我对ES6 Promise API中的某些内容感到困惑。我可以看到一个明确的用例,用于同时提交多个异步作业,并在第一次成功时"解决"。例如,这将适用于这样一种情况,即有多个等效的服务器可用,但有些可能已经停机,而另一些则负载严重且速度缓慢,所以我的目标是从第一个成功的服务器获得响应,而忽略其余的服务器(是的,我知道从服务器的角度来看,这对客户端来说是一种令人讨厌的行为方式,但对最终用户来说很好;)
然而,在我看来,我要么有"全部"行为,要么有"种族"行为。"all"行为似乎要等到所有请求都完成,这意味着我必须等待最慢的请求,即使服务器已经完成(事实上,我可能必须等待超时,在这种情况下这将是一场灾难。
API中是否存在允许"raceToSuccess"类型的行为,或者我是否必须手工构建它。就这一点而言,我将如何手工建造它?
顺便说一下,我在Java 8 CompletableFuture中发现了同样的难题,它似乎是一个非常相似的API。那么,我是不是在哲学层面上错过了什么?
这是一个经典的例子,反转逻辑可以使其更加清晰。在这种情况下,你的"种族"是你希望你的拒绝行为实际上是成功的行为。
function oneSuccess(promises){
return Promise.all(promises.map(p => {
// If a request fails, count that as a resolution so it will keep
// waiting for other possible successes. If a request succeeds,
// treat it as a rejection so Promise.all immediately bails out.
return p.then(
val => Promise.reject(val),
err => Promise.resolve(err)
);
})).then(
// If '.all' resolved, we've just got an array of errors.
errors => Promise.reject(errors),
// If '.all' rejected, we've got the result we wanted.
val => Promise.resolve(val)
);
}
您可以很容易地自己编写。
function raceToSuccess(promises) {
return new Promise(
resolve =>
promises.forEach(
promise =>
promise.then(resolve)
)
);
}
这开启了所有的承诺,当任何一个成功时,都会用它的价值来解决新的承诺。失败的承诺将被忽略。由于新的承诺已经得到解决,随后成功的承诺不会发生任何事情。请注意,如果没有一个输入promise解析,则生成的promise将永远不会解析或拒绝。
这是一个修改后的版本,如果所有输入的承诺都被拒绝,它将返回一个被拒绝的承诺:
function raceToSuccess(promises) {
let numRejected = 0;
return new Promise(
(resolve, reject) =>
promises.forEach(
promise =>
promise .
then(resolve) .
catch(
() => {
if (++numRejected === promises.length) reject();
}
)
)
);
}
我喜欢@loganofsmyth的方法;你可能应该为它的概念清晰性投赞成票。这里有一个变体:
function invertPromise(promise) {
return new Promise(
(resolve, reject) =>
promise.then(reject, resolve)
);
}
function raceToSuccess(promises) {
return invertPromise(
Promise.all(
promises.map(invertPromise)));
}
另一个想法是将失败的承诺转化为既不解决也不拒绝的承诺(换句话说,是永久挂起的),然后使用Promise.race
:
function pendingPromise() { return new Promise(() => { }); }
function killRejected(promise) { return promise.catch(pendingPromise); }
function raceToSuccess(promises) {
return Promise.race(promises.map(killRejected));
}
你可能喜欢也可能不喜欢这种行为。如果输入的promise都没有实现,则返回的promise将永远不会实现或拒绝。还有一种可能是,这些长期悬而未决的承诺不会得到GC的批准,或者一些引擎最终可能会抱怨它们。
我使用的是一个基于Promise.rece()的函数,但有一个转折:它忽略拒绝,除非所有给定的Promise都拒绝:
// ignores any rejects except if all promises rejects
Promise.firstResolve = function (promises) {
return new Promise(function (fulfil, reject) {
var rejectCount = 0;
promises.forEach(function (promise) {
promise.then(fulfil, () => {
rejectCount++;
if(rejectCount == promises.length) {
reject('All promises were rejected');
}
});
});
});
};
它基于Rich Harris的Promise polyfill竞赛方法。我只是让循环promise拒绝有条件:如果所有给定的promise都失败了,它只拒绝主promise,否则它忽略拒绝并解决第一个成功。
用法:
// fastest promise to end, but is a reject (gets ignored)
var promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("foo")
}, 100);
})
// fastest promise to resolve (wins the race)
var promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("bar")
}, 200);
})
// Another, slower resolve (gets ignored)
var promise3 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("baz")
}, 300);
})
Promise.firstResolve([promise1, promise2, promise3])
.then((res) => {
console.log(res) // "bar"
})
.catch(err => {
console.log(err) // "All promises were rejected" (if all promises were to fail)
})
我之所以使用这一方法而不是承诺反转方法,是因为在我看来这更具可读性。
为了以最严格的方式取悦这个问题,下面有一个版本可以解决第一个成功的承诺,但如果所有给定的承诺都失败了,则不会采取任何行动:
// ignores any and all rejects
Promise.firstResolve = function (promises) {
return new Promise(function (fulfil) {
promises.forEach(function (promise) {
promise.then(fulfil, () => {});
});
});
};
(用法同上)
编辑:这实际上与@user663031的建议相同。直到现在我才意识到。
API中是否存在允许";raceToSuccess";行为类型
现在有了。Promise.any
:的第4阶段提案已完成
Promise.any()
接受Promise对象的一个可迭代对象,一旦可迭代对象中的一个Promise实现,就返回一个用该Promise的值解析的Promise。
因此,可以使用以下语法:
// assume getApi returns a Promise
const promises = [
getApi('url1'),
getApi('url2'),
getApi('url3'),
getApi('url4'),
];
Promise.any(promises)
.then((result) => {
// result will contain the resolve value of the first Promise to resolve
})
.catch((err) => {
// Every Promise rejected
});
Promise.any
已经在所有现代浏览器中实现。也有一些聚乙烯填充物可用。
旧主题,但这是我的条目;它本质上是@loganofsmisth的解决方案,但需要进行更多的检查以符合Promise.all()
:建立的约定
- 作为输入的空数组(同步)返回已解析的promise
- 数组中的非promise条目导致第一个此类条目用作解析值
Promise.any = a => {
return !a.length ?
Promise.resolve() :
Promise.all(a.map(
e => (typeof e.then !== 'function') ?
Promise.reject(e) :
e.then(
result => Promise.reject(result),
failure => Promise.resolve(failure)
)
)).then(
allRejected => Promise.reject(allRejected),
firstResolved => Promise.resolve(firstResolved)
);
};
// Testing...
function delayed(timeout, result, rejected) {
return new Promise((resolve, reject) => {
setTimeout(
() => rejected ? reject(result) : resolve(result),
timeout);
});
}
Promise.any([
delayed(800, 'a'),
delayed(500, 'b'),
delayed(250, 'c', true)
]).then(e => {
console.log('First resolved (expecting b):', e);
});
Promise.any([
delayed(800, 'a', true),
delayed(500, 'b', true),
delayed(250, 'c', true)
]).then(null, e => {
console.log('All rejected (expecting array of failures):', e);
});
Promise.any([
delayed(800, 'a'),
delayed(500, 'b'),
delayed(250, 'c', true),
'd',
'e'
]).then(e => {
console.log('First non-promise (expecting d):', e);
});
// Because this is the only case to resolve synchronously,
// its output should appear before the others
Promise.any([]).then(e => {
console.log('Empty input (expecting undefined):', e);
});
我用超时扩展了@loganofsmyth方法,并编写了一个小函数:
- 履行所有承诺
- 等待承诺成功的时间不超过指定的时间量(options.timeOutMs)
- 返回第一个成功的
在下面的片段中,您可以测试它:
const firstThatCompleteSuccessfullyES6 = (options) => {
// return the first promise that resolve
const oneSuccess = (promises) => Promise.all(promises.map(p => {
// If a request fails, count that as a resolution so it will keep
// waiting for other possible successes. If a request succeeds,
// treat it as a rejection so Promise.all immediately bails out.
return p.then(
(val) => { return Promise.reject(val); },
(err) => { return Promise.resolve(err); }
);
})
).then(
// If '.all' resolved, we've just got an array of errors.
(errors) => { return Promise.reject(errors); },
// If '.all' rejected, we've got the result we wanted.
(val) => { return Promise.resolve(val); }
);
// return the promise or reect it if timeout occur first
const timeoutPromise = (ms, promise) => new Promise(function(resolve, reject) {
setTimeout(() => reject(new Error('timeout')), ms);
promise.then(resolve, reject);
});
if (options.subsystems.length < 1) {
return Promise.reject('Parameters error, no subSystems specified');
}
const timedOutSubsystems = options.subsystems.map(function(subsystem){
return timeoutPromise(options.timeOutMs, subsystem(options));
});
const startDate = Date.now();
return oneSuccess(
timedOutSubsystems
)
.then((result) => {
const elapsedTime = Math.abs((startDate - Date.now()) / 1000);
console.log('firstThatCompleteSuccessfully() done, after s: ' + elapsedTime + ': '+ result);
return result;
})
.catch((error) => {
const elapsedTime = Math.abs((startDate - Date.now()) / 1000);
console.error('firstThatCompleteSuccessfully() error/nodata: ' + error);
});
}
// example of use with two promises (subsystem1 & subsystem2) that resolves after a fixed amount of time
const subsystem1 = (options) => new Promise(function(resolve, reject) {
setTimeout(function(){
console.log('subsystem1 finished');
resolve('subsystem 1 OK');
}, 1000);
});
const subsystem2 = (options) => new Promise(function(resolve, reject) {
setTimeout(function(){
console.log('subsystem2 finished');
resolve('subsystem 2 OK');
}, 2000);
});
firstThatCompleteSuccessfullyES6({
subsystems: [subsystem1, subsystem2],
timeOutMs: 2000
})
.then((result) => console.log("Finished: "+result));
为了解决这个问题,我使用了Promise.rice
和Promise.allSettled
。
下一个代码等待Promise.rice
为成功值。但如果没有成功的结果。返回一个包含所有错误的数组。
const PromiseRiceSuccess = <T = unknown>(promises: Promise<T>[]) => {
let done: (reason?: T) => void;
const waitEndAllPromises = new Promise((resolve, reject) => done = reject);
const waitCatchs = promise => Promise.resolve(promise).catch(() => waitEndAllPromises);
Promise.allSettled(promises).then(r => done(r));
return Promise.race(promises.map(waitCatchs));
};
示例:
PromiseRiceSuccess([
Promise.reject(1),
new Promise((r) => setTimeout(() => r(2), 4000)),
]);
// 2
PromiseRiceSuccess([
Promise.reject(1),
new Promise((resolve, reject) => setTimeout(() => reject(2), 4000)),
]);
// Uncaught (in promise) (2) [{…}, {…}]