[英]wait until condition is met or timeout is passed in javascript
我需要让代码休眠,直到满足某些条件或通过 3 秒超时。 然后返回一个简单的字符串。 无论如何我可以做到这一点吗?
// this function needs to return a simple string
function something() {
var conditionOk = false;
var jobWillBeDoneInNMiliseconds = Math.floor(Math.random() * 10000);
setTimeout(function() {
// I need to do something here, but I don't know how long it takes
conditionOk = true;
}, jobWillBeDoneInNMiliseconds);
// I need to stop right here until
// stop here until ( 3000 timeout is passed ) or ( conditionOk == true )
StopHereUntil( conditionOk, 3000 );
return "returned something";
}
这就是我要做的:
我让浏览器滚动到页面底部,然后将调用一些 ajax 函数来获取评论(我无法控制它)。 现在我需要等到评论出现在带有“.comment”类的文档中。
我需要getComments()
函数将评论作为 json 字符串返回。
function getComments() {
window.scrollTo(0, document.body.scrollHeight || document.documentElement.scrollHeight);
var a = (document.querySelectorAll('div.comment'))
// wait here until ( a.length > 0 ) or ( 3 second is passed )
// then I need to collect comments
var comments = [];
document.querySelectorAll('div.comment p')
.forEach(function(el){
comments.push(el.text());
});
return JSON.stringify(comments);
}
getComments();
我遇到了这个问题,没有一个解决方案令人满意。 我需要等到某个元素出现在 DOM 中。 所以我接受了hedgehog125的回答并根据我的需要进行了改进。 我认为这回答了最初的问题。
const sleepUntil = async (f, timeoutMs) => {
return new Promise((resolve, reject) => {
const timeWas = new Date();
const wait = setInterval(function() {
if (f()) {
console.log("resolved after", new Date() - timeWas, "ms");
clearInterval(wait);
resolve();
} else if (new Date() - timeWas > timeoutMs) { // Timeout
console.log("rejected after", new Date() - timeWas, "ms");
clearInterval(wait);
reject();
}
}, 20);
});
}
用法(异步/等待承诺):
try {
await sleepUntil(() => document.querySelector('.my-selector'), 5000);
// ready
} catch {
// timeout
}
用法(.then 承诺):
sleepUntil(() => document.querySelector('.my-selector'), 5000)
.then(() => {
// ready
}).catch(() => {
// timeout
});
您应该能够使用Promise.race
来实现这一点。 这是一个基本示例:
let promise1 = new Promise(resolve => { setTimeout(resolve, 500, 'one'); }); let promise2 = new Promise(resolve => { setTimeout(resolve, 800, 'two'); }); async function fetchAndLogResult() { let result = await Promise.race([promise1, promise2]); console.log(result); } fetchAndLogResult();
这是一个替代版本,虽然不使用async
/ await
,但更简洁:
let promise1 = new Promise(resolve => { setTimeout(resolve, 500, 'one'); }); let promise2 = new Promise(resolve => { setTimeout(resolve, 800, 'two'); }); Promise.race([promise1, promise2]).then(result => console.log(result));
在 JavaScript 中,没有办法等待。 您可以使用settimeout
也可以使用 while 循环(请记住,在这种情况下脚本无法运行,然后页面可能会变得无响应)。
设置超时
// this function needs to return a simple string
function something() {
conditionOk = false;
var jobWillBeDoneInNMiliseconds = Math.floor(Math.random() * 10000);
timeout = setTimeout(function() {
// I need to do something here, but I don't know how long it takes
conditionOk = true;
}, jobWillBeDoneInNMiliseconds);
// I need to stop right here until
// stop here until ( 3000 timeout is passed ) or ( conditionOk == true )
timeWas = new Date();
wait = setInterval(function() {
if (conditionOk) {
// Communicate what you were trying to return using globals
clearInterval(wait);
}
if (new Date() - timeWas > 3000) { // Timeout
// Clear this interval
clearInterval(wait);
}
}, 30);
}
随着一会儿
// this function needs to return a simple string
function something() {
conditionOk = false;
var jobWillBeDoneInNMiliseconds = Math.floor(Math.random() * 10000);
timeout = setTimeout(function() {
// I need to do something here, but I don't know how long it takes
conditionOk = true;
}, jobWillBeDoneInNMiliseconds);
// I need to stop right here until
// stop here until ( 3000 timeout is passed ) or ( conditionOk == true )
timeWas = new Date();
while ((! conditionOk) && (! (new Date() - timeWas > 3000))) { // 3000 = the delay
// Do nothing
}
if (conditionOk) {
return "returned something";
}
else {
return "returned nothing";
}
}
您可能还想看看这个问题: JavaScript sleep/wait before continue
希望这可以帮助!
当我在寻找类似的解决方案时,我在浏览器中打开了这个问题。 可能帖子作者不再需要它,但这里有来自非事件循环世界(php、java、python)的其他人。
这是我在阅读 MDN 后得到的: https ://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await https://developer.mozilla.org/en-US/docs/Web /JavaScript/参考/语句/async_function
这两个描述了需要什么。 为此,您将需要一些辅助函数。 它看起来臃肿,但似乎在 JS 中没有其他方法。 虽然完成了工作:)
// sleep helper function sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms)) } // func that will check your condition function checkCondition(number) { if (number == 5) return true; else return false; } // helper that will wait and then check the condition // await halts the execution flow until sleep is resolved // this is the part that you need async function waitOneSecondAndCheckCondition(number) { const v = await sleep(500); return checkCondition(number); } // since most likely you want to poll the checkCondition // and not waste all the time waiting // this polls conditon every half a second // and in the end return a simple string // either when condition is met or after three seconds async function waitUntilConditionIsMetOrTimeoutIsPassed() { for (let i = 0; i < 6; i++) { let result = await waitOneSecondAndCheckCondition(i); console.log("i is: " + i + " condition result is: " + result); if (!result) continue; else break; } } waitUntilConditionIsMetOrTimeoutIsPassed();
如果在 3 秒内的某个时间点满足条件,则控制台输出:
i is: 0 condition result is: false
i is: 1 condition result is: false
i is: 2 condition result is: false
i is: 3 condition result is: true
a simple string
发生超时时的控制台输出:
i is: 0 condition result is: false
i is: 1 condition result is: false
i is: 2 condition result is: false
i is: 3 condition result is: false
i is: 4 condition result is: false
i is: 5 condition result is: false
a simple string
希望这对你们所有像我一样的JS新手有帮助:)
如果要等到满足条件:
main(); async function main() { let foo = 0; // for demo purposes, artificially increment foo setInterval(() => foo++); console.log('Waiting until foo reaches 1337 ...'); await until(() => foo === 1337); console.log('foo === ' + foo); } function until(condition) { return new Promise((resolve) => { const interval = setInterval(() => { if (condition()) { clearInterval(interval); resolve(); } }); }); }
如果你想等到一定时间过去:
main(); async function main() { console.log('Waiting 2 seconds ...'); await milliseconds(2_000); console.log('Done!'); } function milliseconds(ms) { return new Promise((resolve) => setTimeout(resolve, ms)); }
好的,因为您使用的是ajax
,您可以执行以下操作:
var eventEmitter = new EventEmitter()
eventEmitter.on('myEvent', myFunction)
$.ajax(...).then(function() {
eventEmitter.emit('myEvent', {state: true})
})
setTimeout(function() { eventEmitter.emit('myEvent', {state: false}), 3000);
function myFunction() {
//you can do your checks inside here
}
不使用jQuery
的 ajax:
var xhr = new XMLHttpRequest();
xhr.open('GET', 'myservice/username?id=some-unique-id');
xhr.onload = function() {
if (xhr.status === 200) {
eventEmitter.emit('myEvent', {state: true})
}
else {
alert('Request failed. Returned status of ' + xhr.status);
}
};
xhr.send();
对于 Typescript,我修改了 @pbotas 的答案,如下所示:
export const sleepUntil = async (f: () => boolean, timeoutMs: number) => {
return new Promise((resolve, reject) => {
const timeWas = new Date();
const wait = setInterval(function () {
if (f()) {
console.log('resolved after', +new Date() - +timeWas, 'ms');
clearInterval(wait);
resolve(true);
} else if (+new Date() - +timeWas > timeoutMs) {
// Timeout
console.log('rejected after', +new Date() - +timeWas, 'ms');
clearInterval(wait);
reject(false);
}
}, 20);
});
};
请注意,我正在评估一个布尔条件。
调用此函数:
sleepUntil(() => someBoolean, 5000)
.then(() => {
// ready
})
.catch(() => {
// timeout
});
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