我有一系列的承诺,我想并行调用,但同步解析。
我编写了这段代码来完成所需的任务,但是,我需要创建自己的对象QueryablePromise
包裹原生Promise
我可以同步检查它的已解决状态。
有没有更好的方法来完成这个不需要特殊对象的任务?
请注意。我不想使用Promise.all
因为我不想在处理承诺的效果之前必须等待所有承诺都解决。我不能使用async
我的代码库中的函数。
const PROMISE = Symbol('PROMISE')
const tap = fn => x => (fn(x), x)
class QueryablePromise {
resolved = false
rejected = false
fulfilled = false
constructor(fn) {
this[PROMISE] = new Promise(fn)
.then(tap(() => {
this.fulfilled = true
this.resolved = true
}))
.catch(x => {
this.fulfilled = true
this.rejected = true
throw x
})
}
then(fn) {
this[PROMISE].then(fn)
return this
}
catch(fn) {
this[PROMISE].catch(fn)
return this
}
static resolve(x) {
return new QueryablePromise((res) => res(x))
}
static reject(x) {
return new QueryablePromise((_, rej) => rej(x))
}
}
/**
* parallelPromiseSynchronousResolve
*
* Call array of promises in parallel but resolve them in order
*
* @param {Array<QueryablePromise>} promises
* @praram {Array<fn>|fn} array of resolver function or single resolve function
*/
function parallelPromiseSynchronousResolve(promises, resolver) {
let lastResolvedIndex = 0
const resolvePromises = (promise, i) => {
promise.then(tap(x => {
// loop through all the promises starting at the lastResolvedIndex
for (; lastResolvedIndex < promises.length; lastResolvedIndex++) {
// if promise at the current index isn't resolved break the loop
if (!promises[lastResolvedIndex].resolved) {
break
}
// resolve the promise with the correct resolve function
promises[lastResolvedIndex].then(
Array.isArray(resolver)
? resolver[lastResolvedIndex]
: resolver
)
}
}))
}
promises.forEach(resolvePromises)
}
const timedPromise = (delay, label) =>
new QueryablePromise(res =>
setTimeout(() => {
console.log(label)
res(label)
}, delay)
)
parallelPromiseSynchronousResolve([
timedPromise(20, 'called first promise'),
timedPromise(60, 'called second promise'),
timedPromise(40, 'called third promise'),
], [
x => console.log('resolved first promise'),
x => console.log('resolved second promise'),
x => console.log('resolved third promise'),
])
<script src="https://codepen.io/synthet1c/pen/KyQQmL.js"></script>
为任何帮助干杯。
Using a for await...of https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of循环,如果你已经有了承诺数组,你可以很好地做到这一点:
const delay = ms => new Promise(resolve => { setTimeout(resolve, ms); });
const range = (length, mapFn) => Array.from({ length }, (_, index) => mapFn(index));
(async () => {
const promises = range(5, index => {
const ms = Math.round(Math.random() * 5000);
return delay(ms).then(() => ({ ms, index }));
});
const start = Date.now();
for await (const { ms, index } of promises) {
console.log(`index ${index} resolved at ${ms}, consumed at ${Date.now() - start}`);
}
})();
由于不能使用异步函数,因此可以模仿以下效果for await...of
通过使用将承诺链接在一起Array.prototype.reduce() https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce,并为每个链同步调度回调:
const delay = ms => new Promise(resolve => { setTimeout(resolve, ms); });
const range = (length, mapFn) => Array.from({ length }, (_, index) => mapFn(index));
const asyncForEach = (array, cb) => array.reduce(
(chain, promise, index) => chain.then(
() => promise
).then(
value => cb(value, index)
),
Promise.resolve()
);
const promises = range(5, index => {
const ms = Math.round(Math.random() * 5000);
return delay(ms).then(() => ms);
});
const start = Date.now();
asyncForEach(promises, (ms, index) => {
console.log(`index ${index} resolved at ${ms}, consumed at ${Date.now() - start}`);
});
错误处理
由于承诺被声明为并行实例化,因此我假设任何单个承诺的错误都不会传播到其他承诺,除非通过通过构建的任何潜在的脆弱链asyncForEach()
(如上)。
但我们也希望在将 Promise 链接在一起时避免它们之间的交叉传播错误asyncForEach()
。这是一种稳健地安排错误回调的方法,其中错误只能从原始承诺传播:
const delay = ms => new Promise(resolve => { setTimeout(resolve, ms); });
const maybe = p => p.then(v => Math.random() < 0.5 ? Promise.reject(v) : v);
const range = (length, mapFn) => Array.from({ length }, (_, index) => mapFn(index));
const asyncForEach = (array, fulfilled, rejected = () => {}) => array.reduce(
(chain, promise, index) => {
promise.catch(() => {}); // catch early rejection until handled below by chain
return chain.then(
() => promise,
() => promise // catch rejected chain and settle with promise at index
).then(
value => fulfilled(value, index),
error => rejected(error, index)
);
},
Promise.resolve()
);
const promises = range(5, index => {
const ms = Math.round(Math.random() * 5000);
return maybe(delay(ms).then(() => ms)); // promises can fulfill or reject
});
const start = Date.now();
const settled = state => (ms, index) => {
console.log(`index ${index} ${state}ed at ${ms}, consumed at ${Date.now() - start}`);
};
asyncForEach(
promises,
settled('fulfill'),
settled('reject') // indexed callback for rejected state
);
这里唯一需要注意的是,回调中抛出的任何错误都会传递给asyncForEach()
除了在数组最后一个索引的回调中抛出的错误之外,将被链中的错误处理所吞噬。
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