取消 JavaScript 承诺

Question

我正在尝试取消承诺，如下所示：

function example(cancel = Promise.reject()) {
    return new Promise((resolve, reject) => {
        const timer = setTimeout(() => resolve('jack-jack'), 5000);
        cancel.then((res) => {
            clearTimeout(timer);
            reject('cancelled'); 
        }, ()=>{})
    });
}
var cancel=Promise.reject(); 
example(cancel).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
cancel=Promise.resolve();

但是我无法取消它。我在这里做错了什么？

Answer 1

您已将拒绝的承诺分配给变量，将拒绝的承诺传递给您的函数，并将已解决的承诺分配给变量。变量取的两个值是不相关的，你不能改变一个已解决的 Promise 的状态。

传递一个你可以解决的承诺：

let cancel;
let cancelPromise = new Promise((resolve) => {
    cancel = resolve;
});

example(cancelPromise).then(…).catch(…);
console.log('attempting cancellation of promise');
cancel();

function example(cancel = Promise.reject()) {
    return new Promise((resolve, reject) => {
        const timer = setTimeout(() => resolve('jack-jack'), 5000);
        cancel.then((res) => {
            clearTimeout(timer);
            reject('cancelled'); 
        }, ()=>{})
    });
}

let cancel;
let cancelPromise = new Promise((resolve) => {
    cancel = resolve;
});

example(cancelPromise)
    .then((res) => console.log('res handled:' + res))
    .catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
cancel();

Answer 2

因为你传递了被拒绝的承诺。如果您希望 cancel.then() 块运行，请传递已解决的承诺。

function example(cancel = Promise.resolve()) {
  return new Promise((resolve, reject) => {
    console.log(cancel);
    const timer = setTimeout(() => {
      resolve('jack-jack'), 5000
    });
    cancel.then((res) => {
      console.log('CANCELLED');
      clearTimeout(timer);
      reject('cancelled');
    }, () => {})
  });
}
var cancel = Promise.resolve();
example(cancel).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
cancel = Promise.resolve();

Answer 3

在您的代码中，您已经向函数传递了一个完整的（被拒绝的）Promise。而cancel=Promise.resolve(); 在attempting cancellation of promise 之后不会对传递给example 的promise 产生任何影响，因为您只是创建了一个新的resolved Promise。

如果您想取消正在运行的进程，那么您可能需要选择这样的解决方案：

function example(helper) {
  return new Promise((resolve, reject) => {
    helper.cancel = function() {
      clearTimeout(timer)
      reject('cancelled');
    }
    const timer = setTimeout(() => resolve('jack-jack'), 5000);

  });
}
var helper = {};
example(helper).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));

console.log('attempting cancellation of promise');
helper.cancel()

Answer 4

由于cancel设置为reject，那么then部分

cancel.then((res) => { //you cannot use then for reject, as reject cannot be resolved.
    clearTimeout(timer);
     reject('cancelled'); 
}, ()=>{})

永远不会被执行，因此你必须解决它，而不是拒绝它

function example(cancel = Promise.reject()) {
    return new Promise((resolve, reject) => {
        const timer = setTimeout(() => resolve('jack-jack'), 5000);
        cancel.then((res) => { //you cannot use then for reject
            clearTimeout(timer);
            reject('cancelled'); 
        }, ()=>{})
    });
}

var cancel = Promise.resolve(); // just change to resolve

example(cancel).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));

console.log('attempting cancellation of promise');
cancel=Promise.resolve(); // this will have no effect as you have already passed a reject