delphi计时器的滴答声比计时器服务中断例程更快 [英] delphi timer ticks faster than timer service interrupt routine
问题描述
您好，我被要求为某人维护一个基于Delphi 5的程序，该程序正在使用计时器对象每隔50毫秒进行一次滴答，并且在每次启动时都会运行单线程代码块。我只是想知道，如果执行该代码块的时间长于计时器滴答间隔，会发生什么情况？例如，是否会引起访问冲突之类的问题？默认情况下，Delphi如何处理这种情况？非常感谢。
Hi I have been asked to maintain a Delphi 5 based program for someone, and the program is using a timer object to tick every 50 milli-seconds and upon each time-up it runs the block of single threaded code. I am just wondering, what would happen if the time taken to execute this block of code is longer than the timer tick interval, would this be bad? For example could it cause problems like access violation? How does Delphi handle this kind of situation by default? Thanks a lot.
推荐答案
计时器提示不会中断您的代码。
Ticks of the timer do not interrupt your code.
计时器刻度线以窗口消息的形式传递。窗口消息只能在您检查消息队列中是否有新消息时到达。当您的计时器事件处理程序返回并且程序恢复其事件循环时，这会自动发生，但是您可以通过调用 Application.ProcessMessages 显式触发它。不过不要这样说；
Timer ticks are delivered in the form of window messages. Window messages can only arrive when you check the message queue for new messages. That happens automatically when your timer event handler returns and your program resumes its event loop, but you can trigger it explicitly by calling Application.ProcessMessages. Don't call that, though; it seldom solves problems in the long run.
如果不检查计时器刻度处理程序中的消息队列，则处理程序将永远不会开始运行
If you don't check the message queue in your timer-tick handler, then your handler will never start running a second time while it's still handling a previous tick.
即使您 do 检查队列，所有发生的事情是滴答处理程序将被称为递归。毕竟，它们都在单个线程中运行。但是，递归计时器处理可能不是您想要的，因此，我再次建议反对在消息处理程序中检查消息。
Even if you do check the queue, all that happens is that the tick handler will be called recursively. After all, it's all running in a single thread. Recursive timer handling probably isn't what you want to happen, though, so I'll again counsel against checking for messages in a message handler.
此外，如果您的计时器处理程序需要很长时间才能运行，则计时器消息永远不会堆积。定时消息是伪造的，因为它们实际上不会定期地添加到消息队列中。相反，在您的程序检查队列中是否有更多消息时，操作系统会合成计时器消息。如果队列中没有更高优先级的消息，并且计时器间隔已经过去，则操作系统将返回 wm_Timer 消息。如果您不检查更多消息，则队列中将没有计时器消息。特别是，队列中不会有多个计时器消息。
Furthermore, timer messages can never "pile up" if your timer handler takes a long time to run. Timer messages are "fake" in that they don't actually get added to the message queue at regular intervals. Instead, the OS will synthesize a timer message at the time your program checks the queue for more messages. If there are no higher-priority messages in the queue, and the timer interval has elapsed, then the OS will return a wm_Timer message. If you don't check for more messages, then there will be no timer message in the queue. In particular, there will not be multiple timer messages in the queue.
进一步阅读：
- Stack Overflow: How does the message queue work in Win32?
- Dr. Dobbs: Inside the Windows Messaging System
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