简单讲，handler就是两个功能
插入消息，enqueuemessage，msg，when
从消息队列中遍历所有消息，比对msg.when和当前的when，找到合适的位置插入
处理消息，looper.loop会从messagequeue中调用next。取消息，如果消息还没到时间该执行，就会比对时间，下次轮询就通过binder写入，native函数休眠，到时间唤醒执行。
handler内存泄漏
GCRoot 一般是静态变量或者常量可以作为GCROOT
GCROOT 是ThreadLocal，存在于Looper中，Looper被加载就存在，
handler持有activity或者fragment，handler又被message持有，message的target属性，message被messagequeue持有，messagequeue被looper中的threadlocal持有
java中匿名内部类会默认持有外部类的引用
打断持有链
- handler.removemesage handler.removecallbacks
- handler使用static修饰。
主线程的Looper不允许退出
处理消息，looper取出来后，调用message.tager.dispatchemesage后面调用了handler的handlemessage方法。
还有个callback对象，如果有callback，dispatch会先执行callback的处理，calllback返回true，后面就不处理了，callback返回false就给handler的handlemessage处理了
Meesage对象创建
message创建用的obtain，池化，频繁的创建销毁会导致内存不稳定，抖动，造成卡顿 oom等问题
message pool的最大缓存50
阻塞和休眠,阻塞是被动的，休眠是主动的，阻塞不会让出cpu，休眠会，thread.yield会让出cpu。
子线程主线程通信
handler，livedata，eventbus，flow，rxjava，broadcast，观察者模式不能跨线程
最终都是handler完成的。
Handler监听卡顿
原理是在looper内完成的，looper处理消息的时候，会打印内容，就是Printer，looper可以设置它。
Message消息的分类
同步消息，普通的消息都是同步消息
异步消息，创建handler的时候设置async为true即可
同步屏障，需要通过反射调用，app层无法直接调用，是messagequeue提供的posSyncBarrier方法实现的，返回一个token，时msg的arg1值，用它来取消同步屏障。和普通消息的区别是，msg。target属性为null。
刷新UI的消息是异步消息，发送前先插入了一个同步屏障消息，异步消息处理完成后，要将同步屏障消息移除队列
消息入队
handler消息加入队列，有一系列方法，如下：
// 发送空消息public final boolean sendEmptyMessage(int what){}public final boolean sendEmptyMessageDelayed(int what,long delay){}public final boolean sendEmptyMessageAtTime(int what,long when){}// 发送消息public final boolean sendMessage(@NonNull Message msg){}public final boolean sendMessageDelayed(@NonNull Message msg,long time){}public final boolean sendMessageAtTime(@NonNull Message msg,long when){}public final boolean sendMessageAtFrontOfQueue(Message msg) {}// post发送public final boolean post(@NonNull Runnable r) {}public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {}public final boolean postAtTime( @NonNull Runnable r, @Nullable Object token, long uptimeMillis) {}public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {}public final boolean postDelayed(Runnable r, int what, long delayMillis) {}public final boolean postDelayed( @NonNull Runnable r, @Nullable Object token, long delayMillis) {}public final boolean postAtFrontOfQueue(@NonNull Runnable r) {}// enqueuepublic final boolean executeOrSendMessage(@NonNull Message msg) {}private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg, long uptimeMillis) {}最终都是掉用的enqueueMessage加入队列
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg, long uptimeMillis) { msg.target = this; // 绑定消息处理对象 msg.workSourceUid = ThreadLocalWorkSource.getUid(); if (mAsynchronous) { // 根据handler创建是否是异步的，来将消息标记为异步消息 msg.setAsynchronous(true); } // 调用messagequeue的方法，加入队列。 return queue.enqueueMessage(msg, uptimeMillis); }下面看下消息如何入队的
boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } synchronized (this) { if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); } 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; // 当前队列的头 Message p = mMessages; boolean needWake; if (p == null || when == 0 || when < p.when) { // New head, wake up the event queue if blocked. // 消息队列为null，或者消息是立刻执行的，或者要执行的时间先于头消息的时间，将当前消息作为新的队列的头 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(); // 遍历队列，找到合适的位置，将当前消息插入进去 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; } // 如果需要的话，唤醒队列，开始处理消息，就是MessageQueue的next方法开始执行 // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true; }取消息以及消息处理取消息1
取消息入口是Looper的loop方法处理的
public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } if (me.mInLoop) { Slog.w(TAG, "Loop again would have the queued messages be executed" + " before this one completed."); } me.mInLoop = true; // 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); me.mSlowDeliveryDetected = false; // 无限循环，通过loopOnce取消息 for (;;) { if (!loopOnce(me, ident, thresholdOverride)) { return; } } }处理消息
private static boolean loopOnce(final Looper me, final long ident, final int thresholdOverride) { // 取消息 Message msg = me.mQueue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return false; } // 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 { // 这里取出msg的target属性，就是handler对象，进行消息的处理。注意：屏障消息是没有target的。 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 (me.mSlowDeliveryDetected) { if ((dispatchStart - msg.when) <= 10) { Slog.w(TAG, "Drained"); me.mSlowDeliveryDetected = false; } } else { if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery", msg)) { // Once we write a slow delivery log, suppress until the queue drains. me.mSlowDeliveryDetected = true; } } } // 这里会打印那些执行慢的消息 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(); return true; }取消息2
再看下实际的取消息的方法，MessageQueue的next方法
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(); } // 这个方法会在没有消息的时候阻塞 nativePollOnce(ptr, nextPollTimeoutMillis); synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; // 消息头保存 Message msg = mMessages; if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. // 这里处理同步屏障消息，如果当前消息是异步消息，就跳出循环，否则继续循环 do { prevMsg = msg; msg = msg.next; } while (msg != null && !msg.isAsynchronous()); // 循环的作用，找出队列里的异步消息，存储在msg里，或者将同步屏障消息存储在msg中，prevMsg存储的是同步消息 } if (msg != null) { if (now < msg.when) { // 下次的唤醒时间 // 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(); // 将取出的消息交给handler处理。 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; } }handler的消息处理
是从dispatchMessage方法开始的，里面进行消息处理
public void dispatchMessage(@NonNull Message msg) {// 检查message是否设置了callback对象（runnable类型的）if (msg.callback != null) {// 执行其run方法handleCallback(msg);} else {// 检查handler是否设置了callback对象if (mCallback != null) {if (mCallback.handleMessage(msg)) {// 如果返回true，后续就不执行了。return;}}// 自定义handler的时候，实现该方法处理消息handleMessage(msg);}}IdleHandler是什么？干什么？继续看MessageQueue类。
IdleHandler是MessageQueue的静态内部接口。如下
public static interface IdleHandler { /** * Called when the message queue has run out of messages and will now * wait for more. Return true to keep your idle handler active, false * to have it removed. This may be called if there are still messages * pending in the queue, but they are all scheduled to be dispatched * after the current time. */ boolean queueIdle(); }当前线程的消息对立内当前没有消息要处理时，会取出idlehander执行任务，因为执行在主线程，禁止执行耗时操作。返回true表示执行完并不会移除该对象，false执行完一次就移除。
而且，执行时机是不确定的。
执行的地方在next方法内部。
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(); } 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; } // 执行到这里，说明没有找到message对象需要执行了，且线程没有退出。 // 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; } // 取出idlehandlers 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 { // 执行idlehandler的方法 keep = idler.queueIdle(); } catch (Throwable t) { Log.wtf(TAG, "IdleHandler threw exception", t); } if (!keep) { // 需要移除的话，在此处进行移除 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; } }