【转】从源码分析Handler的postDelayed为什么可以延时?

时间:2023-03-08 17:08:08

版权声明:本文为博主原创文章,未经博主允许不得转载。 https://blog.****.net/aliankg/article/details/70842494
Thread/Hander/Looper是Android在Java线程基础之上提供的线程通信/消息处理机制,这个众所周知,不再细说。Handler提供了两个发送延迟处理任务的api:

/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis) /**
* Causes the Runnable r to be added to the message queue, to be run
* after the specified amount of time elapses.
* The runnable will be run on the thread to which this handler
* is attached.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
*
* @param r The Runnable that will be executed.
* @param delayMillis The delay (in milliseconds) until the Runnable
* will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the Runnable will be processed --
* if the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean postDelayed(Runnable r, long delayMillis)

问题在于,这两个delay的精度到底能有多大?如何理解?很多APP的定时处理机制都是使用这两个api递归抛延迟任务来实现的。所以有必要研究一下框架层的实现,心中有数。Android这套消息循环机制工作在最上层,距离Linux kernel的时间管理甚远。本文仍然采用跟踪分析代码的方式,基于android7.1.1。

postDelayed()实际上封装了sendMessageDelayed(),第一时间便殊途同归:

    public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
} public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

postDelayed()首先通过getPostMessage()将传入的Runnable对象封装成一个Message,调用sendMessageDelayed(),而sendMessageDelayed()增加了一个delay时间参数的健壮性检查,然后转化成绝对时间,调用sendMessageAtTime()。至此,再多说一句:最简单的sendMessage()和post()实际上也是sendMessageDelayed(0)的封装。所以,Handler五花八门的post/send api们本质上无差别。只是为了让使用者在简单的情况下避免手动封装Message,只需提供一个Runnable即可。Handler调用关系整理如下:

post()/postDelayed()/sendMessage()->sendMessageDelayed()->sendMessageAtTime()->enqueueMessage()
postAtTime()->sendMessageAtTime()->enqueueMessage()
postAtFrontOfQueue()->sendMessageAtFrontOfQueue()->enqueueMessage()

最后都以enqueueMessage()告终

enqueueMessage()->MessageQueue.enqueueMessage(Message msg, long when)

如前所述,这时候when已经转化成绝对系统时间。转入消息队列类MessageQueue看一下enqueueMessage()这个方法:

    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;
}

这个方法比较简单,采用线程安全的方式将Message插入到消息队列中,插入的新消息有三种可能成为消息队列的head:

(1)消息队列为空;

(2)参数when为0,因为此时when已经转成绝对时间,所以只有AtFrontOfQueue系列的API才会满足这个条件;

(3)当前的head Message执行时间在when之后,即消息队列中无需要在此Message之前执行的Message。

接下来就要看看消息循环(Looper)如何使用when,这是本文问题的关键。关键的方法,Looper.loop(),启动线程消息循环:

    /**
* 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(); for (;;) {
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;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
} 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();
}
}

从for(;;)可以看到一次循环开始于从消息队列中去取一个消息,MessageQueue.next(),如果next()返回null,则loop()会返回,本次消息循环结束。取出消息之后,通过Handler.dispatchMessage()处理消息:

msg.target.dispatchMessage(msg);

也就是说,取下一个消息的实际执行时间取决于上一个消息什么时候处理完。再看MessageQueue.next()做了什么:

    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;
}
}

看到next()实际上也有一个for(;;),而出口只有两个:消息队列已经退出,返回null;找到了一个合适的消息,将其返回。如果没有合适的消息,或者消息队列为空,会block或者由IdleHandler处理,不在本文问题范畴,暂不展开。主要看找到合适的消息的逻辑:

                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;
}

可以看到,如果在消息队列中顺序找到了一个消息msg(前文分析过,消息队列的插入是由when顺序排列,所以如果当前的消息没有到执行时间,其后的也一定不会到),当前的系统时间小于msg.when,那么会计算一个timeout,以便在到执行时间时wake up;如果当前系统时间大于或等于msg.when,那么会返回msg给Looper.loop()。所以这个逻辑只能保证在when之前消息不被处理,不能够保证一定在when时被处理。很好理解:
(1)在Loop.loop()中是顺序处理消息,如果前一个消息处理耗时较长,完成之后已经超过了when,消息不可能在when时间点被处理。

(2)即使when的时间点没有被处理其他消息所占用,线程也有可能被调度失去cpu时间片。

(3)在等待时间点when的过程中有可能入队处理时间更早的消息,会被优先处理,又增加了(1)的可能性。

所以由上述三点可知,Handler提供的指定处理时间的api诸如postDelayed()/postAtTime()/sendMessageDelayed()/sendMessageAtTime(),只能保证在指定时间之前不被执行,不能保证在指定时间点被执行。

from:https://blog.****.net/zhanglianyu00/article/details/70842494