模拟同步 XmlHttpRequest

发布于 2024-08-12 06:27:22 字数 546 浏览 6 评论 0原文

我读过一些其他相关问题（包装异步 JavaScript 函数以使其同步的模式 &进行异步JavaScript 中的事件同步可能还有更多），但我只是想确保穷尽所有可能性。

是否可以使用 setInterval 或 setTimeout 将异步 XmlHttpRequest“转换”为准同步请求？

这个想法是，Ajax 请求成功后，将设置一个变量，该变量将作为 while 循环（已调用 setInterval 或 setTimeout，以及适当的回调函数）退出的信号。或者我从根本上误解了 setInterval 和/或 setTimeout 的能力（或限制？）？

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蘑菇王子 2024-08-19 06:27:22

确实，您不想按照您所描述的方式使用 setInterval 和 setTimeout 。您真正想做的只是熟悉嵌套函数，您可以在其中以或多或少同步的方式进行编写。

例如：

XHR.get(your_data, function()
   {
      //what you would have done "synchronously"
   });

虽然您可以使用 setInterval 和/或 setTimeout（在函数体中再次调用 setTimeout）来“轮询”成功代码，但这种方法明显不如首先处理回调而不是轮询它。它会带来延迟，占用 CPU 资源，并且无法跨多个 XHR 请求进行扩展，等等。

XHR 将在完成时调用您的函数，运行一个函数询问“我们完成了吗？我们完成了吗？”是没有意义的。同时。另一方面，如果您希望阻止某些周期性行为，直到响应返回（例如，一段需要该数据运行的动画），则完全可以接受将阻止代码包含在 if 语句中：

var tick = window.setTimeout(tock, 20);
var tock = function()
{
   if (response_done)
   {
      // dependent code
   }
   tick = window.setTimeout(tock, 20);
}
XHR.get(your_data, function() { /*Handle response*/ response_done = true; });

It's true, you don't want to use setInterval and setTimeout in the way you've described. What you really want to do is just get comfortable with nested functions, where you can write in a more or less synchronous way.

For example:

XHR.get(your_data, function()
   {
      //what you would have done "synchronously"
   });

While you can use setInterval and/or setTimeout (with calls to setTimeout again in the function body) to "poll" for a success code, that approach is dramatically inferior to just handling the callback in the first place instead of polling for it. It introduces latency, runs away with the CPU, and doesn't scale across multiple XHR requests, to name a few shortcomings.

XHR will call your function when it completes, it makes little sense to run a function asking "Are we done yet? Are we done yet?" in the meantime. On the other hand if there is some periodic behavior you are wanting to BLOCK until the response comes back (a piece of animation that needs that data to run, for example) it is totally acceptable to surround the blocking code in an if statement:

var tick = window.setTimeout(tock, 20);
var tock = function()
{
   if (response_done)
   {
      // dependent code
   }
   tick = window.setTimeout(tock, 20);
}
XHR.get(your_data, function() { /*Handle response*/ response_done = true; });

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ζ澈沫 2024-08-19 06:27:22

如果我正确回答了您的问题，您可以使用以下代码实现类似的效果：

var computationInterval =
    window.setInterval(function() {
        // do a computation cycle, as if you're in the body of a while loop
    }, 100);

$.get('http://example.com/areWeThereYet', function() {
    // break the intervals (the pseudo-cycle)
    window.clearInterval(computationInterval);
});

If I'm getting your question right, you could achieve a similar effect using the following code:

var computationInterval =
    window.setInterval(function() {
        // do a computation cycle, as if you're in the body of a while loop
    }, 100);

$.get('http://example.com/areWeThereYet', function() {
    // break the intervals (the pseudo-cycle)
    window.clearInterval(computationInterval);
});

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