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!
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?
来源：https://stackoverflow.com/questions/29871633/how-would-i-create-a-callback-around-an-xml-request