为什么Promise对象会阻止渲染？ [英] Why does the Promise object block rendering?
问题描述
我正在测试Promise对象，并编写了一些代码来模拟长时间运行的同步任务。我正在比较Promise和setTimeout-请参见小提琴：
I was testing the Promise object and wrote some code that simulates a long running task that is synchronous. I was comparing Promise and setTimeout - see fiddle:
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="style.css">
</head>
<body>
<h2>Promise vs setTimeout</h2>
<div><button id="settimeout-test">setTimeout with slow running function</button></div>
<div><button id="promise-test">Promise and slow running function</button></div>
<div><button id="clear">Clear Results</button></div>
<h5>Results</h5>
<div id="result"></div>
<script>
const slow = function() {
let nu = Date.now();
while (Date.now() - nu < 1000) {}
}
const getSlowPromise = () => new Promise(resolve => {
slow();
resolve();
});
const resultsElement = document.getElementById('result')
const log = (message) => {
resultsElement.innerText += message;
}
const settimeoutButton = document.getElementById('settimeout-test');
settimeoutButton.addEventListener('click', () => {
const now = Date.now();
log(`\nsetTimeout test starts after ${Date.now() - now} ms`);
setTimeout(() => {
slow();
log(`\nSlow function completes after ${Date.now() - now} ms`);
}, 0);
log(`\nEvent listener completes after ${Date.now() - now} ms`);
});
const promiseButton = document.getElementById('promise-test');
promiseButton.addEventListener('click', () => {
const now = Date.now();
log(`\nsetTimeout test starts after ${Date.now() - now} ms`);
getSlowPromise().then(res => log(`\nPromise completes after ${Date.now() - now} ms`));
log(`\nevent listener completes after ${Date.now() - now} ms`);
})
const clear = () => resultsElement.innerText = '';
const clearButton = document.getElementById('clear');
clearButton.addEventListener('click', () => clear());
</script>
</body>
</html>
我认为Promise和setTimeout的行为类似，将代码添加到任务队列中，然后继续执行。结果的顺序是相同的，但是长时间运行的任务的承诺似乎会阻止渲染，直到长时间运行的任务完成。有人可以解释吗？
I thought Promise and setTimeout would behave in a similar way, add the code to the task queue and then continue the execution. The order of the results are the same but the promise with a long running task seems to block the rendering until the long running task completes. Can someone explain this?
该示例在Chrome中运行得最好。
The example runs best in Chrome.
更新：
我不是要使长时间运行的任务并行运行，我只是想了解为什么Promise和setTimeout在我的示例中表现不同。但是，如果您确实想并行运行任务，那么Web Workers / Worker线程就是Quentin建议的方式。
Update: I'm not trying to make the long running task to run in parallel, I just want to understand why Promise and setTimeout behave differently in my example. But if you do want to run the task in parallel then Web Workers / Worker threads is the way to go as Quentin suggests.
但是我的问题的答案似乎是正如Bergi在评论中所写，Promise构造函数是同步的。
这是更长的解释
But the answer to my question seems to be that Promise constructors are synchronous as Bergi writes in a comment. Here is a longer explanation
推荐答案
while（Date.now（ ）-nu< 1000）{} 不会模拟长期运行的同步任务。 是长期同步运行的任务。
while (Date.now() - nu < 1000) {} doesn't simulate a long running task that is syncronous. It is a long running task that is syncronous.
承诺是管理异步代码的一种方法，而不是使代码异步的方法，也不是模拟多线程的方法。
Promises are a way to manage asynchronous code, not a way to make code asynchronous, nor a way to simulate multithreading.
为此，您需要查看工人。
- Web Workers 用于浏览器。
- Node.js的工作线程。
- Web Workers for browsers.
- Worker Threads for Node.js.
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