作
            ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
            if (filters == null) {
                return;
            }
			// receiver对应的action存在，则从action对应的receiver里remove
            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) {
							// 找到了，则从action对应的receiver列表移除
                                receivers.remove(k);
                                k--;
                            }
                        }
                        if (receivers.size() <= 0) {
						// 该action没有receiver了，则将这项remove
                            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) {
		// 异步发送广播，广播接收器的onReceive在主线程执行
        synchronized (mReceivers) {
			// 取出intent的相关属性,action,type,data,categories
            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);

			// 根据action找出其对应的所有receiver
            ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
            if (entries != null) {
                if (debug) Log.v(TAG, "Action list: " + entries);

				// 遍历该action对应的receivers
                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) {
					// broadcasting是一个临时标志，用于防止重复加入
                        if (debug) {
                            Log.v(TAG, "  Filter's target already added");
                        }
                        continue;
                    }

					// 对每个receiver和发送的intent做匹配，匹配过程在这里暂不展开
                    int match = receiver.filter.match(action, type, scheme, data,
                            categories, "LocalBroadcastManager");
                    if (match >= 0) {
						// 要广播的intent和该receiver的过滤器匹配
                        if (debug) Log.v(TAG, "  Filter matched!  match=0x" +
                                Integer.toHexString(match));
                        if (receivers == null) {
                            receivers = new ArrayList<ReceiverRecord>();
                        }
						// 先放入数组，并置标志位
                        receivers.add(receiver);
                        receiver.broadcasting = true;
                    } else {
                        if (debug) {// debug信息
                            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) {
				// 找到了该intent对应的receiver
                    for (int i=0; i<receivers.size(); i++) {
					// 先清除标志
                        receivers.get(i).broadcasting = false;
                    }
					// 放入待广播容器mPendingBroadcasts
                    mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
                    if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
					//抛到主线程执行，executePendingBroadcasts
                        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)) {
		//同步执行，在哪个线程发送的广播，在哪个线程执行
            executePendingBroadcasts();
        }
    }

    private void executePendingBroadcasts() {
	// 真正发送的地方，操作的数据结构是mPendingBroadcasts
        while (true) {
            BroadcastRecord[] brs = null;
            synchronized (mReceivers) {
                final int N = mPendingBroadcasts.size();
                if (N <= 0) {
                    return;
                }
				// 在该同步块内，先将mPendingBroadcasts的内容拷贝到
				// 局部变量brs中，然后清空mPendingBroadcasts
				// 之后退出该同步代码块，我想作用是为了提高多线程下的
				// 效率
                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++) {
				// 调用onReceive
                    br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
                }
            }
        }
    }
}

小结

• 典型的发布订阅模式，注册，取消注册，发布
• 广播消息的传播范围是当前进程，无安全问题，高效
• 和EventBus等第三方开源库比较，属同类，作用范围是当前进程，高效，安全。所以个人觉得在没有非常必要的情况下没必要在工程中引入类似第三方库，android自带的local broadcast足以满足需求。

扩展

• 在发送广播时，intentFilter是如何过滤出匹配的receiver的呢？