Android Handler MessageQueue Looper 消息机制原理

提到Android里的消息机制，便会提到Message、Handler、Looper、MessageQueue这四个类，我先简单介绍以下这4个类

之间的爱恨情仇。

Message

消息的封装类，里边存储了消息的详细信息，以及要传递的数据

Handler

主要用在消息的发送上，有即时消息，有延迟消息，内部还提供了享元模式封装了消息对象池，能够有效的减少重复对象的创建，留更多的内存做其他的事，

Looper

这个类内部持有一个MessageQueue对象，当创建Looper的时候，同时也会创建一个MessageQueue，然后Looper的主要工作就不断的轮训MessageQueue，轮到天荒地老的那种

MessageQueue

内部持有一个Message对象，采用单项链表的形式来维护消息列队。并且提供了入队，出队的基础操作

举个现实中的栗子，Message就相当于包装好的快递盒子，Handler就相当于传送带，MessageQueue就相当于快递车，Looper就相当于快递员，联想一下，来个快递盒子，biu丢到传送带上，传送带很智能，直接传送到快递三轮车里，然后快递小哥送一波～，日夜交替，不分昼夜的工作，好家伙，007工作制

消息机制的初始化

好，我们把这4个家伙从头到位分析一遍，要想使用Android的消息，首先要创建Looper对象，Android系统已经帮我们在UI线程内创建好了一个，我们可以看一下

public final class ActivityThread extends ClientTransactionHandler {
    /**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false, startSeq);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }
}

ActivityThread这个类大家应该不陌生吧，没错，他就是我们App的主线程管理类，我们看到他调用了 prepareMainLooper 来初始化，然后 loop，天荒地老的那种loop，这个loop，我们最后聊

我们看一下Looper内部提供的 prepareMainLooper 实现

public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}
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));
}
private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

上边涉及到了3个方法，我都贴出来了，首先 quitAllowed 这个参数代表该Looper是否可以退出，我们主线程内的Looper是不允许退出的，所以封装了 prepareMainLooper 方法和 prepare 方法已做区分，我们项目中平时用的都是 prepare 方法，因为是子线程，所以允许退出Looper，大家在子线程内用完记得调用quit哦~

这里我们看Looper内部是通过ThreadLocal维护的Looper对象，也就是说每个线程都是相互独立的。而且Looper做了限制，每个线程内部只能存在一个Looper对象，等同于每个线程内只能有一个MessageQueue

最后在Looper的构造方法内，创建了一个MessageQueue对象，整个Looper的初始化就结束了

创建消息

我们准备好了Looper和MessageQueue后，就可以创建消息啦，接下来我们创建一个消息吧

//直接new对象，不推荐的方式
Message msg = new Message();
//推荐：内部是一个复用对象池
Message message = handler.obtainMessage();
message.what = 1;
message.obj = "hello world";

发送消息（入队）

我们发送消息的时候，都是会借助Handler的sendMessage就可以把消息发送到列队里了，我们往下看是如何完成的入队操作吧，首先我们平时都是创建一个Handler，然后调用sendMessage就可以了

Handler handler = new Handler();
handler.sendMessage(message);

我们先看一下Handler的构造方法

public Handler() {
    this(null, false);
}
public Handler(@Nullable Callback callback, boolean async) {
    //FIND_POTENTIAL_LEAKS一直都是false，所以不用关心这个逻辑
    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());
        }
    }
    //得到当前线程下的Looper对象
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    //从Loopper内部获取一个列队
    mQueue = mLooper.mQueue;
    // 回调对象，我们平时写的时候，一般都是用类集成的方式重写 handleMessage 方法
    mCallback = callback;
    //标示当前Handler是否支持异步消息
    mAsynchronous = async;
}

其实构造方法很简单呐，就是获取Looper对象，然后初始化列队和回调对象就完事了，我们继续看sendMessage然后看消息的入队吧

public final boolean sendMessage(@NonNull Message msg) {
    return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull 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;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
}

通过内部的重载方法，一直调用到sendMessageAtTime方法，在这里得到Handler内部的MessageQueue对象，然后调用了 enqueueMessage 方法准备入队

private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
        long uptimeMillis) {
    msg.target = this;
    msg.workSourceUid = ThreadLocalWorkSource.getUid();

    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

这里调用了MessageQueue的enqueueMessage方法真正入队，我们继续看一下

boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }

    synchronized (this) {
        //如果当前退出状态，则回收消息，并返回消息入队失败
        if (mQuitting) {
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            return false;
        }

        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        //设置链表的头
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            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;
                }
            }
            //把msg放到链表最后
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

通过这个方法，我们了解到MessageQueue是通过Message的单链结构存储的，然后每次入队的时候，都会

通过这个enqueueMessage方法向链表的最末尾添加数据。

最后我们聊一下Looper下的loop方法吧

接下来我们看一下

public static void loop() {
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;

    // Make sure the identity of this thread is that of the local process,
    // and keep track of what that identity token actually is.
    Binder.clearCallingIdentity();
    final long ident = Binder.clearCallingIdentity();

    // Allow overriding a threshold with a system prop. e.g.
    // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
    final int thresholdOverride =
            SystemProperties.getInt("log.looper."
                    + Process.myUid() + "."
                    + Thread.currentThread().getName()
                    + ".slow", 0);

    boolean slowDeliveryDetected = false;

    for (;;) {
        //queue的next会阻塞
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }

        // This must be in a local variable, in case a UI event sets the logger
        final Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }
        // Make sure the observer won't change while processing a transaction.
        final Observer observer = sObserver;

        final long traceTag = me.mTraceTag;
        long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
        long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
        if (thresholdOverride > 0) {
            slowDispatchThresholdMs = thresholdOverride;
            slowDeliveryThresholdMs = thresholdOverride;
        }
        final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
        final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

        final boolean needStartTime = logSlowDelivery || logSlowDispatch;
        final boolean needEndTime = logSlowDispatch;

        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }

        final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
        final long dispatchEnd;
        Object token = null;
        if (observer != null) {
            token = observer.messageDispatchStarting();
        }
        long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
        try {
            //派发消息，执行回调handleMessage
            msg.target.dispatchMessage(msg);
            if (observer != null) {
                observer.messageDispatched(token, msg);
            }
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } catch (Exception exception) {
            if (observer != null) {
                observer.dispatchingThrewException(token, msg, exception);
            }
            throw exception;
        } finally {
            ThreadLocalWorkSource.restore(origWorkSource);
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        if (logSlowDelivery) {
            if (slowDeliveryDetected) {
                if ((dispatchStart - msg.when) <= 10) {
                    Slog.w(TAG, "Drained");
                    slowDeliveryDetected = false;
                }
            } else {
                if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                        msg)) {
                    // Once we write a slow delivery log, suppress until the queue drains.
                    slowDeliveryDetected = true;
                }
            }
        }
        if (logSlowDispatch) {
            showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
        }

        if (logging != null) {
            logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
        }

        // Make sure that during the course of dispatching the
        // identity of the thread wasn't corrupted.
        final long newIdent = Binder.clearCallingIdentity();
        if (ident != newIdent) {
            Log.wtf(TAG, "Thread identity changed from 0x"
                    + Long.toHexString(ident) + " to 0x"
                    + Long.toHexString(newIdent) + " while dispatching to "
                    + msg.target.getClass().getName() + " "
                    + msg.callback + " what=" + msg.what);
        }

        msg.recycleUnchecked();
    }
}

Looper内的loop方法别看这么多，大多数都是日志相关的处理。其实他就两件事

第一件事就是从列队中通过next取出Message对象

第二件事就是通过Message对象上绑定的target对象dispatchMessage方法，来分发消息

我们接下来看一下dispatchMessage方法，然后在看MessageQueue的next

public void dispatchMessage(@NonNull Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

灰常简单，判断CallBack对象。然后调用handleMessage就完事了，我们的Activity就收到数据了。

接下来我们看看MessageQueue的next是怎么获取列队内的消息的把。

Message next() {
    // Return here if the message loop has already quit and been disposed.
    // This can happen if the application tries to restart a looper after quit
    // which is not supported.
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }

    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }
        //没有消息的时候，或者有延迟消息的时候会进行睡眠
        nativePollOnce(ptr, nextPollTimeoutMillis);

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                //当前时间小于消息内记录的时间，然后计算一个睡眠时间，跳出循环执行睡眠
                if (now < msg.when) {
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                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);
        }

        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = null; // release the reference to the handler

            boolean keep = false;
            try {
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }

            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }

        // Reset the idle handler count to 0 so we do not run them again.
        pendingIdleHandlerCount = 0;

        // While calling an idle handler, a new message could have been delivered
        // so go back and look again for a pending message without waiting.
        nextPollTimeoutMillis = 0;
    }
}

首先MessageQueue的消息是用单链表的形式存储，然后next函数做的事情就是死循环获取消息，

在获取消息的时候判断一下消息是否符合执行时间，如果不符合执行时间，就进入睡眠状态等待消息。

如果符合执行时间就直接返回Message给Looper进行分发，如果Message链表都为空。则睡眠时间是-1

代表无休止的睡眠。在无休止睡眠的状态下，enqueueMessage的nativeWake方法，会进行一次唤醒，唤醒后next函数继续执行，判断返回消息给Looper执行消息分发