Nodejs/Q:链式顺序承诺 [英] Nodejs / Q : Chaining promises sequentially

问题描述

我想做一些非常简单的事情，但是我一点都不懂...

I want to do something really simple, but i don't understand a little thing ...

var Q = require('q');

var funcs = ["first", "second", "third", "fourth"];

function main(){

// really don't know how to chain sequentially here ...
    var result = Q();

    funcs.forEach(function (f) {
        result = treat(f).then(f);
    });

}

function treat(t){

    var deferred = Q.defer();

    setTimeout(function(){
        deferred.resolve("treated "+ t); 
    },2000);

    return deferred.promise;
}

main();

我希望funcs数组中的每个元素都被顺序处理"，然后输出将类似于:

I would like each element of my funcs array to be "treated" sequentially, the output would then be something like :

treated first
//2 seconds later
treated second
//2 seconds later
treated third
//2 seconds later
treated fourth

我无法实现这一目标:(应该很简单，我什么都没发现:(

I cannot achieve that :( it should be simple , i don't catch something :(

推荐答案

从您的示例来看，我认为您已经看到

Judging from your example, I would assume that you already saw Sequences part of Q readme, but failed to understand it.

当每个函数的输出作为输入传递给下一个函数时，原始示例使用瀑布"模型:

Original example used "waterfall" model, when the output of each function passed as an input to the next one:

var funcs = [foo, bar, baz, qux];

var result = Q(initialVal);
funcs.forEach(function (f) {
    result = result.then(f);
});
return result;

但是您只想按顺序执行我们的所有功能，因此只需将每个函数与其变量绑定即可:

But you just want to execute all our functions in sequence, so you could simply bind each function with its variables:

var args = ["first", "second", "third", "fourth"];

var result = Q();
args.forEach(function (t) {
    result = result.then(treat.bind(null, t));
});
return result;

在我的示例中，treat函数将被顺序调用4次，并且result promise将使用最新的treat调用的值进行解析(所有先前调用的结果都将被忽略).

In my example treat function will be called 4 times sequentially, and result promise will be resolved with the value of latest treat call (results of all previous calls will be ignored).

诀窍在于，.then方法接受一个处理程序，该处理程序将在当前的Promise解析后返回并返回一个新的Promise.因此，您应该将一个函数传递给.then，该函数应在执行链的下一步中调用. treat.bind(null, t)将treat函数与属性t绑定.换句话说，它将返回一个新函数，该函数将调用treat，并将t作为其第一个参数.

The trick is that .then method accepts a handler which will be called after current promise will be resolved and returns a new promise. So, you should pass to .then a function which should be called on the next step of your execution chain. treat.bind(null, t) binds treat function with attribute t. In other words, it returns a new function, which will invoke treat, passing t as its first argument.

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