2015-01-18 12:00
在android中做延时处理一般用handler.postDelayed()和view.postDelayed(action,delay)来实现，view.postDelayed也是通过handlder.postDelayed来实现的，不过有一些特殊处理的地方。
handler.postDelayed
handler处理延时逻辑是通过发送延时消息来处理的
//source
public final boolean postDelayed(Runnable r, long delayMillis){
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
sendMessageDelayed:
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
sendMessageAtTime:
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);
}
将延时消息放入到消息队列中。SystemClock.uptimeMillis()是开机到现在的时间（毫秒），不包括睡眠的时间。不用System.currentTimeMillis()的原因是System.currentTimeMillis()的时间是可以被System.setCurrentTimeMillis修改的，如果被修改了则发生难以想象的后果。
enqueueMessage调用MessageQueue.enqueueMessage(msg,uptimeMillis)
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;
}
此处会将传入的消息对象根据触发时间（when）插入到message queue中。然后判断是否要唤醒等待中的队列。
. 如果插在队列中间。说明该消息不需要马上处理，不需要由这个消息来唤醒队列。
. 如果插在队列头部（或者when=0），则表明要马上处理这个消息。如果当前队列正在堵塞，则需要唤醒它进行处理。
通过nativeWake方法唤醒队列。
looper执行MessageQueue中的消息：
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
try {
msg.target.dispatchMessage(msg);
} finally {
...
}
msg.recycleUnchecked();
}
}
只是调用了MessageQueue.next()方法。可能会阻塞。
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;
}
}
该方法会先调用nativePollOnce阻塞，然后进入死循环。nativePollOnce()的作用类似与object.wait()，使用了native方法来对线程进行精确时间的唤醒，
如果head Message是有延迟而且延迟时间没到的（now < msg.when），计算下时间间隔（nextPollTimeoutMillis），设置timeout为两者之差，进入下一次循环。
如果Message无延时或已到达执行时间，则直接返回该Message.
如果当前的Message阻塞住了MessageQueue（pendingIdleHandlerCount <= 0），把全局变量mBlocked改为true，在下一个Message放入队时会判断这个message的位置。如果在queue的头部，则用nativeWake唤醒线程。
总结：[msg loop] -->调用MessageQueue.next(),若头部的消息需要被执行在返回该消息，send该消息之后继续调用next方法获取下一个消息，若头部的不需要被执行则阻塞住队列，若有等待执行的Message,计算一下剩余时间,继续调用nativePollOnce()阻塞，无限循环到阻塞时间到或者下一次有Message进队。下个消息进队时若不需要延时在放在Queue的头部，否则在放在Queue的中部。
view.postDelayed
view的延时也是调用了handler的postDelayed.
public boolean postDelayed(Runnable action, long delayMillis) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.postDelayed(action, delayMillis);
}
// Postpone the runnable until we know on which thread it needs to run.
// Assume that the runnable will be successfully placed after attach.
getRunQueue().postDelayed(action, delayMillis);
return true;
}
若view的AttachInfo不为空，则调用AttachInfo中的handler的postDelayed。若AttachInfo为空，则先将action放入RunQueue中。RunQueue为HandlerActionQueue，用来存放view没有handler时的action。
执行action：
public void executeActions(Handler handler) {
synchronized (this) {
final HandlerAction[] actions = mActions;
for (int i = 0, count = mCount; i < count; i++) {
final HandlerAction handlerAction = actions[i];
handler.postDelayed(handlerAction.action, handlerAction.delay);
}
mActions = null;
mCount = 0;
}
}
还是需要传入一个Handler，利用handler来执行action。当view被关联到window时，会执行该队列中的Action.
AttachInfo是View的一个附加信息存储类，当view被关联到window时会被赋值。
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