如何创建围绕XML请求的回调？ [英] How would I create a callback around an XML request?

问题描述

我一直在试图理解回调在Swift中如何工作。我已经讨论了几个例子（喜欢这个）帮助我更好地理解回调，但我没有运气正确实现一个。

我有一个函数接受一个URL，从web api下载一些XML数据，然后将其解析为对象。同时，我有一个UILabel，它正在等待XML请求中的一些数据。

下面是我想设置的函数的一个部分示例与回调。为了清楚起见，只是假设它只返回单个数据点，稍后将分配给 UILabel ：

XMLUtility .swift

  // global 
 var weekForecasts = [DayForecast]（）
 
 class XMLUtility {
 
 func retrieveDataFromXML（myUrl：String）{
 
如果let url = NSURL（string：myUrl）{
 if let data = NSData （contentsOfURL：url）{
 var error：NSError？ 
 var cleansData = filterData（data）
 
如果let doc = AEXMLDocument（xmlData：cleansData，error：& error）{
 
 // ...一些工作解析xml //// 
 
 
 for day in date {
 
 // ...一些工作分配值///// 
 
 weekForecasts.append（thisDay）
 
} 
} 
} 
} 
  $ p> 
 
 
问题发生在我的ViewController ...我有一些UILabels正在等待来自XML数据请求的值。当ViewController加载时，XML尚未处理，标签未能接收到值。
 
 
 
这是一个简单的例子，我在我的ViewController中做的： / p> 
 
 
  ViewController.swift  
  @IBOutlet weak var currentTemperatureLabel：UILabel！ 
 
 override func viewDidLoad（）{
 super.viewDidLoad（）
 
 
 
 currentTemperatureLabel.text = //我得到一个值的我的[DayForecasts] 
} 
  
 
 
我理解为什么会是这样，一个新手的了解如何解决问题。我相信我需要使用回调，但基于我所看到的例子，我不知道如何实现一个。 
 
 
 
我的问题：
 
 
 
根据提供的示例，如何转换我的 retrieveDataFromXML 方法转换为回调。另外，如何从我的ViewController调用函数来访问数据。
 
 
 
任何帮助将非常感谢！ 
解决方案
 
  func retrieveDataFromXML（myUrl：String，completion：（（Array< DayForecast> - > Void））{
 
如果let url = NSURL（string：myUrl）{
 if let data = NSData（contentOfURL：url）{
 var error：NSError？ 
 var cleansData = filterData（data）
 var weekForecasts = [DayForecast]（）//局部变量
 
如果let doc = AEXMLDocument（xmlData：cleansData，error：& error ）{
 
 // ...做一些工作从xml创建对象
 for date in date {
 
 // ...一些工作分配值// /// 
 
 weekForecasts.append（thisDay）
 
} 
 //将本地数组传递给完成块，它接受
 // Array< ; DayForecast>作为其参数
完成（weekForecasts）
} 
} 
} 
} 
  
称为
  //在此示例中，它在viewDidLoad 
中调用func viewDidLoad（）{
 var urlString =urlstring
 retrieveDataFromXML（urlString，{（result） - > Void in 
 //结果是weekForecasts 
 
 / / UI元素只能在主线程上更新，所以获取主
 //线程并更新该线程上的UI元素
 dispatch_async（dispatch_get_main_queue（），{
 self.currentTemperatureLabel。 text = result [0] //或任何您想要的索引
 return 
}）
}）
} 
  
这是你的问题是什么？
 
I've been trying to understand how callbacks work in Swift. I've gone over quite a few examples (like this one) that have helped me to better understand callbacks, but I haven't had any luck in properly implementing one.

I have a function that accepts a URL, downloads some XML data from a web api and then parses it into objects. At the same time I have a UILabel that is waiting for some data from the XML request.

Below is a partial example of my function that I'd like to set up with a callback. For the sake of clarity just assume it only returns a single data point which which will be assigned to a UILabel later:

XMLUtility.swift
// global
var weekForecasts = [DayForecast]()

class XMLUtility {

    func retrieveDataFromXML(myUrl: String) {

        if let url = NSURL(string: myUrl) {
            if let data = NSData(contentsOfURL: url) {
                var error: NSError?
                var cleanedData = filterData(data)

                if let doc = AEXMLDocument(xmlData: cleanedData, error: &error) {

                //... does some work parsing xml ////


                for day in date {

                   //... some work assigning values /////

                   weekForecasts.append(thisDay)

                }         
            }   
        }  
    } 
The problem occurs in my ViewController... I have some UILabels that are waiting for values from the XML data request. When the ViewController loads, the XML hasn't processed yet and the label failed to receive a value.

Here's a simplified example of what I am doing in my ViewController:

ViewController.swift
 @IBOutlet weak var currentTemperatureLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()



    currentTemperatureLabel.text = // I get a value out of my [DayForecasts]
}
I understand why this is the case, and I have a novice understanding of how to solve the problem. I believe I need to use a callback but, based on the examples I have seen so far, I am not sure how to implement one. 

My question:

Given the example provided, how would I convert my retrieveDataFromXML method into a callback. Additionally, how do I call the function from my ViewController to access the data.

Any help on this would be greatly appreciated! 
解决方案
func retrieveDataFromXML(myUrl: String, completion: ((Array<DayForecast>) -> Void)) {

    if let url = NSURL(string: myUrl) {
        if let data = NSData(contentsOfURL: url) {
            var error: NSError?
            var cleanedData = filterData(data)
            var weekForecasts = [DayForecast]() //local variable

            if let doc = AEXMLDocument(xmlData: cleanedData, error: &error) {

                //... does some work creating objects from xml
                for day in date {

                   //... some work assigning values /////

                   weekForecasts.append(thisDay)

                }    
                //pass the local array into the completion block, which takes
                //Array<DayForecast> as its parameter
                completion(weekForecasts) 
            }
        }
    }  
}
called like 
//in this example it is called in viewDidLoad
func viewDidLoad() {
    var urlString = "urlstring"
    retrieveDataFromXML(urlString, {(result) -> Void in
        //result is weekForecasts

        //UI elements can only be updated on the main thread, so get the main 
        //thread and update the UI element on that thread
        dispatch_async(dispatch_get_main_queue(), {
            self.currentTemperatureLabel.text = result[0] //or whatever index you want
            return
        })
    })
}
Is this what your question was asking for?

                        
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