等到flag = true [英] Wait until flag=true

问题描述

我有这样的javascript函数：

I have javascript function like this:

function myFunction(number) {

    var x=number;
    ...
    ... more initializations
    //here need to wait until flag==true
    while(flag==false)
    {}

    ...
    ... do something

}

问题是javascript一直停留在我的程序中。所以我的问题是如何在函数中间等待，直到flag为true而没有busy-wait？

The problem is that the javascript is stuck in the while and stuck my program. so my question is how can I wait in the middle of the function until flag is true without "busy-wait"?

推荐答案

因为浏览器中的javascript是单线程的（除了这里没有涉及的webworkers）并且javascript执行的一个线程在另一个可以运行之前运行完成，所以你的语句：

Because javascript in a browser is single threaded (except for webworkers which aren't involved here) and one thread of javascript execution runs to completion before another can run, your statement:

while(flag==false) {}

将简单永远运行（或直到浏览器抱怨无响应的javascript循环），页面将显示为挂起，没有其他javascript将有机会运行，因此标志的值永远不会被更改。

will simply run forever (or until the browser complains about a non-responsive javascript loop), the page will appear to be hung and no other javascript will ever get a chance to run, thus the flag's value can never be changed.

如需更多解释， Javascript是一种事件驱动语言 。这意味着它运行一段Javascript，直到它将控制权返回给解释器。然后，只有当它返回到解释器时，Javascript才会从事件队列中获取下一个事件并运行它。

For a little more explanation, Javascript is an event driven language. That means that it runs a piece of Javascript until it returns control back to the interpreter. Then, only when it returns back to the interpreter, Javascript gets the next event from the event queue and runs it.

所有事情，如计时器和网络事件都贯穿整个事件队列。因此，当计时器触发或网络请求到达时，它不会中断当前运行的Javascript。相反，一个事件被放入Javascript事件队列中，然后，当当前运行的Javascript完成时，下一个事件将从事件队列中拉出并轮到它运行。

All things like timers and network events run through the event queue. So, when a timer fires or a network request arrives, it does not ever "interrupt" the currently running Javascript. Instead, an event gets put in the Javascript event queue and then, when the currently running Javascript finishes, the next event is pulled from the event queue and it gets its turn to run.

因此，当您执行无限循环（例如）（flag == false）{} 时，当前运行的Javascript永远不会完成，因此下一个事件永远不会被拉从事件队列中，因此 flag 的值永远不会被更改。它们的关键是 Javascript不是中断驱动的 。当计时器触发时，它不会中断当前运行的Javascript，运行其他一些Javascript，然后让当前运行的Javascript继续。它只是放入事件队列中等待，直到当前正在运行的Javascript完成才能运行。

So, when you do an infinite loop such as while(flag==false) {}, the currently running Javascript never finishes and thus the next event is never pulled from the event queue and thus the value of flag never gets changed. They key here is that Javascript is not interrupt driven. When a timer fires, it does not interrupt the currently running Javascript, run some other Javascript and then let the currently running Javascript continue. It just gets put in the event queue waiting until the currently running Javascript is done to get its turn to run.

什么您需要做的是重新考虑代码的工作方式，并找到一种不同的方法来触发标志值更改时要运行的代码。 Javascript被设计为事件驱动的语言。所以，你需要做的是弄清楚你可以注册哪些事件感兴趣，这样你就可以监听可能导致标志改变的事件，你可以检查该事件的标志，或者你可以从任何代码都可能改变标志，或者你可以实现一个回调函数，无论代码改变哪个代码都可以调用你的回调，只要负责更改标志值的代码将其值改为 true ，它只调用回调函数，因此当标志设置为 true 时，想要运行的代码将在正确的时间运行。这比尝试使用某种计时器来持续检查标志值要高效得多。

What you need to do is rethink how your code works and find a different way to trigger whatever code you want to run when the flag value changes. Javascript is designed as an event-driven language. So, what you need to do is figure out what events you can register an interest in so you can either listen for the event that might cause the flag to change and you can examine the flag on that event or you can trigger your own event from whatever code might change the flag or you can implement a callback function that whatever code changes that flag can call your callback whenever the piece of code responsible for changing the flag value would change it's value to true, it just calls the callback function and thus your code that wants to run when the flag gets set to true will get to run at the right time. This is much, much more efficient than trying to use some sort of timer to constantly check the flag value.

function codeThatMightChangeFlag(callback) {
    // do a bunch of stuff
    if (condition happens to change flag value) {
        // call the callback to notify other code
        callback();
    }
}

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