也来解释一下android的MessageQueue源码

干茂才
2023-12-01

    大三了,老想自己写点市面上没有的东西,这些天在尝试写一个可以用鼠标操作手机的android应用,前段时间苦于找不到能在屏幕上产生触摸事件的方法,找了N久,发现一个Instrumentation类,觉得靠谱,就拿过来试一试。用的时候发现要在不同的线程里使用,于是我想是不是做个像android消息队列的结构,无信息要处理的时候线程等待,一旦有信息就立刻处理。

    但是连猪跑都没见过,自己写一个谈何容易?想起在博客上看到的名言:“read the fucking source code!”一股“老子要读源码了”的屌丝优越感油然而生,于是对android的MessageQueue类做了下面的翻译和注释,翻译的过程中发现网上已经有了相似的东西,决知要躬行,最后还是自己又逐行读了一遍,其中不乏不懂之处,真心希望高手发现欠妥之处或看的我的疑问后不吝赐教!


/*
 * Copyright (C) 2006 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 com.finetu.bo;

import java.util.ArrayList;

import android.os.Binder;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.os.SystemClock;
import android.util.AndroidRuntimeException;
import android.util.Config;
import android.util.Log;

/**
 * Low-level class holding the list of messages to be dispatched by a
 * {@link Looper}.  Messages are not added directly to a MessageQueue,
 * but rather through {@link Handler} objects associated with the Looper.
 * 
 * <p>You can retrieve the MessageQueue for the current thread with
 * {@link Looper#myQueue() Looper.myQueue()}.
 * 
 * 底层类,用于维护一个message的列表,被一个Looper对象使用。Message对象并不直接加进MessageQueue中,
 * 但可以认为是通过Handler对象与Looper关联。
 * 
 * 调用Looper.mayQueue()可以得到当前线程的Message对象。
 */
public class MessageQueueOfAndroid {
    Message mMessages;
    //handler对象列表,个人理解为观察者列表
    private final ArrayList mIdleHandlers = new ArrayList();
    private boolean mQuiting = false;
    boolean mQuitAllowed = true;
    
    /**
     * Callback interface for discovering when a thread is going to block
     * waiting for more messages.
     * 
     * 当进程将要等待更多Message而挂起时的回调接口。
     */
    public static interface IdleHandler {
        /**
         * Called when the message queue has run out of messages and will now
         * wait for more.  Return true to keep your idle handler active, false
         * to have it removed.  This may be called if there are still messages
         * pending in the queue, but they are all scheduled to be dispatched
         * after the current time.
         * 
         * 当消息队列处理完所有的message需要等待更多message时调用本方法。当需要保持空闲Handler(这里的所
         * 谓空闲Handler个人理解为做为观察者的Handler)活动状态时返回true,反之要移除时返回false。这个方
         * 法有被调用时在消息队列中可能仍然有待处理的消息,但这些消息会在下一时刻被处理。
         */
        boolean queueIdle();
    }

    /**
     * Add a new {@link IdleHandler} to this message queue.  This may be
     * removed automatically for you by returning false from
     * {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is
     * invoked, or explicitly removing it with {@link #removeIdleHandler}.
     * 
     * <p>This method is safe to call from any thread.
     * 
     * @param handler The IdleHandler to be added.
     * 
     * 添加一个新的绑定当前消息队列的Handler对象。该对象被调用后可能会因为IdleHandler.queueIdle()方法返
     * 回值为false而被自动移除,或者被removeIdlerHandler()方法显式移除。
     * 
     * 任何线程调用该方法都是安全的。
     * 
     * 参数:handler 要添加的Handler对象。
     */
    public final void addIdleHandler(IdleHandler handler) {
        if (handler == null) {
            throw new NullPointerException("Can't add a null IdleHandler");
        }
        synchronized (this) {
        	//向Handler列表中添加Handler对象 。
            mIdleHandlers.add(handler);
        }
    }

    /**
     * Remove an {@link IdleHandler} from the queue that was previously added
     * with {@link #addIdleHandler}.  If the given object is not currently
     * in the idle list, nothing is done.
     * 
     * @param handler The IdleHandler to be removed.
     * 
     * 从加入了Handler的队列中删除Handler对象,如果参数中的对象在列表中未找到,则什么也不做。
     * 
     */
    public final void removeIdleHandler(IdleHandler handler) {
        synchronized (this) {
            mIdleHandlers.remove(handler);
        }
    }
    //空构造函数
    MessageQueueOfAndroid() {
    }
    /**
     * 
     */
    final Message next() {
        boolean tryIdle = true;

        while (true) {
            long now;//记录当前时间
            Object[] idlers = null;//Handler列表
    
            // Try to retrieve the next message, returning if found.
            //尝试取出下一条消息,如果成功则返回
            synchronized (this) {
                now = SystemClock.uptimeMillis();//把系统时间赋给now
                Message msg = pullNextLocked(now);//从当前的消息队列中拿出当前允许拿出的一个Message对象
                if (msg != null) return msg;//对象不为空,返回
                if (tryIdle && mIdleHandlers.size() > 0) {//拿到Handler的数组
                    idlers = mIdleHandlers.toArray();
                }
            }
    
            // There was no message so we are going to wait...  but first,
            // if there are any idle handlers let them know.
            //没有取出下一条消息,需要等待,但是首先,如果Handler数组中有Handler对象,则通知这些对象,
            boolean didIdle = false;//标记是否执行过Handler回调的标志位
            if (idlers != null) {
                for (Object idler : idlers) {
                    boolean keep = false;
                    try {
                        didIdle = true;//已有Handler调用了自己的queueIdle()方法,记录状态。
                        //消息队列需要等待,此处一次调用Handler对象的queueIdle()方法,
                        //使得需要移出的Handler被移除,无需移出的被保存
                        keep = ((IdleHandler)idler).queueIdle();
                    } catch (Throwable t) {
                        Log.wtf("MessageQueue", "IdleHandler threw exception", t);
                    }

                    if (!keep) {
                        synchronized (this) {
                            mIdleHandlers.remove(idler);//移除Handler的操作
                        }
                    }
                }
            }
            
            // While calling an idle handler, a new message could have been
            // delivered...  so go back and look again for a pending message.
            //当有Handler调用了回调函数时,可能恰好有一个消息加入了消息队列,所以需要让最上层的while循环
            //重新循环一遍看有没有就绪的Messaged对象。(此处笔者不明白为何直接continue就可以做到这种功能,
            //因为对于有无新的Message,didIdle应该会有不同的状态,单位和这两种不同的状态可以代表有无Message对象,笔者不明白)
            if (didIdle) {
                tryIdle = false;
                continue;
            }

            synchronized (this) {
                // No messages, nobody to tell about it...  time to wait!
            	//没有消息了,此时已经不需要通知任何Handler对象,队列开始等待
                try {
                    if (mMessages != null) {//消息队列非空,说明有消息在一段可预知的时间后可以获取到,此时只需等待这一段有限的时间即可
                        if (mMessages.when-now > 0) {
                            Binder.flushPendingCommands();//Binder操作,刷新什么不知道--"
                            this.wait(mMessages.when-now);//等待已知的有限长时间
                        }
                    } else {//否则,不知道何时会有新的Message可以拿到
                        Binder.flushPendingCommands();
                        this.wait();//持续等待
                    }
                }
                catch (InterruptedException e) {
                }
            }
        }
    }
    /**
     * 在message链表中取出当前时间下可取出的下一个Message对象,在一些特殊情况下,Message可以设置delay,来人
     * 为延迟一段时间,这时不能立即取出队头的message对象,此处直接返回null。
     */
    final Message pullNextLocked(long now) {
        Message msg = mMessages;
        if (msg != null) {
            if (now >= msg.when) {
                mMessages = msg.next;
                if (Config.LOGV) Log.v(
                    "MessageQueue", "Returning message: " + msg);
                return msg;
            }
        }

        return null;
    }
    /**通过when即时间,在message链表中插入新的message对象,参数msg.target==null作为执行quit操作的标识,
     * 插入前将msg的when属性用参数when赋值,若原本的链表为空,或when是最小的,则加在链表头,否则循环,找到一个合适
     * 的位置插入msg,最后返回true。
     */
    final boolean enqueueMessage(Message msg, long when) {
        if (msg.when != 0) {
            throw new AndroidRuntimeException(msg
                    + " This message is already in use.");
        }
        if (msg.target == null && !mQuitAllowed) {
            throw new RuntimeException("Main thread not allowed to quit");
        }
        synchronized (this) {
            if (mQuiting) {
                RuntimeException e = new RuntimeException(
                    msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            } else if (msg.target == null) {
                mQuiting = true;
            }

            msg.when = when;
            //Log.d("MessageQueue", "Enqueing: " + msg);
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {
                msg.next = p;
                mMessages = msg;
                this.notify();
            } else {
                Message prev = null;
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
                msg.next = prev.next;
                prev.next = msg;
                this.notify();
            }
        }
        return true;
    }
    //重写的删除消息方法
    final boolean removeMessages(Handler h, int what, Object object,
            boolean doRemove) {
        synchronized (this) {
            Message p = mMessages;
            boolean found = false;

            // Remove all messages at front.
            while (p != null && p.target == h && p.what == what
                   && (object == null || p.obj == object)) {
                if (!doRemove) return true;
                found = true;
                Message n = p.next;
                mMessages = n;
                p.recycle();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && n.what == what
                        && (object == null || n.obj == object)) {
                        if (!doRemove) return true;
                        found = true;
                        Message nn = n.next;
                        n.recycle();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
            
            return found;
        }
    }
    //重写的删除消息方法
    final void removeMessages(Handler h, Runnable r, Object object) {
        if (r == null) {
            return;
        }

        synchronized (this) {
            Message p = mMessages;

            // Remove all messages at front.
            while (p != null && p.target == h && p.callback == r
                   && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycle();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && n.callback == r
                        && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycle();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }
    //删除所有的Handler和Message
    final void removeCallbacksAndMessages(Handler h, Object object) {
        synchronized (this) {
            Message p = mMessages;

            // Remove all messages at front.
            while (p != null && p.target == h
                    && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycle();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycle();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }

    /*
    private void dumpQueue_l()
    {
        Message p = mMessages;
        System.out.println(this + "  queue is:");
        while (p != null) {
            System.out.println("            " + p);
            p = p.next;
        }
    }
    */
    //把控制权交给下一个等待的线程(对线程理解不深,仅供参考)
    void poke()
    {
        synchronized (this) {
            this.notify();
        }
    }
}

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