[英]Async generator class stuck on infinite loop javascript
我正在尝试使以下异步生成器工作:
class MyIterator {
constructor(m) {
this.collection = m;
}
async *[Symbol.iterator]() {
for (let item of this.collection) {
const resultItem = await Promise.resolve(item)
console.log("item: ", resultItem)
yield resultItem
}
}
}
(async () => {
const iterator = new MyIterator([1,2,3])
let times = 0
for await (let thing of iterator) {
console.log("thing: ", thing)
// this is here to avoid an infinite loop
times++
if (times > 1000) break
}
})()
但它最终会陷入无限循环,并且thing
总是未定义的。
item: 1
thing: undefined
item: 2
thing: undefined
item: 3
thing: undefined (x999)
我尝试过类似的代码,但这次没有Promise/async
行为,它似乎工作得很好。
class MyIterator {
constructor(m) {
this.collection = m;
}
*[Symbol.iterator]() {
for (let item of this.collection) {
console.log("item: ", item)
yield item
}
}
}
const iterator = new MyIterator([1,2,3])
for (let thing of iterator) {
console.log("thing: ", thing)
}
item: 1
thing: 1
item: 2
thing: 2
item: 3
thing: 3
for await..of
构造将尝试迭代异步迭代器。
使用@@asyncIterator
众所周知的符号定义异步迭代器:
class MyIterator { constructor(m) { this.collection = m; } async *[Symbol.asyncIterator]() { //<-- this is async for (let item of this.collection) { const resultItem = await Promise.resolve(item) //console.log("item: ", resultItem) yield resultItem } } } (async () => { const iterator = new MyIterator([1,2,3]) let times = 0 for await (let thing of iterator) { //no infinite loop console.log("thing: ", thing) } })()
for await..of
也可以使用产生承诺的普通迭代:
const promiseArray = [Promise.resolve("a"), Promise.resolve("b"), Promise.resolve("c")]; (async function() { for await(const item of promiseArray) { console.log(item); } })()
尝试创建作为异步方法/函数的常规迭代器不起作用。
如果你想保留@@iterator
定义的方法,最好的选择是让它产生 Promise:
class MyIterator { constructor(m) { this.collection = m; } *[Symbol.iterator]() { // not async for (let item of this.collection) { yield Promise.resolve(item); //produce a promise } } } (async () => { const iterator = new MyIterator([1,2,3]) let times = 0 for await (let thing of iterator) { console.log("thing: ", thing) } })()
虽然,如果任何承诺拒绝,这可能是一个不好的做法:
const wait = (ms, val) => new Promise(res => setTimeout(res, ms, val)); const fail = (ms, val) => new Promise((_, rej) => setTimeout(rej, ms, val)); const arr = [ wait(100, 1), wait(150, 2), fail(0, "boom"), wait(200, 3) ]; (async function(){ try { for await (const item of arr) { console.log(item); } } catch (e) { console.error(e); } })() /* result in the browser console: Uncaught (in promise) boom 1 2 boom */
但是,请注意,这些之间存在语义差异:
value
和done
属性的 object。 const syncIterable = { [Symbol.iterator]() { return { next() { return {value: 1, done: true} } } } } const syncIterator = syncIterable[Symbol.iterator](); console.log("sync IteratorResult", syncIterator.next());
const asyncIterable = { [Symbol.asyncIterator]() { return { next() { return Promise.resolve({value: 2, done: true}); } } } } const asyncIterator = asyncIterable[Symbol.asyncIterator](); asyncIterator.next().then(result => console.log("async IteratorResult", result));
value
promise: const promiseSyncIterable = { [Symbol.iterator]() { return { next() { return {value: Promise.resolve(3), done: true} } } } } const promiseSyncIterator = promiseSyncIterable[Symbol.iterator](); const syncPromiseIteratorResult = promiseSyncIterator.next(); console.log("sync IteratorResult with promise", syncPromiseIteratorResult); syncPromiseIteratorResult.value.then(value => console.log("value of sync IteratorResult with promise", value));
关于命名的旁注: MyIterator
不是迭代器。 一个迭代器是一个 object 和一个next()
方法,它产生一个 IteratorResult。 您可以迭代的 object 有一个@@iterator
(或@@asyncIterable
)方法,它被称为可迭代(或分别称为异步迭代)。
正如@VLAZ在对我的问题的评论中指出的那样,我使用的是Symbol.iterator
而不是Symbol.asyncIterator
。 以下实现按预期工作:
class MyIterator {
constructor(m) {
this.collection = m;
}
async *[Symbol.asyncIterator]() {
for (let item of this.collection) {
const resultItem = await Promise.resolve(item)
console.log("item: ", resultItem)
yield resultItem
}
}
}
(async () => {
const iterator = new MyIterator([1,2,3])
for await (let thing of iterator) {
console.log("thing: ", thing)
}
})()
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