理解 Promise.all

Question

考虑以下我从

中获取的代码

function getExample() {
    var a = promiseA(…);
    var b = a.then(function(resultA) {
        // some processing
        return promiseB(…);
    });
    return Promise.all([a, b]).spread(function(resultA, resultB) {
        // more processing
        return // something using both resultA and resultB
    });
}

以及我创建的代码演示 https://jsfiddle.net/Lsobypup/

这个想法是运行多个承诺和 return 一些基于其结果的复合值。

我不明白的是为什么在上面的代码中 promiseA 运行s 只有一次？在我看来，使用 Promise.all([a, b]) 它应该首先运行当评估 a 时，然后再次评估 b 时，因为它取决于 a。但正如演示所示，这并没有发生。

Promise.all 中有什么魔法可以让这一切发生吗？这种行为的规则是什么？

Answer 1

var b = a.then(function(resultA) {
        // some processing
        return promiseB(…);
    });

这是链接 a 的结果，这意味着如果 a 处于完成状态，回调将立即被调用。您对 promise a 的解决首先发生，因为它在 all() 调用中首先遇到。一旦兑现，最终价值始终如一。

根据这个MDN reference：

Internally, a promise can be in one of three states:

Pending, when the final value is not available yet. This is the only state that may transition to one of the other two states.

Fulfilled, when and if the final value becomes available. A fulfillment value becomes permanently associated with the promise. This may be any value, including undefined.

Rejected, if an error prevented the final value from being determined. A rejection reason becomes permanently associated with the promise. This may be any value, including undefined, though it is generally an Error object, like in exception handling.

Answer 2

很适合使用bluebird模块 http://bluebirdjs.com/docs/api/promise.props.html

const Promise = require("bluebird");
Promise.props({
    pictures: getPictures(),
    comments: getComments(),
    tweets: getTweets()
})
.then(function(result) {
    console.log(result.tweets, result.pictures, result.comments);
});

理解 Promise.all

understanding Promise.all

javascript

es6-promise