安卓基础-activity-handler更深入一层的学习

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本文对最近面试过程中遇到的handler没有答出来的问题做一次系统性的学习

以前学handler还是在做framework的时候，有些细节也没有考虑过

Handler概览

Handler构造

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/**
 * Default constructor associates this handler with the {@link Looper} for the
 * current thread.
 *
 * If this thread does not have a looper, this handler won't be able to receive messages
 * so an exception is thrown.
 */
public Handler() {
    this(null, false);
}

/**
 * Constructor associates this handler with the {@link Looper} for the
 * current thread and takes a callback interface in which you can handle
 * messages.
 *
 * If this thread does not have a looper, this handler won't be able to receive messages
 * so an exception is thrown.
 *
 * @param callback The callback interface in which to handle messages, or null.
 */
public Handler(Callback callback) {
    this(callback, false);
}

/**
 * Use the provided {@link Looper} instead of the default one.
 *
 * @param looper The looper, must not be null.
 */
public Handler(Looper looper) {
    this(looper, null, false);
}

/**
 * Use the provided {@link Looper} instead of the default one and take a callback
 * interface in which to handle messages.
 *
 * @param looper The looper, must not be null.
 * @param callback The callback interface in which to handle messages, or null.
 */
public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
}

/**
 * Use the {@link Looper} for the current thread
 * and set whether the handler should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
 *
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
public Handler(boolean async) {
    this(null, async);
}

/**
 * Use the {@link Looper} for the current thread with the specified callback interface
 * and set whether the handler should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
 *
 * @param callback The callback interface in which to handle messages, or null.
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
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());
        }
    }

    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

/**
 * Use the provided {@link Looper} instead of the default one and take a callback
 * interface in which to handle messages.  Also set whether the handler
 * should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by conditions such as display vsync.
 *
 * @param looper The looper, must not be null.
 * @param callback The callback interface in which to handle messages, or null.
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

handler的构造函数其实就分成两种，一种构造最后走到双参数构造里面，一种走到三参数构造里面。

区别在于双参数里面是通过 mLooper = Looper.myLooper();来获取looper。

双参数中looper的来源

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/**
 * Return the Looper object associated with the current thread.  Returns
 * null if the calling thread is not associated with a Looper.
 */
public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}

Looper.myLooper()的方法是如上，会从threadlocal中直接get出来。

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

这个threadlocal是存在于Looper成员变量区域中

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/** Initialize the current thread as a looper.
  * This gives you a chance to create handlers that then reference
  * this looper, before actually starting the loop. Be sure to call
  * {@link #loop()} after calling this method, and end it by calling
  * {@link #quit()}.
  */
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));
}

在prepare的过程中会创建新的Looper塞到对应的线程threadlocal中

双参数和三参数的共同点

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mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;

可以看出messageQueue是在looper内部的一个变量

async 的作用

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* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.

从备注里面可以看到，如果async为true的话，会在message中setAsynchronous

Message#setAsynchronous()

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/**
 * Sets whether the message is asynchronous, meaning that it is not
 * subject to {@link Looper} synchronization barriers.
 * <p>
 * Certain operations, such as view invalidation, may introduce synchronization
 * barriers into the {@link Looper}'s message queue to prevent subsequent messages
 * from being delivered until some condition is met.  In the case of view invalidation,
 * messages which are posted after a call to {@link android.view.View#invalidate}
 * are suspended by means of a synchronization barrier until the next frame is
 * ready to be drawn.  The synchronization barrier ensures that the invalidation
 * request is completely handled before resuming.
 * </p><p>
 * Asynchronous messages are exempt from synchronization barriers.  They typically
 * represent interrupts, input events, and other signals that must be handled independently
 * even while other work has been suspended.
 * </p><p>
 * Note that asynchronous messages may be delivered out of order with respect to
 * synchronous messages although they are always delivered in order among themselves.
 * If the relative order of these messages matters then they probably should not be
 * asynchronous in the first place.  Use with caution.
 * </p>
 *
 * @param async True if the message is asynchronous.
 *
 * @see #isAsynchronous()
 */
public void setAsynchronous(boolean async) {
    if (async) {
        flags |= FLAG_ASYNCHRONOUS;
    } else {
        flags &= ~FLAG_ASYNCHRONOUS;
    }
}

这个方法备注里面写的还是比较清楚的

message设置了这个方法之后，将会不遵从looper的同步屏障。

像一些方法，view#invalidation，该操作传入messagequeue的时候会同步往message queue中引入同步屏障，直到满足一些条件。

在invalidation这种情况出现的时候，在invalidate之后post的message将会被同步屏障拦截，直到下一帧被绘制完毕。

同步屏障会确保invalidation操作执行完毕之后在执行别的同步消息。

异步消息则会被免除于同步屏障之外，异步消息一般是中断，输入事件，或者别的需要在其他工作停滞状态时独立分开处理的信号等。

请注意，尽管异步消息之间总是按顺序传递，但与同步消息相比，异步消息的传递可能是无序的。如果这些消息的相对顺序很重要，那么它们可能一开始就不应该是异步的。谨慎使用。

最后这句话就用谷歌翻译了。..

handler 另外四种构造方式

public static Handler createAsync(@NonNull Looper looper)

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public static Handler createAsync(@NonNull Looper looper) {
    if (looper == null) throw new NullPointerException("looper must not be null");
    return new Handler(looper, null, true);
}

这个就是创建异步的handler

public static Handler createAsync(@NonNull Looper looper, @NonNull Callback callback)

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@NonNull
 public static Handler createAsync(@NonNull Looper looper, @NonNull Callback callback) {
     if (looper == null) throw new NullPointerException("looper must not be null");
     if (callback == null) throw new NullPointerException("callback must not be null");
     return new Handler(looper, callback, true);
 }

上一种handler的变体

public static Handler getMain()

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/** @hide */
@NonNull
public static Handler getMain() {
    if (MAIN_THREAD_HANDLER == null) {
        MAIN_THREAD_HANDLER = new Handler(Looper.getMainLooper());
    }
    return MAIN_THREAD_HANDLER;
}

这个方法提供给系统使用

public static Handler mainIfNull(@Nullable Handler handler)

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/** @hide */
@NonNull
public static Handler mainIfNull(@Nullable Handler handler) {
    return handler == null ? getMain() : handler;
}

这个方法提供给系统使用

创造message的几种方式

public final Message obtainMessage()

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/**
 * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
 * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
 *  If you don't want that facility, just call Message.obtain() instead.
 */
public final Message obtainMessage()
{
    return Message.obtain(this);
}

从全局的message pool里面获取一个新的message，比直接创建实例更有效率，并且已经设置过target = this了

obtaionMessage的一些变种

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/**
  * Same as {@link #obtainMessage()}, except that it also sets the what member of the returned Message.
  * 
  * @param what Value to assign to the returned Message.what field.
  * @return A Message from the global message pool.
  */
 public final Message obtainMessage(int what)
 {
     return Message.obtain(this, what);
 }
 
 /**
  * 
  * Same as {@link #obtainMessage()}, except that it also sets the what and obj members 
  * of the returned Message.
  * 
  * @param what Value to assign to the returned Message.what field.
  * @param obj Value to assign to the returned Message.obj field.
  * @return A Message from the global message pool.
  */
 public final Message obtainMessage(int what, Object obj)
 {
     return Message.obtain(this, what, obj);
 }

 /**
  * 
  * Same as {@link #obtainMessage()}, except that it also sets the what, arg1 and arg2 members of the returned
  * Message.
  * @param what Value to assign to the returned Message.what field.
  * @param arg1 Value to assign to the returned Message.arg1 field.
  * @param arg2 Value to assign to the returned Message.arg2 field.
  * @return A Message from the global message pool.
  */
 public final Message obtainMessage(int what, int arg1, int arg2)
 {
     return Message.obtain(this, what, arg1, arg2);
 }
 
 /**
  * 
  * Same as {@link #obtainMessage()}, except that it also sets the what, obj, arg1,and arg2 values on the 
  * returned Message.
  * @param what Value to assign to the returned Message.what field.
  * @param arg1 Value to assign to the returned Message.arg1 field.
  * @param arg2 Value to assign to the returned Message.arg2 field.
  * @param obj Value to assign to the returned Message.obj field.
  * @return A Message from the global message pool.
  */
 public final Message obtainMessage(int what, int arg1, int arg2, Object obj)
 {
     return Message.obtain(this, what, arg1, arg2, obj);
 }

Message#obtain()

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/**
 * Return a new Message instance from the global pool. Allows us to
 * avoid allocating new objects in many cases.
 */
public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

obtain方法其实就是判断pool是否有值，有值的话返回出来，没值就新增。

sPool的赋值过程如下

Message#recycle() => Message#recycleUnchecked() => sPool = this

这也代表着Message其实是一个单链表节点，其会静态缓存一系列的被recycle的Message，这也代表平时使用完Message之后，记得recycle一下

Post

Post其实就是核心中的核心，其有很多post的方法

第一种：public final boolean post(Runnable r)

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/**
 * Causes the Runnable r to be added to the message queue.
 * The runnable will be run on the thread to which this handler is 
 * attached. 
 *  
 * @param r The Runnable that will be executed.
 * 
 * @return Returns true if the Runnable was successfully placed in to the 
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.
 */
public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

这里调用getPostMessage方法，将runnable改为Message，然后执行sendMessageDelayed

第二种：public final boolean postAtTime(Runnable r, long uptimeMillis)

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public final boolean postAtTime(Runnable r, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

该方法和是在固定的时间点执行

第三种：public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)

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public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}

和第二种方式类似，token的作用是用于cancel这个message

第四种：public final boolean postDelayed(Runnable r, long delayMillis)

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public final boolean postDelayed(Runnable r, long delayMillis)
{
    return sendMessageDelayed(getPostMessage(r), delayMillis);
}

该方式和第一种类似，加上了delaymillis

第五种：public final boolean postDelayed(Runnable r, Object token, long delayMillis)

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public final boolean postDelayed(Runnable r, Object token, long delayMillis)
{
    return sendMessageDelayed(getPostMessage(r, token), delayMillis);
}

加上了token的第四种方式

第六种：public final boolean postAtFrontOfQueue(Runnable r)

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public final boolean postAtFrontOfQueue(Runnable r)
{
    return sendMessageAtFrontOfQueue(getPostMessage(r));
}

往头部插入message，虽好不要使用，特殊场景除外，会造成顺序问题以及别的影响

第七种：public final boolean executeOrSendMessage(Message msg)

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public final boolean executeOrSendMessage(Message msg) {
    if (mLooper == Looper.myLooper()) {
        dispatchMessage(msg);
        return true;
    }
    return sendMessage(msg);
}

如果当前的线程就是handler的线程，直接执行message，否则塞入该handler的线程

sendMessageDelayed

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/**
 * Enqueue a message into the message queue after all pending messages
 * before (current time + delayMillis). You will receive it in
 * {@link #handleMessage}, in the thread attached to this handler.
 *  
 * @return Returns true if the message was successfully placed in to the 
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.  Note that a
 *         result of true does not mean the message will be processed -- if
 *         the looper is quit before the delivery time of the message
 *         occurs then the message will be dropped.
 */
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

事实上sendMessageDelayed也是执行了sendMessageAtTime

SystemClock.uptimeMillis()是指从开机到现在的毫秒数，并非System.currentTimeMillis()这种从1970年1月1日 UTC到现在的毫秒数

sendMessageAtTime

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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);
}

如果messageQueue不存在就报错，否则就执行enqueueMessage操作，mq的初始化在handler构造器里面赋值的。

enqueueMessage

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private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

如果消息是异步消息，设置一下状态，然后塞到MessageQueue里面

Looper概览

刚才看完handler相关，其实looper大部分的已经经过了。在提一下。

prepare()

prepare事实上有一个参数，是是否允许退出，mainlooper不允许，其他的是允许的

prepare核心是创建一个looper塞到threadlocal里面

myLooper()

Looper.myLooper()就是当前线程的looper

核心方法loop()

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/**
 * 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
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    //获取该looper对应的mq
    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.
    //重设置远端的uid和pid，用当前本地进程的uid和pid替代
    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);
    //用于监控慢函数
    boolean slowDeliveryDetected = false;

    for (;;) {
        Message msg = queue.next(); // might block
        //next之后拿到的msg是null，这时候代表其实是要退出的，因为普通时候这个next是会阻塞的
        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);
        }
        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;
        try {
            //target就是handler，传给handler去处理
            msg.target.dispatchMessage(msg);
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            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);
        }
        //清理msg
        msg.recycleUnchecked();
    }
}

这里就是进入到消息循环中去了，它不断地从消息队列mQueue中去获取下一个要处理的消息msg，如果消息的target成员变量为null，就表示要退出消息循环了，否则的话就要调用这个target对象的dispatchMessage成员函数来处理这个消息，这个target对象的类型为Handler

Looper核心没有太多东西，主要是Loop。

还有一个问题，loop是谁执行的？

主线程的looper其实在main函数里面执行的。

子线程需要改写一下，手动调用

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*  class LooperThread extends Thread {
*      public Handler mHandler;
*
*      public void run() {
*          Looper.prepare();
*
*          mHandler = new Handler() {
*              public void handleMessage(Message msg) {
*                  // process incoming messages here
*              }
*          };
*
*          Looper.loop();
*      }

MessageQueue

MessageQueue可以说是最重要的，其next方法核心才是loop不阻塞的根源。

构造

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MessageQueue(boolean quitAllowed) {
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
}

quitAllowed主线程是false，子线程是true。

nativeInit是native的构造方法，其会在native这一层构造一个nativeMessageQueue。

同时NativeMessageQueue也会创建一个Native的Looper，该looper的实现在jni层并非java层的looper

创建完之后会将nativeMQ存入到mPtr中

JNI层的Looper

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Looper::Looper(bool allowNonCallbacks) :
	mAllowNonCallbacks(allowNonCallbacks),
	mResponseIndex(0) {
	int wakeFds[2];
	int result = pipe(wakeFds);
	......
 
	mWakeReadPipeFd = wakeFds[0];
	mWakeWritePipeFd = wakeFds[1];
 
	......
 
#ifdef LOOPER_USES_EPOLL
	// Allocate the epoll instance and register the wake pipe.
	mEpollFd = epoll_create(EPOLL_SIZE_HINT);
	......
 
	struct epoll_event eventItem;
	memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
	eventItem.events = EPOLLIN;
	eventItem.data.fd = mWakeReadPipeFd;
	result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem);
	......
#else
	......
#endif
 
	......
}

这个Looper重要的就是

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int wakeFds[2];
int result = pipe(wakeFds);
......
 
mWakeReadPipeFd = wakeFds[0];
mWakeWritePipeFd = wakeFds[1];

使用Pipe创建了一个管道对应的两个句柄，一个读一个写，中间还需要加一层等待状态的机制。就是Epoll，Epoll是select/poll的增强版本。

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mEpollFd = epoll_create(EPOLL_SIZE_HINT);

这段代码是注册了一个EpollFd

使用Pipe和Epoll的核心关键作用，也就是这个native的Looper的作用。

这个native的Looper的作用是，当Java层的消息队列中没有消息时，就使Android应用程序主线程进入等待状态，而当Java层的消息队列中来了新的消息后，就唤醒Android应用程序的主线程来处理这个消息。

MessageQueue#next()

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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();
        }
        //关键点方式
        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;
            //同步屏障相关
            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());
            }
            //找到了这个msg，进行处理
            if (msg != null) {
                //时间延时问题
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    //改变一下poll time out
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    //之前的msg不是空，直接拼接
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    //之前的msg是空，直接赋值
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    //返回msg
                    return msg;
                }
            } else {
                // No more messages.
                //没有msg，mills改成-1
                nextPollTimeoutMillis = -1;
            }

            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }
            //处理idlehandler
            // 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.
        //逐个处理idlehandler
        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;
    }
}

next的一整体主要是返回了msg，以前也只会这些，其实还有idleHandler的处理，处理就是直接处理的过程，根据queueIdle的返回值，false就会移除，不是false就不移除。

最核心的是nativePollOnce()和nextPollTimeoutMillis的方法把控

nativePollOnce(mPtr, nextPollTimeoutMillis)

mPtr：之前的native的mq的应用。
nextPollTimeoutMillis：表示如果当前消息队列中没有消息，它要等待的时候

nextPollTimeoutMillis

postDelay的message什么时候会执行呢？这个其实就是取决于这个nextPollTimeoutMillis参数
取出来一条消息发现这条消息是未来需要执行的，计算好时间，会作为nextPollTimeoutMills的参数启动下一次nativePollOnce

另外可以看到，刚开始的时候nextPollTimeoutMillis 是0，其实就代表第一次的时候直接poll。

另外还可以看到，当没有消息的时候，nextPollTimeoutMillis是-1，这时候代表空闲

nativePollOnce 的native代码

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static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
        jint ptr, jint timeoutMillis) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->pollOnce(timeoutMillis);
}
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void NativeMessageQueue::pollOnce(int timeoutMillis) {
    mLooper->pollOnce(timeoutMillis);
}
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int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
	int result = 0;
	for (;;) {
		......
 
		if (result != 0) {
			......
 
			return result;
		}
 
		result = pollInner(timeoutMillis);
	}
}
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int Looper::pollInner(int timeoutMillis) {
	......
 
	int result = ALOOPER_POLL_WAKE;
 
	......
 
#ifdef LOOPER_USES_EPOLL
	struct epoll_event eventItems[EPOLL_MAX_EVENTS];
	int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
	bool acquiredLock = false;
#else
	......
#endif
 
	if (eventCount < 0) {
		if (errno == EINTR) {
			goto Done;
		}
 
		LOGW("Poll failed with an unexpected error, errno=%d", errno);
		result = ALOOPER_POLL_ERROR;
		goto Done;
	}
 
	if (eventCount == 0) {
		......
		result = ALOOPER_POLL_TIMEOUT;
		goto Done;
	}
 
	......
 
#ifdef LOOPER_USES_EPOLL
	for (int i = 0; i < eventCount; i++) {
		int fd = eventItems[i].data.fd;
		uint32_t epollEvents = eventItems[i].events;
		if (fd == mWakeReadPipeFd) {
			if (epollEvents & EPOLLIN) {
				awoken();
			} else {
				LOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
			}
		} else {
			......
		}
	}
	if (acquiredLock) {
		mLock.unlock();
	}
Done: ;
#else
	......
#endif
 
	......
 
	return result;
}

nativePollOnce => NativeMessageQueue::android_os_MessageQueue_nativePollOnce => Looper::pollOnce => Looper::pollInner

过程虽然长，核心就是将nextPollTimeoutMillis传到native层，然后进行设置一次epoll_wait等待操作。

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void Looper::awoken() {
	......
 
	char buffer[16];
	ssize_t nRead;
	do {
		nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
	} while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
}

、
wake操作如上，直接读信道

话虽然是这么讲，但是难免会有队列中只有一个未来发生的msg，此时头插一个新的msg。此时会直接wake。

messageQueue#IdleHandler

之前看到在设置nextPollTimeoutMillis = -1的时候和msg在未来才会发生的时候，并不会return，而是会继续走下去。

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// 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);

这里会遍历mIdleHandlers的接口，然后生成mPendingIdleHandlers，注意，很明显，idleHandler最大数量是4，也就是多余的事实上是会被去掉的

之后便会执行idlehandlers里面的pendingIdleHandlers

MessageQueue#同步屏障

next方法中获得消息之后

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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());
}

这里会有一个鉴别是否是开启同步屏障。

同步屏障其实就是一个message的target为空的msg而已。

如果target为空，这个if就会走进去，他会不断地往后遍历，直到寻找到异步msg，或者遍历到尾部。

可是每次native

postSyncBarrier & removeSyncBarrier

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public int postSyncBarrier() {
    return postSyncBarrier(SystemClock.uptimeMillis());
}
​
private 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++;
        final Message msg = Message.obtain();
        msg.markInUse();
        msg.when = when;
        msg.arg1 = token;
​
        Message prev = null;
        Message p = mMessages;
        if (when != 0) {
            while (p != null && p.when <= when) {
                prev = p;
                p = p.next;
            }
        }
        if (prev != null) { // invariant: p == prev.next
            msg.next = p;
            prev.next = msg;
        } else {
            msg.next = p;
            mMessages = msg;
        }
        return token;
    }
}

这个方法会构造一个没有target的msg传入到mq中，并且获取一个token

由于塞入一个同步屏障且没有异步msg在mq中的话，主线程会一直休眠，直到处理一个异步mq。因此不需要的时候记得务必要remove

同步屏障会导致idlehandler无法被调用

当mMessages是同步屏障，且后续没有异步消息，那么获取异步消息和获取同步消息这两步都会失败了，即nextPollTimeoutMillis会被赋值为-1，表示无限制的休眠

pendingIdleHandlerCount 默认是-1，所以会尝试着赋值。其中，由于同步屏障的存在，所以mMessages肯定不为空，一旦屏障的时间比现在要早，那么就不会进行赋值，之后也就不会走idle的实例运行。

判断mq是否是idle状态

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public boolean isIdle() {
    synchronized (this) {
        final long now = SystemClock.uptimeMillis();
        return mMessages == null || now < mMessages.when;
    }
}

就是当前信息是空且是未来才会发生，这时候就是idle状态

enqueueMessage

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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) {
        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;
        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;
                }
            }
            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;
}

每次都会按照时间进行重排序

block有两种情况，一种是当前mq就是空或者老的msg的whn是在过去，这时候需要塞入尾部或者说重创一个头，然后如果mq是阻塞状态，就需要唤醒

第二种是塞入的是异步消息并且mq的消息头是同步屏障，且mq的队列内部没有异步消息，这时候也需要一次唤醒