Handler机制原理图
常见问题：
1.一个线程可以有多少个Handler？
答：可以有无数个，因为我们可以无限的去创建Handler
2.一个线程有几个Looper？如何保证？
答：一个线程只能有一个Looper，例如主线程在ThreadActivity的main函数中，调用 Looper.prepareMainLooper()，初始化Looper，将ThreadLocal对象和Looper存入ThreadLocalMap中，确保每个线程只有一个looper对象
//Looper 的prepare函数
private static void prepare(boolean quitAllowed) {
// 当同一个线程多次调用prepare方法时，就会抛出异常
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
// 将Looper对象存入ThreadLocalMap中
sThreadLocal.set(new Looper(quitAllowed));
}
3.Handler作为内部类使用会内存泄漏？为什么其他内部类不会？
内部类持有外部类的引用，handler.sendMessage(msg)-->enqueueMessage()，假如某一时刻外部类即将被销毁，而handler还没有处理完消息，msg持有handler，handler持有外部类，导致外部类无法被回收，进而导致内存泄漏；其他内部类不会是因为其他内部类没有延时操作。
// Handler的方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { // 将msg和Handler绑定，以便消息处理时候，可以找到正确的handler
msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); }
// 将msg加入到消息队列 return queue.enqueueMessage(msg, uptimeMillis); }
4.为何主线程可以new Handler？如何在子线程中new Handler？
答：主线程中可以直接new是因为主线程在ThreadActivity的main函数中已经做了Looper初始化，并调用了loop()方法，可以使消息处理机制运行起来；倘若想要在子线程中new Handler，需要自己调用Looper的prepare()和Loop()方法，如果还想退出，可以调用Looper的quit()函数。
5.子线程中维护L
6.既然可以存在多个handler，往MessageQueue中添加数据发消息时怎么保证消息不混乱？
答：往消息队列中添加消息时，enqueueMessage()使用同步代码块，将添加操作锁住
7.我们在使用Message的时候怎么创建它；
答：使用静态方法obtain(),获取消息，消息队列默认的长度是50，可以回收利用
8.使用handler的postDelay后，消息队列会有什么变化？
答：如果没有消息会计算nextPollTimeoutMillis，进入nativePollOnce休眠，等待时间，然后取出消息，回调handleMessage。
9.Looper死循环为什么不会导致应用卡死？
答：每个app都有一个自己的虚拟机，点击图标流程：launcher-->application-->zygote-->虚拟机-->ActivityThread，执行Looper的loop方法，会有一个死循环，会调用queue.next();调用next(), nativePollOnce(ptr, nextPollTimeoutMillis); 如果没有消息处理，会使线程休眠，不至于一直循环，等有消息时再去唤醒，使loop方法继续执行。
for (;;) {
// 这行代码会使线程处于休眠状态
Message msg = queue.next(); // might block
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 {
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);
}
msg.recycleUnchecked();
}
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;
}
// 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;
}
}