1、简介：Handler机制主要为线程间通信而生，主要是为了解决子线程执行完耗时操作后，怎么回调到主（UI）线程的问题。
2、主要组成部分：Handler、Looper、Message和MessageQueue
Handler负责发送Message到MessageQueue
Looper负责从MessageQueue读取消息，并通过Message的target，调用handler的消息分发处理
Handler负责处理消息回调
3、Handler send 消息 和 post 有什么区别？
先看下 Handler post的源码：
public final boolean post(@NonNull Runnable r) {return sendMessageDelayed(getPostMessage(r), 0);
}public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {if (delayMillis < 0) {delayMillis = 0;}return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
我们从源码可以看到无论是 send方式 还是 post方式 最终都是调用 sendMessageAtTime 这个 方法，不同的是post方式，会通过getPostMessage(r)获得一个Message，代码如下：
private static Message getPostMessage(Runnable r) {Message m = Message.obtain();m.callback = r;return m;
}
这里会把Runnable对象赋值给Message 的 callback，我们再来看看Handler最终处理消息分发的方法：
/*** Handle system messages here.*/
public void dispatchMessage(@NonNull Message msg) {if (msg.callback != null) {handleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}handleMessage(msg);}
}
如果Message 中 callback 不为空，回调Message的callback；如果Handler对象中的mCallback不为空，回调这个mCallback的方法；只有前两个为空，才会回调handlerMessage方法。
第三种是我们最常用的方式，Handler无参构造，其中第二种是 Handler有参构造，源码如下：
public Handler() {this(null, false);
}public Handler(@Nullable 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;
}
总结：send 方式 和 post 方式 本质没有区别，都是发送Message，只不过在最后dispatchMessage分发的时候处理不一样。
4、子线程能直接新建Handler并使用吗？子线程能通过Handler直接向UI线程发消息的原因是什么？主线程怎么向子线程发消息？
（1）、子线程能并不能直接新建Handler并使用，原因可以看Handler的源码：
public Handler(@Nullable 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;
}
在构造Handler方法中 Handler会持有该线程的Looper 和 Looper中的MessageQueue，Handler的使用必须是结合Looper，而子线程中并没有绑定Looper，所以会报错。
（2）、为什么主线程确不用创建Looper，直接使用Handler呢？我们可以从源码角度来看，实际上主线程创建的时候程序已经自动帮我们创建Looper了
应用的启动 离不开 ActivityThread、ActivityManageService 两个进程，我们来看程序入口main方法：
public static void main(String[] args) {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");// Install selective syscall interceptionAndroidOs.install();// 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();// Make sure TrustedCertificateStore looks in the right place for CA certificatesfinal File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());TrustedCertificateStore.setDefaultUserDirectory(configDir);Process.setArgV0("<pre-initialized>");Looper.prepareMainLooper();// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.// It will be in the format "seq=114"long startSeq = 0;if (args != null) {for (int i = args.length - 1; i >= 0; --i) {if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {startSeq = Long.parseLong(args[i].substring(PROC_START_SEQ_IDENT.length()));}}}ActivityThread thread = new ActivityThread();thread.attach(false, startSeq);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler();}if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");
}
我们可以看到Looper.prepareMainLooper(); 和 Looper.loop(); 这两个关键的方法，一个是创建Looper，一个是启动Looper循环。再跟进prepareMainLooper（） 这个方法：
public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}
再看看 prepare这个方法：
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));}
PS：ThreadLocal是为了提供线程内部的局部变量，方便在本线程内随时随地的读取，并且与其他线程隔离
主线程的prepare(false),所以是不可退出的Looper
每一个线程 只有一个Looper、一个Looper只有一个MessageQueue。
说明主线程的Looper是在程序入口处创建的，所以才不用自己取新建一个Looper
（3）、主线程要想向子线程发送消息，需要在子线程中调用Looper.prepare(true) 这个方法，并且之后再启动looper循环可以在run中写类似代码：
Looper.prepare();
handler=new Handler(){@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);//做事情,做完后要退出循环handler.getLooper().quitSafely();}
};
Looper.loop();
5、Handler同步屏障
（1）Message消息主要分为三种：同步消息、屏障消息、异步消息
（2）同步屏障就是通过屏障消息来屏蔽所有同步消息，处理后续异步消息
（3）核心代码我们可以先看看 MessageQueue 取消息的代码：
@UnsupportedAppUsageMessage 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 iterationint nextPollTimeoutMillis = 0;for (;;) {if (nextPollTimeoutMillis != 0) {Binder.flushPendingCommands();}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) {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.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 handlerboolean 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;}}
最核心的是的判断是这段代码
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());}
屏障消息主要是起一个屏蔽作用，所以不需要有handler 处理消息分发，所以target为null。
如果有屏障消息，且消息不是异步的则继续取下一个，知道遇到异步消息。
（4）异步消息如何设置：
可以直接调用Message的 方法
public void setAsynchronous(boolean async) {if (async) {flags |= FLAG_ASYNCHRONOUS;} else {flags &= ~FLAG_ASYNCHRONOUS;}}
然后Handler构造方法的时候发现也可以设置 Handler的mAsynchronous，但是hide方法，我们可以通过反射去调用，也是可以的。
再来看看 Handler发送消息，插入MessageQueue队列的代码：
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);}
（5）如何设置同步屏障
屏障消息和一般的消息区别就是没有 target，可以看下MessageQueue的源码
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.nextmsg.next = p;prev.next = msg;} else {msg.next = p;mMessages = msg;}return token;}}
按照时间顺序插入到队列里面
（6）同步屏障的应用
举例：屏幕刷新机制
Android 每隔16.6ms会刷新一次屏幕，每个Activity对应一个 DecorView 根布局View树，初始化过程中DecorView会被添加到
viewRootImp setView的过程：
viewRootImp.setView() —> viewRootImp.requestLayout() —>
viewRootImp.scheduleTraversals() —> viewRootImp.doTraversal() —> viewRootImp.performTraversals()—>
这里我们主要看scheduleTraversals这个方法：
@UnsupportedAppUsagevoid scheduleTraversals() {if (!mTraversalScheduled) {mTraversalScheduled = true;mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);if (!mUnbufferedInputDispatch) {scheduleConsumeBatchedInput();}notifyRendererOfFramePending();pokeDrawLockIfNeeded();}}
其中 mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier(); 这行代码就是设置同步屏障
mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null); 这行代码跟踪
@TestApipublic void postCallback(int callbackType, Runnable action, Object token) {postCallbackDelayed(callbackType, action, token, 0);}@TestApipublic void postCallbackDelayed(int callbackType,Runnable action, Object token, long delayMillis) {if (action == null) {throw new IllegalArgumentException("action must not be null");}if (callbackType < 0 || callbackType > CALLBACK_LAST) {throw new IllegalArgumentException("callbackType is invalid");}postCallbackDelayedInternal(callbackType, action, token, delayMillis);}
private void postCallbackDelayedInternal(int callbackType,Object action, Object token, long delayMillis) {if (DEBUG_FRAMES) {Log.d(TAG, "PostCallback: type=" + callbackType+ ", action=" + action + ", token=" + token+ ", delayMillis=" + delayMillis);}synchronized (mLock) {final long now = SystemClock.uptimeMillis();final long dueTime = now + delayMillis;mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);if (dueTime <= now) {scheduleFrameLocked(now);} else {Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);msg.arg1 = callbackType;msg.setAsynchronous(true);mHandler.sendMessageAtTime(msg, dueTime);}}}
看到最后 发现发送了一个异步消息
由于同步屏障的做 异步消息优先于同步消息，异步消息先执行，保证了屏幕刷新的及时性和优先级。
6、MessageQueue相关操作，插入和取出
MessageQueue的底层数据结构是单向链表
（1）插入操作
通过源码可以看到添加消息到队列是根据Message中的when来保证顺序性的，这个when的值是由SystemClock.uptimeMilis()+delayMillis 决定的，这个uptimeMilis是相对于系统启动的相对时间，为什么不用SystemClock. currentTimeMillis()，因为这个时间可以通过调整系统时间进行修改，可能会不准确。
可以看到Handler发送消息源码：
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);}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);}
然后我们再来看看MessageQueue中 enqueueMessage方法，是怎么插入队列的
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.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;}
可以看出 如果 if(p==null || when==0|| when<p.when)， 代表此队列是空队列，就是第一次，或者消息不是延时消息，需要立即执行，其中needWake= mBlocked，下面判断是否线程需要唤醒，有可能线程是阻塞状态，然后需要唤醒。
否则代表此消息是延时消息，然后按照延时顺序插入
（2）取出操作
看看Looper 中 loop 循环操作源码：
/*** Run the message queue in this thread. Be sure to call* {@link #quit()} to end the loop.*/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 (;;) {Message msg = queue.next(); // might blockif (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 loggerfinal 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 {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();}}
MessageQueue的 next方法就是取出操作
再来看看 next对应源码：
@UnsupportedAppUsageMessage 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) {/**代表队列为空，一半是Looper调用了 quit方法，可以看到mPtr的定义 used by native code,代表被 nativie层使用，关联的是C++层的MessageQueue，其中阻塞操作就是C++层的MessageQueue来操作的
*/return null;}int pendingIdleHandlerCount = -1; // -1 only during first iterationint nextPollTimeoutMillis = 0;for (;;) {if (nextPollTimeoutMillis != 0) {Binder.flushPendingCommands();}/**阻塞方法，阻塞方法，通过native层的epoll监听文件描述符的写入事件来实现的。nextPollTimeoutMillis = -1，代表一直阻塞，直到被唤醒，0代表不会阻塞，立即返回，>0代表阻塞时间不超过nextPollTimeoutMillis ，期间有程序唤醒立即返回
*/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;//同步屏障相关代码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) {if (now < msg.when) {/** 如果消息此刻还没有到时间，设置阻塞时间nextPollTimeoutMillis，进入下次循环的时候会调用nativePollOnce(ptr, nextPollTimeoutMillis)进行阻塞*/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// 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 {// 没有消息，会一直阻塞，直到被唤醒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.
//下面是IdleHandler的处理监听 ,IdleHandler默认数量为-1，只有当 消息为空，或者当前消息还没有到//时间才会赋值IdleHandlerCountif (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);}//这里会调用idlerd的 queueIdle方法，会判断是否移除该空闲监听// 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 handlerboolean 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;}}
上面代码我做了详细注释，这里我特别讲下这个IdleHandle 的使用场景
比如在framework层中，需要系统GC回收 就用到了这个监听，只有在cpu空闲的时候才执行GC操作，
void scheduleGcIdler() {if (!mGcIdlerScheduled) {mGcIdlerScheduled = true;Looper.myQueue().addIdleHandler(mGcIdler);}mH.removeMessages(H.GC_WHEN_IDLE);
}final class GcIdler implements MessageQueue.IdleHandler {@Overridepublic final boolean queueIdle() {doGcIfNeeded();return false;}
}
比如需要懒加载，例子：测量某控件的高宽，等到UI线程消息执行完再来执行相关代码：
override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)Looper.myQueue().addIdleHandler {//todofalse}}
(3) 为什么loop不会阻塞线程
Android是基于事件驱动的，即所有Activity的生命周期都是通过Handler事件驱动的。loop方法中会调用MessageQueue的next方法获取下一个message，当没有消息时，基于Linux pipe/epoll机制会阻塞在loop的queue.next()中的nativePollOnce()方法里，并不会消耗CPU。
我的理解：
其实这里可以从循环外面的代码来说确实卡住了，但对于循环里面的代码并没有卡住。
while(true){
这里没卡住
}
这里卡住了