据我所知,在ES7/ES2016中,在代码中放置多个await将类似于用promise链接.then(),这意味着它们将一个接一个地执行,而不是并行执行。例如,我们有以下代码:
await someCall();
await anotherCall();
我是否正确理解只有当someCall()完成时才会调用另一个otherCall()?并行调用它们最优雅的方式是什么?
我想在Node中使用它,所以也许有一个异步库的解决方案?
编辑:我对这个问题中提供的解决方案不满意:由于异步生成器中的非并行等待承诺,所以速度变慢,因为它使用生成器,我问的是一个更通用的用例。
// A generic test function that can be configured
// with an arbitrary delay and to either resolve or reject
const test = (delay, resolveSuccessfully) => new Promise((resolve, reject) => setTimeout(() => {
console.log(`Done ${ delay }`);
resolveSuccessfully ? resolve(`Resolved ${ delay }`) : reject(`Reject ${ delay }`)
}, delay));
// Our async handler function
const handler = async () => {
// Promise 1 runs first, but resolves last
const p1 = test(10000, true);
// Promise 2 run second, and also resolves
const p2 = test(5000, true);
// Promise 3 runs last, but completes first (with a rejection)
// Note the catch to trap the error immediately
const p3 = test(1000, false).catch(e => console.log(e));
// Await all in parallel
const r = await Promise.all([p1, p2, p3]);
// Display the results
console.log(r);
};
// Run the handler
handler();
/*
Done 1000
Reject 1000
Done 5000
Done 10000
*/
虽然设置p1、p2和p3并不是严格地并行运行它们,但它们不会阻碍任何执行,您可以通过捕获捕获上下文错误。
您可以调用多个异步函数,而无需等待它们。这将并行执行它们。执行此操作时,将返回的Promise保存在变量中,并在某个时刻单独或使用Promise.all()等待它们,然后处理结果。
还可以使用try…包装函数调用。。。catch处理单个异步操作的失败并提供回退逻辑。
下面是一个示例:观察日志,在单个异步函数开始执行时打印的日志会立即打印,即使第一个函数需要5秒才能解析。
函数someLongFunc(){return new Promise((resolve,reject)=>{console.log('执行功能1')setTimeout(分辨率,5000)})}函数另一个LongFunc(){return new Promise((resolve,reject)=>{console.log('执行功能2')setTimeout(分辨率,5000)})}异步函数main(){让某个LongFuncPromise,另一个LongFunc Promiseconst start=Date.now()尝试{someLongFuncPromise=someLongFunc()}捕获(ex){console.error('功能1期间出现问题')}尝试{anotherLongFuncPromise=另一个LongFunc()}捕获(ex){console.error('功能2期间出现问题')}等待某人LongFuncPromise等待另一个LongFuncPromiseconst totalTime=Date.now()-开始console.log('执行完成时间',totalTime)}main()
TL;博士
对并行函数调用使用Promise.all,当发生错误时,应答行为不正确。
首先,立即执行所有异步调用并获取所有Promise对象。其次,对Promise对象使用await。这样,当您等待第一个Promise来解决其他异步调用时,其他异步调用仍在进行。总的来说,您将只等待最慢的异步调用。例如:
// Begin first call and store promise without waiting
const someResult = someCall();
// Begin second call and store promise without waiting
const anotherResult = anotherCall();
// Now we await for both results, whose async processes have already been started
const finalResult = [await someResult, await anotherResult];
// At this point all calls have been resolved
// Now when accessing someResult| anotherResult,
// you will have a value instead of a promise
JSbin示例:http://jsbin.com/xerifanima/edit?js安慰
注意:只要第一个等待调用发生在所有异步调用之后,那么等待调用是在同一条线上还是在不同的线上并不重要。见JohnnyHK的评论。
更新:根据@bergi的回答,这个答案在错误处理中的时间不同,它不会在错误发生时抛出错误,而是在所有承诺都执行后抛出错误。我将结果与@jonny的提示进行比较:[result1,result2]=Promise.all([async1(),async2()]),检查以下代码片段
常量correctAsync500ms=()=>{return new Promise(解析=>{setTimeout(resolve,500,'correct500msResult');});};常量correctAsync100ms=()=>{return new Promise(解析=>{setTimeout(resolve,100,'correct100msResult');});};常量拒绝同步100ms=()=>{return new Promise((resolve,reject)=>{setTimeout(拒绝,100,'拒绝100毫秒错误');});};const asyncInArray=异步(fun1,fun2)=>{const label='测试数组中的异步函数';尝试{console.time(标签);常量p1=fun1();常量p2=fun2();const result=[await p1,wait p2];console.timeEnd(标签);}捕获(e){console.error('错误为',e);console.timeEnd(标签);}};常量asyncInPromiseAll=async(fun1,fun2)=>{const label='使用Promise.all测试异步函数';尝试{console.time(标签);let[value1,value2]=等待Promise。all([fun1(),fun2()]);console.timeEnd(标签);}捕获(e){console.error('错误为',e);console.timeEnd(标签);}};(异步()=>{console.group('无错误的同步功能');console.log('无错误的同步函数:启动')等待异步InArray(correctAsync500ms,correctAsync 100ms);等待异步InPromiseAll(correctAsync500ms,correctAsync 100ms);console.groupEnd();console.group('带错误的同步功能');console.log('带有错误的同步函数:start')等待异步InArray(correctAsync500ms,rejectAsync100ms);等待异步InPromiseAll(correctAsync500ms,rejectAsync100ms);console.groupEnd();})();
更新:
最初的答案使正确处理拒绝承诺变得困难(在某些情况下是不可能的)。正确的解决方案是使用Promise.all:
const [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);
原答覆:
只需确保在等待其中一个函数之前调用这两个函数:
// Call both functions
const somePromise = someCall();
const anotherPromise = anotherCall();
// Await both promises
const someResult = await somePromise;
const anotherResult = await anotherPromise;
await Promise.all([someCall(),anotherCall()]);正如已经提到的那样,它将充当线程围栏(在CUDA这样的并行代码中非常常见),因此它将允许其中的所有承诺在不相互阻止的情况下运行,但将阻止执行继续,直到解决所有问题。
另一种值得分享的方法是Node.js异步,如果任务直接与API调用、I/O操作等有限资源的使用相关联,它还允许您轻松控制通常需要的并发量。
// create a queue object with concurrency 2
var q = async.queue(function(task, callback) {
console.log('Hello ' + task.name);
callback();
}, 2);
// assign a callback
q.drain = function() {
console.log('All items have been processed');
};
// add some items to the queue
q.push({name: 'foo'}, function(err) {
console.log('Finished processing foo');
});
q.push({name: 'bar'}, function (err) {
console.log('Finished processing bar');
});
// add some items to the queue (batch-wise)
q.push([{name: 'baz'},{name: 'bay'},{name: 'bax'}], function(err) {
console.log('Finished processing item');
});
// add some items to the front of the queue
q.unshift({name: 'bar'}, function (err) {
console.log('Finished processing bar');
});
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我创建了一个助手函数waitAll,也许它可以让它更甜。它目前只在nodejs中工作,在浏览器chrome中不工作。
//const parallel = async (...items) => {
const waitAll = async (...items) => {
//this function does start execution the functions
//the execution has been started before running this code here
//instead it collects of the result of execution of the functions
const temp = [];
for (const item of items) {
//this is not
//temp.push(await item())
//it does wait for the result in series (not in parallel), but
//it doesn't affect the parallel execution of those functions
//because they haven started earlier
temp.push(await item);
}
return temp;
};
//the async functions are executed in parallel before passed
//in the waitAll function
//const finalResult = await waitAll(someResult(), anotherResult());
//const finalResult = await parallel(someResult(), anotherResult());
//or
const [result1, result2] = await waitAll(someResult(), anotherResult());
//const [result1, result2] = await parallel(someResult(), anotherResult());