在promise链上使用setTimeout [英] using setTimeout on promise chain

问题描述

在这里，我试图围绕promises.Here第一次请求我获取一组链接。在下一个请求我获取第一个链接的内容。但我想在返回下一个promise对象之前做出延迟。所以我在它上面使用了setTimeout。但是它给了我以下JSON错误（ 没有setTimeout（）它可以正常工作 ）

Here i am trying to wrap my head around promises.Here on first request i fetch a set of links.and on next request i fetch the content of first link.But i want to make a delay before returning next promise object.So i use setTimeout on it.But it gives me the following JSON error (without setTimeout() it works just fine)

SyntaxError：JSON.parse：
第1行第1行的意外字符JSON数据

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

我想知道它失败的原因？

i would like to know why it fails?

let globalObj={};
function getLinks(url){
    return new Promise(function(resolve,reject){

       let http = new XMLHttpRequest();
       http.onreadystatechange = function(){
            if(http.readyState == 4){
              if(http.status == 200){
                resolve(http.response);
              }else{
                reject(new Error());
              }
            }           
       }
       http.open("GET",url,true);
       http.send();
    });
}

getLinks('links.txt').then(function(links){
    let all_links = (JSON.parse(links));
    globalObj=all_links;

    return getLinks(globalObj["one"]+".txt");

}).then(function(topic){


    writeToBody(topic);
    setTimeout(function(){
         return getLinks(globalObj["two"]+".txt"); // without setTimeout it works fine 
         },1000);
});

推荐答案

为了保持承诺链，你不能使用 setTimeout（）你的方式，因为你没有从 .then（）处理程序返回一个承诺 - 你从 setTimeout（）回调中返回它，这对你没有好处。

To keep the promise chain going, you can't use setTimeout() the way you did because you aren't returning a promise from the .then() handler - you're returning it from the setTimeout() callback which does you no good.

相反，你可以做一个像这样的简单的小延迟函数：

Instead, you can make a simple little delay function like this:

function delay(t, v) {
   return new Promise(function(resolve) { 
       setTimeout(resolve.bind(null, v), t)
   });
}

并且，然后像这样使用它：

And, then use it like this:

getLinks('links.txt').then(function(links){
    let all_links = (JSON.parse(links));
    globalObj=all_links;

    return getLinks(globalObj["one"]+".txt");

}).then(function(topic){
    writeToBody(topic);
    // return a promise here that will be chained to prior promise
    return delay(1000).then(function() {
        return getLinks(globalObj["two"]+".txt");
    });
});

这里你要从 .then（）返回一个承诺处理程序，因此它被正确链接。

Here you're returning a promise from the .then() handler and thus it is chained appropriately.

您还可以向Promise对象添加延迟方法然后直接在你的承诺上使用 .delay（x）方法：

You can also add a delay method to the Promise object and then directly use a .delay(x) method on your promises like this:

function delay(t, v) {
   return new Promise(function(resolve) { 
       setTimeout(resolve.bind(null, v), t)
   });
}

Promise.prototype.delay = function(t) {
    return this.then(function(v) {
        return delay(t, v);
    });
}


Promise.resolve("hello").delay(500).then(function(v) {
    console.log(v);
});

或者，使用已经 .del的蓝鸟承诺库。 ay（）内置方法。

Or, use the Bluebird promise library which already has the .delay() method built-in.

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