又一年对Android消息机制（Handler&Looper）的思考

private Handler mHandler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
    }
};

Message message = Message.obtain();
message.setData(bundle);
message.setTarget(mHandler);
message.sendToTarget();
//或者mHandler.handleMessage(message);

public Handler() {
    this(null, false);
}
public Handler(Callback callback) {
    this(callback, false);
}
public Handler(Looper looper) {
    this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
}
public Handler(boolean async) {
    this(null, async);
}
public Handler(Callback callback, boolean async) {
    //检测handler是否是静态的，否则会造成内存泄露；
    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());
        }
    }
    //获取当前线程的Looper否则抛异常
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}
public interface Callback {
    public boolean handleMessage(Message msg);
}
/**
 * looper:处理消息的装置
 * callback：处理消息回调，为空则调用handler的handleMessage方法
 * async：是否开启消息同步设置
 */
public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

/**
 * Sends this Message to the Handler specified by {@link #getTarget}.
 * Throws a null pointer exception if this field has not been set.
 */
public void sendToTarget() {
    target.sendMessage(this);
}

//Handler.java
public final boolean sendMessage(Message msg){
    return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis){
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//在这个时候delay已经变成when，时间有延迟变为执行
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;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    //将要发送的Message绑上该Handler的标签
    msg.target = this;
    //如果构造设置了async，那么会为所有的消息打上异步的标签
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    //将消息将入Handler构造函数中初始化的MessageQueue队列
    return queue.enqueueMessage(msg, uptimeMillis);
}

//MessageQueue.java
/**
 * Posts a synchronization barrier to the Looper's message queue.
 * @hide
 */
//发送一个同步的屏障到Looper的消息队列中
public int postSyncBarrier() {
    return postSyncBarrier(SystemClock.uptimeMillis());
}
/**
 * when:消息的执行时间
 */
int postSyncBarrier(long when) {
    // Enqueue a new sync barrier token.
    // We don't need to wake the queue because the purpose of a barrier is to stall it.
    synchronized (this) {
        final int token = mNextBarrierToken++;
        //msg的target为空
        final Message msg = Message.obtain();
        msg.markInUse();
        msg.when = when;
        msg.arg1 = token;
        Message prev = null;
        Message p = mMessages;
        //找到一个执行时间比msg更晚或相同同时执行的消息
        if (when != 0) {
            while (p != null && p.when <= when) {
                prev = p;
                p = p.next;
            }
        }
        //如果有那么插入到这个消息的前面，否则这个消息就是连表头部的消息
        if (prev != null) { // invariant: p == prev.next
            msg.next = p;
            prev.next = msg;
        } else {
            msg.next = p;
            mMessages = msg;
        }
        return token;
    }
}

/**
 * when:消息的执行时间
 */
public void removeSyncBarrier(int token) {
    // Remove a sync barrier token from the queue.
    // If the queue is no longer stalled by a barrier then wake it.
    synchronized (this) {
        Message prev = null;
        //链表的头结点
        Message p = mMessages;
        //从头结点开始遍历，找到target=null，而且token相等的Message
        while (p != null && (p.target != null || p.arg1 != token)) {
            prev = p;
            p = p.next;
        }
        //指定的消息队列同步屏障令牌尚未发布或已被删除。
        if (p == null) {
            throw new IllegalStateException("The specified message queue synchronization "
                    + " barrier token has not been posted or has already been removed.");
        }
        final boolean needWake;
        //如果获取的屏障不是头结点，那么删除该结点，此时不需要唤醒
        if (prev != null) {
            prev.next = p.next;
            needWake = false;
        } else {
            //获取的屏障是头结点，而且下一个结点不是屏障那么就需要唤醒
            mMessages = p.next;
            needWake = mMessages == null || mMessages.target != null;
        }
        //回收这个消息
        p.recycleUnchecked();
        // If the loop is quitting then it is already awake.
        // We can assume mPtr != 0 when mQuitting is false.
        // 如果需要唤醒而且没有退出消息的循环，那么执行唤醒
        if (needWake && !mQuitting) {
            nativeWake(mPtr);
        }
    }
}

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class

final MessageQueue mQueue;
final Thread mThread;

//Looper.java
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");
    }
     //Looper类会创建一个新的Looper对象,并放入全局的sThreadLocal中。
    sThreadLocal.set(new Looper(quitAllowed));
}
public static Looper myLooper() {
    return sThreadLocal.get();
}
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 Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}

Looper.prepare();
Looper looper = Looper.myLooper();

public static void main(String[] args) {
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
    SamplingProfilerIntegration.start();

    // CloseGuard defaults to true and can be quite spammy.  We
    // disable it here, but selectively enable it later (via
    // StrictMode) on debug builds, but using DropBox, not logs.
    CloseGuard.setEnabled(false);

    Environment.initForCurrentUser();

    // Set the reporter for event logging in libcore
    EventLogger.setReporter(new EventLoggingReporter());

    // Make sure TrustedCertificateStore looks in the right place for CA certificates
    final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
    TrustedCertificateStore.setDefaultUserDirectory(configDir);

    Process.setArgV0("<pre-initialized>");
    //创建主线程的Looper对象
    Looper.prepareMainLooper();

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

    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);
    //开启循坏队列监听并接受&处理消息
    Looper.loop();

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

/**
 * Run the message queue in this thread. Be sure to call
 * {@link #quit()} to end the loop.
 */
public static void loop() {
    //获取当前线程的Looper对象实例
    final Looper me = myLooper();
    //调用loop()方法之前必须在当前线程通过Looper.prepare()方法创建Looper对象实例。
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    //获取Looper实例的中的消息队列
    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();
    /***部分代码省略***/
    //开启循环队列-不断
    for (;;) {
        //从队列中取出消息，有可能产生阻塞
        Message msg = queue.next(); // might block
        //如果获取不到消息那么退出
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }
        /***部分代码省略***/
        //将消息分发给注册它的handler
        try {
            msg.target.dispatchMessage(msg);
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        /***部分代码省略***/
        msg.recycleUnchecked();
    }
}

/**
 * Handle system messages here.
 */
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

//MessageQueue.java
@SuppressWarnings("unused")
private long mPtr; // used by native code
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.
    //如果消息循环已经退出并被处理，请返回此处。 
    //如果应用程序退出后不尝试重新启动循环程序，则可能会发生这种情况。
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }
    //挂起空闲处理程序计数，一次next方法只设置一次；
    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    //不断循环直到获取到Message为止；
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }
        //唤醒消息队列；
        //nextPollTimeoutMillis=-1,表示等待；
        //nextPollTimeoutMillis=0,表示唤醒；
        ////nextPollTimeoutMillis>0,表示nextPollTimeoutMillis时间之后唤醒；
        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;
            //如果遇到同步屏障，那么找到离头部最近的异步消息；
            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) {
                //如果消息不为空，而且在当前时间之后执行那么设置下次唤醒时间为Math.min(msg.when - now, Integer.MAX_VALUE)；
                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 {
                    // Got a message.
                    //如果消息现在执行，那么消息队列设置不需要阻塞，并且设置改消息为已使用并在链表中删除；
                    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.
                //获取不到消息那么nextPollTimeoutMillis = -1，将会进行空闲等待；
                nextPollTimeoutMillis = -1;
            }
            //已经处理当前获取的消息是否进行退出，调用了MessageQueue的quit方法；
            // 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.
            //如果是第一次空闲，则获取发现线程何时阻塞的回调数量；
            //挂起空闲处理程序仅在队列为空或将来要处理队列中的第一条消息（可能是同步屏障）时才运行；
            //mIdleHandlers是添加了等待空闲的程序回调接口的列表；
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            //没有挂起空闲处理程序；
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }
            //等待空闲的程序回调接口的数组如果为空，那么创建新的数组；
            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.
        //运行等待空闲时间的回调；
        //我们只会在第一次迭代时到达此代码块,因为后续的pendingIdleHandlerCount会置为0；
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            //释放mPendingIdleHandlers的引用对象；
            mPendingIdleHandlers[i] = null; // release the reference to the handler
            boolean keep = false;
            try {
                //进行接口回调，返回是否该回调要继续保持；
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }
            //如果不需要保持那么删除这个等待空闲程序；
            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }
        //将空闲处理程序计数重置为0，这样我们本地next方法就不再运行它们；
        // 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.
        //在调用空闲处理程序时，可能已经传递了一条新消息；
        //因此，请返回并再次查找未决的消息，而无需等待；
        nextPollTimeoutMillis = 0;
    }
}

//ViewRootImpl.java
//请求View开始进行布局
@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        //检查是否是主线程让
        checkThread();
        mLayoutRequested = true;
        //进行绘制的任务队列
        scheduleTraversals();
    }
}
void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        //往handler的消息队列中插入同步屏障，并保存token
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        //添加异步消息任务mTraversalRunnable
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}
void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        //移除同步屏障
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        if (mProfile) {
            Debug.startMethodTracing("ViewAncestor");
        }
        //开始进行绘制
        performTraversals();
        if (mProfile) {
            Debug.stopMethodTracing();
            mProfile = false;
        }
    }
}