蓝鸟，承诺然后（） [英] Bluebird, promises and then()

问题描述

我一直只使用蓝鸟几天，但我想查看我所有的旧代码和 promisify 它：）

I've been only using bluebird for a few days but I want to go over all my old code and promisify it :)

我的问题是我仍然没有完全掌握然后（）命令的流程。

My problem is that I still don't fully grasp the flow of then() commands.

考虑这两个块：

A

methodThatReturnsAPromise().then(task2).then(task3);

B

var promise = methodThatReturnsAPromise();
promise.then(task2)
promise.then(task3);

1. task3 会得到 task2 的结果？在B中他们都得到了第一个承诺的结果？

1. in scenario A task3 will get the result of task2? In B they all get the result of the first promise?

第二个与蓝鸟运行 Promise.all 有什么不同？

How does the second one differ from running Promise.all from bluebird?

在使用 catch时，这些A / B / Promise.all 有何不同方法（我在哪里放）。

How do these A/B/Promise.all differ when it comes to using the catch method (where do I put it).

对不起，这是一堆问题一个。

Sorry it's a bunch of questions in one.

推荐答案

欢迎来到美好的承诺世界。

Welcome to the wonderful world of promises.

您的断言在 1 是对的。我们可以使用 Promise.resolve 模拟一个在Bluebird中解析的promise。

Your assertion in 1 is correct. We can simulate a promise resolving in Bluebird using Promise.resolve on a value.

让我们看看：

让我们得到一个返回承诺的函数：

Let's get a function that returns a promise:

function foo(){
    return Promise.resolve("Value");    
}

foo().then(alert);

这个简短的片段会提醒价值如我们可以看到。

This short snippet will alert "Value" as we can see.

现在，让我们创建两个承诺，每个承诺提醒并返回不同的值。

Now, let's create two more promises, each that alert and return different values.

function task2(e){
    alert("In two got " + e);
    return " Two ";
}
function task3(e){
    alert("In three got " + e);
    return " Three ";
}

所以，你可以在你的第一个代码它确实会在链中解析，每个代码都包含前一部分的值。

So, as you can see in your first code it will indeed resolve in a chain, each with the value of the previous part.

在第二个示例中，task2和task3将获得相同的值并且也将一起执行（即，任务3不会等待任务2）。你可以看到这里。

In the second example, both task2 and task3 will get the same value and will also execute together (that is, task 3 will not wait for task 2). You can see that here.

Promise.all（或者只是从然后履行处理程序返回一个数组，然后使用 .spread ）用于等待多个结果全部完成。在您的示例中，您将在多个部分中连接单个结果。

Promise.all (or just returning an array from a then fulfillment handler and then using .spread) is used for waiting for multiple results to all complete. On your example, you're hooking on a single result in multiple parts.

您始终把catch放在你想要捕获错误的位置。正如你通常在同步代码中那样。只记得总是投入承诺或承诺的代码。

You always put catch where you want the error to be caught. As you would normally in synchronous code. Just remember to always throw in a promise or in promisified code.

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