android消息处理源码分析

//保存looper对象，在android中每创建一个消息队列，就有一个并且是唯一一个与之对应的looper对象 
static final threadlocal<looper> sthreadlocal = new threadlocal<looper>();
//主线程的looper
private static looper smainlooper;
//消息队列
final messagequeue mqueue;
final thread mthread;

//子线程中通过调用该方法来创建消息队列
public static void prepare() {
    prepare(true);
}

private static void prepare(boolean quitallowed) {
    if (sthreadlocal.get() != null) {
        throw new runtimeexception("only one looper may be created per thread");
    }
    sthreadlocal.set(new looper(quitallowed));
}


//主线程调用该方法来创建消息队列
public static void preparemainlooper() {
    prepare(false);
    synchronized (looper.class) {
        if (smainlooper != null) {
            throw new illegalstateexception("the main looper has already been prepared.");
        }
        smainlooper = mylooper();
    }
}

//实例化looper，创建消息队列，获取当前线程
private looper(boolean quitallowed) {
    mqueue = new messagequeue(quitallowed);
    mthread = thread.currentthread();
}

//调用loop方法开启消息循环 
public static void loop() { 
    //获取当前的looper对象，若为null，抛出异常 
    final looper me = mylooper();
    if (me == null) {
        throw new runtimeexception("no looper; looper.prepare() 
            wasn't called on this thread."); 
     } 
    //获取当前的消息队列，进入循环 
    final messagequeue queue = me.mqueue; 
    for (;;) { 
        //调用next()方法从消息队列中获取消息，如果为null，结束循环；否则，继续执行(有可能会阻塞) 
        message msg = queue.next(); 
        if (msg == null) { 
            return; 
        } 
        ...... 
        try { 
            //调用handler的dispatchmessage(msg)分发消息  
            msg.target.dispatchmessage(msg); 
        } finally {
        ...... 
        } 
        //回收消息资源  
        msg.recycleunchecked(); 
    }
}

//消息循环退出
public void quit() {
    mqueue.quit(false);
}

public void quitsafely() {
    mqueue.quit(true);
}

/*
*messagequeue中enqueuemessage方法的目的有两个：
*1.插入消息到消息队列
*2.唤醒looper中等待的线程(如果是即时消息并且线程是阻塞状态)
*/
boolean enqueuemessage(message msg, long when) {
    //发送该消息的handler为null，抛出异常
    if (msg.target == null) {
        throw new illegalargumentexception("message must have a target.");
    }
    //此消息正在被使用
    if (msg.isinuse()) {
        throw new illegalstateexception(msg + " this message is already in use.");
    }

    synchronized (this) {
        //此消息队列已经被放弃了
        if (mquitting) {
            illegalstateexception e = new illegalstateexception(
                    msg.target + " sending message to a handler on a dead thread");
            msg.recycle();
            return false;
        }
        msg.markinuse();
        msg.when = when;
        //消息队列的第一个元素，messagequeue中的成员变量mmessages指向的就是该链表的头部元素。
        message p = mmessages;
        boolean needwake;
        if (p == null || when == 0 || when < p.when) {
            //如果此队列中头部元素是null(空的队列，一般是第一次)，或者此消息不是延时的消息，则此消息需要被立即处理，
            //将该消息作为新的头部，并将此消息的next指向旧的头部。如果是阻塞状态则需要唤醒。
            msg.next = p;
            mmessages = msg;
            needwake = mblocked;
        } else {
            //如果此消息是延时的消息，则将其添加到队列中，
            //原理就是链表的添加新元素，按照时间顺序来插入的，这样就得到一条有序的延时消息链表  
            needwake = mblocked && p.target == null && msg.isasynchronous();
            message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needwake && p.isasynchronous()) {
                    needwake = false;
                }
            }
            msg.next = p;
            prev.next = msg;
        }
        if (needwake) {
            nativewake(mptr);
        }
    }
    return true;
}

message next() {
    //与native方法相关,当mptr为0时返回null,退出消息循环
    final long ptr = mptr; 
    if (ptr == 0) {
        return null;
    }

    int pendingidlehandlercount = -1;
    //0不进入睡眠,-1进入睡眠 
    int nextpolltimeoutmillis = 0;  
    for (;;) {
        if (nextpolltimeoutmillis != 0) {
            //处理当前线程中待处理的binder进程间通信请求
            binder.flushpendingcommands();  
        }
        //native方法,nextpolltimeoutmillis为-1时进入睡眠状态
        //阻塞方法，主要是通过native层的epoll监听文件描述符的写入事件来实现的。
        //如果nextpolltimeoutmillis=-1，一直阻塞不会超时。
        //如果nextpolltimeoutmillis=0，不会阻塞，立即返回。
        //如果nextpolltimeoutmillis>0，最长阻塞nextpolltimeoutmillis毫秒(超时)，如果期间有程序唤醒会立即返回
        nativepollonce(ptr, nextpolltimeoutmillis); 
        synchronized (this) {
            final long now = systemclock.uptimemillis();
            message prevmsg = null;
            message msg = mmessages;
            if (msg != null && msg.target == null) {
                do {
                    prevmsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isasynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // next message is not ready.  set a timeout to wake up when it is ready.
                    nextpolltimeoutmillis = (int) math.min(msg.when - now, integer.max_value);
                } else {
                    //正常取出消息，设置mblocked = false代表目前没有阻塞
                    mblocked = false;
                    if (prevmsg != null) {
                        prevmsg.next = msg.next;
                    } else {
                        mmessages = msg.next;
                    }
                    msg.next = null;
                    msg.markinuse();
                    return msg;
                }
            } else {
                // no more messages.更新到睡眠状态
                nextpolltimeoutmillis = -1;
            }

                // process the quit message now that all pending messages have been handled.
                if (mquitting) {
                    dispose();
                    return null;
                }

                // if first time idle, then get the number of idlers to run.
                // idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingidlehandlercount < 0
                        && (mmessages == null || now < mmessages.when)) {
                    pendingidlehandlercount = midlehandlers.size();
                }
                if (pendingidlehandlercount <= 0) {
                    // no idle handlers to run.  loop and wait some more.
                    mblocked = true;
                    continue;
                }

                if (mpendingidlehandlers == null) {
                    mpendingidlehandlers = new idlehandler[math.max(pendingidlehandlercount, 4)];
                }
                mpendingidlehandlers = midlehandlers.toarray(mpendingidlehandlers);
            }
        }            
        //非睡眠状态下处理idlehandler接口 
        for (int i = 0; i < pendingidlehandlercount; i++) { 
            final idlehandler idler = mpendingidlehandlers[i]; 
            // release the reference to the handler 
            mpendingidlehandlers[i] = null; 
            boolean keep = false; 
            try { 
               keep = idler.queueidle(); 
            } catch (throwable t) { 
                log.wtf(tag, "idlehandler threw exception", t); 
            } 
            if (!keep) { 
                synchronized (this) { 
                    midlehandlers.remove(idler); 
                } 
            } 
        } 
        pendingidlehandlercount = 0; 
        nextpolltimeoutmillis = 0;
    }        
}

/*
*通过handler类向线程的消息队列发送消息，
*每个handler对象中都有一个looper对象和messagequeue对象
*/
public handler(callback callback, boolean async) {
    if (find_potential_leaks) {
        final class<? extends handler> klass = getclass();
        if ((klass.isanonymousclass() || klass.ismemberclass() || klass.islocalclass()) &&
                (klass.getmodifiers() & modifier.static) == 0) {
            log.w(tag, "the following handler class should be static or leaks might occur: " +
                klass.getcanonicalname());
        }
    }
    //获取looper对象
    mlooper = looper.mylooper(); 
    if (mlooper == null) {...}
    //获取消息队列
    mqueue = mlooper.mqueue;  
    mcallback = callback;
    masynchronous = async;
}

/*
*多种sendmessage方法,最终都调用了同一个方法sendmessageattime()
*/
public boolean sendmessageattime(message msg, long uptimemillis) {
    messagequeue queue = mqueue;
    if (queue == null) {
        runtimeexception e = new runtimeexception(
                this + " sendmessageattime() called with no mqueue");
        log.w("looper", e.getmessage(), e);
        return false;
    }
    //向消息队列中添加消息
    return enqueuemessage(queue, msg, uptimemillis); 
}
    
/*
*1.当message中的callback不为null时，执行message中的callback中的方法。这个callback时一个runnable接口。
*2.当handler中的callback接口不为null时，执行callback接口中的方法。
*3.直接执行handler中的handlemessage()方法。
*/
public void dispatchmessage(message msg) {
    // 消息callback接口不为null,执行callback接口
    if (msg.callback != null) {
        handlecallback(msg);
    } else {
        if (mcallback != null) {
            //handler callback接口不为null,执行接口方法
            if (mcallback.handlemessage(msg)) {
                return;
            }
        }
        //处理消息
        handlemessage(msg); 
    }
}