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) {}
// enqueue
public 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;
}
}