Javascript 等待请求完成 Node.js
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Wait for request to finish Node.js
提问by OstlerDev
I am currently having an issue with figuring out how to wait for the request to finish before returning any data. I do not believe I can do this with a Callback and I have not been able to figure out a good way of using the EventEmitter to do it. The reason I cannot use a callback is because my flow currently works like this.
我目前在弄清楚如何在返回任何数据之前等待请求完成时遇到问题。我不相信我可以通过回调来做到这一点，而且我无法找到使用 EventEmitter 来做到这一点的好方法。我不能使用回调的原因是我的流程目前是这样工作的。
Request comes into server > Generate XML > Contact remote API for details to finish generating XML > Finish Generating XML > Return request to client
请求进入服务器 > 生成 XML > 联系远程 API 以获取详细信息以完成生成 XML > 完成生成 XML > 将请求返回给客户端
The code I currently have looks very similar to the code included below.
我目前拥有的代码看起来与下面包含的代码非常相似。
Web Server:
网络服务器：
var xml = require('./XMLGenerator');
response.writeHead(200, {'Content-Type': 'text/xml'});
response.write(xml.generateXML());
response.end();
XML Generator:
XML 生成器：
function generateXML(){
// Code to generate XML
var API = require('./API');
var response = API.getItems("5");
for(var i = 1; i <= response.length; i++)
{
// more code to generate further XML using the API response
}
// Finish generating and return the XML
}
API Grabber:
API 抓取器：
function getItems(sort_by, amount) {
var request = require("request")
var url = "https://url.com/api/get_items.json?amount=" + amount;
request({
url: url,
json: true
}, function (error, response, body) {
console.log('we got here!');
if (!error && response.statusCode === 200) {
var items = body.data.items;
console.log(items);
return items;
} else {
console.log("Error connecting to the API: " + url);
return;
}
})
}
When running the code and testing directly it returns "undefined" meaning that the request has not been made yet. I just need to know a way to make the XML generator wait for the request to finish before continuing on with the generation. (there may be minor errors in the psudeo code I typed up as it is not an exact copy paste from the source, it does however work in this flow)
当运行代码并直接测试时，它返回“未定义”，表示尚未发出请求。我只需要知道一种让 XML 生成器在继续生成之前等待请求完成的方法。（我输入的伪代码中可能存在小错误，因为它不是来自源代码的精确复制粘贴，但它确实在此流程中工作）
Am I just using bad practices, or is this the correct way that I should be attempting this?
我只是使用了不好的做法，还是我应该尝试这样做的正确方法？
EDIT: The problem is not loading the module/API file, that loads perfectly fine. The problem is that the request takes about 2 seconds to complete, and that node moves on before the request completes.
编辑：问题不在于加载模块/API 文件，加载非常好。问题是请求大约需要 2 秒才能完成，并且该节点在请求完成之前继续前进。
回答by slebetman
You need to use callbacks. Change your API grabber to this:
您需要使用回调。将您的 API 采集器更改为：
function getItems(amount, callback) {
// some code...
request({
url: url,
json: true
}, function (error, response, body) {
// some code...
if (!error && response.statusCode === 200) {
// some code
callback(items); <-- pass items to the callback to "return" it
}
})
}
Then change the xml generator to also accept callbacks:
然后将 xml 生成器更改为也接受回调：
function generateXML(callback){
// Code to generate XML
var API = require('./API');
API.getItems("5",function(response){
for(var i = 1; i <= response.length; i++)
{
// more code to generate further XML using the API response
}
// Finish generating and return the XML
callback(xml_result); // <-- again, "return" the result via callback
});
}
Then in your server code do:
然后在您的服务器代码中执行以下操作：
var xml = require('./XMLGenerator');
response.writeHead(200, {'Content-Type': 'text/xml'});
xml.generateXML(function(xmlstring){
response.write(xmlstring);
response.end();
});
回答by Zee
Do these changes since requestis async, use callbacks.
执行这些更改，因为request是异步的，请使用回调。
API.getItems("5", function(rs){
var response = rs;
for(var i = 1; i <= response.length; i++)
{
// more code to generate further XML using the API response
}
// Finish generating and return the XML
}
});
...
function getItems(sort_by, amount, callback) {...
...
callback(items); //Instead of return items;
...
You cannot returnfrom an asynccall, which is requestmodule in your case. In such cases you can either use promisesor callbacks. This is a solution with callback.
你不能return从一个async电话，这是request你的情况模块。在这种情况下，您可以使用promise或callbacks. 这是一个带有回调的解决方案。
The actual problem of it returning undefinedis it doesnot wait for var response = API.getItems("5");to execute completely and executes the next line and hence you get responseas undefined. I hope you get the point.\
它返回的实际问题undefined是亘古不变的等待var response = API.getItems("5");完全执行，执行的下一行，因此你得到response的undefined。我希望你明白这一点。\
Also I hope
我也希望
response.writeHead(200, {'Content-Type': 'text/xml'});
response.write(xml.generateXML());
response.end();
is somewhere inside some callback of an API or http.createServer.
位于 API 或http.createServer.
