Javascript 如何检查 Promise 是否挂起
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How to check if a Promise is pending
提问by Jeanluca Scaljeri
I have this situation in which I would like to know what the status is of a promise. Below, the function startonly calls someTestif it is not running anymore (Promise is not pending). The startfunction can be called many times, but if its called while the tests are still running, its not going to wait and returns just false
我有这种情况，我想知道承诺的状态是什么。下面，该函数start仅someTest在不再运行时调用（Promise 未挂起）。该start函数可以被多次调用，但如果在测试仍在运行时调用它，它不会等待并返回false
class RunTest {
start() {
retVal = false;
if (!this.promise) {
this.promise = this.someTest();
retVal = true;
}
if ( /* if promise is resolved/rejected or not pending */ ) {
this.promise = this.someTest();
retVal = true;
}
return retVal;
}
someTest() {
return new Promise((resolve, reject) => {
// some tests go inhere
});
}
}
I cannot find a way to simply check the status of a promise. Something like this.promise.isPendingwould be nice :) Any help would be appreciated!
我找不到一种方法来简单地检查承诺的状态。类似的东西this.promise.isPending会很好:) 任何帮助将不胜感激！
采纳答案by Amit
You can attach a thenhandler that sets a doneflag on the promise (or the RunTestinstance if you prefer), and test that:
您可以附加一个在承诺（或您喜欢的实例）上then设置done标志的处理程序RunTest，并测试：
if (!this.promise) {
this.promise = this.someTest();
this.promise.catch(() => {}).then(() => { this.promise.done = true; });
retVal = true;
}
if ( this.promise.done ) {
this.promise = this.someTest();
this.promise.catch(() => {}).then(() => { this.promise.done = true; });
retVal = true;
}
Notice the empty catch()handler, it's crucial in order to have the handler called regardless of the outcome of the promise.
You probably want to wrap that in a function though to keep the code DRY.
请注意空catch()处理程序，无论 Promise 的结果如何，都必须调用处理程序。您可能希望将其包装在一个函数中以保持代码干燥。
回答by Daniel Graham
class RunTest {
constructor() {
this.isRunning = false;
}
start() {
console.log('isrunning', this.isRunning);
var retVal = false;
if(!this.isRunning) {
this.promise = this.someTest();
this.promise.catch().then(() => { this.isRunning = false; });
retVal = true;
}
return retVal;
}
someTest() {
this.isRunning = true;
return new Promise((resolve, reject) => {
setTimeout(function() {
//some tests go inhere
resolve();
}, 1000);
});
}
};
var x = new RunTest();
x.start(); //logs false
x.start(); //logs true
setTimeout(function() {
//wait for a bit
x.start(); //logs false
}, 2000);
