swift - order of functions - which code runs when? - swift

Question

I have an issue with my code and I think it could be related to the order in which code is called.
import WatchKit
import Foundation
class InterfaceController: WKInterfaceController {
private var tasks = [Task]()
override func willActivate() {
let taskUrl = "http://myjsonurl.com"
downloadJsonTask(url: taskUrl)
print(tasks.count) // EMPTY
super.willActivate()
}
func downloadJsonTask(url: String) {
var request = URLRequest(url: URL(string: url)!)
request.cachePolicy = URLRequest.CachePolicy.reloadIgnoringLocalCacheData
URLSession.shared.dataTask(with: request) { data, urlResponse, error in
guard let data = data, error == nil, urlResponse != nil else {
print("something is wrong")
return
}
do
{
let decoder = JSONDecoder()
let downloadedTasks = try decoder.decode(Tasks.self, from: data)
self.tasks = downloadedTasks.tasks
print(downloadedTasks.tasks.count) //4
} catch {
print("somehting went wrong after downloading")
}
}.resume()
}
}
I'm defining the private var tasks and fill it with the downloadJsonTask function but after the function ran the print(tasks.count) gives 0.
When I call print(downloadedTasks.tasks.count) it gives 4.
I think that in sequence of time the tasks variable is empty when I print it and it is filled later on.

Answer 1

When you are trying to print number of tasks in willActivate(), function downloadJsonTask(url: String) hasn't been completed yet, so you have empty array because tasks haven't been set yet.
You should add completion handler to downloadJsonTask just like this:
(don't forget to pass completion as parameter of function)
func downloadJsonTask(url: String, completion: #escaping () -> Void) {
var request = URLRequest(url: URL(string: url)!)
request.cachePolicy = URLRequest.CachePolicy.reloadIgnoringLocalCacheData
URLSession.shared.dataTask(with: request) { data, urlResponse, error in
guard let data = data, error == nil, urlResponse != nil else {
print("something is wrong")
completion()
return
}
do {
let decoder = JSONDecoder()
let downloadedTasks = try decoder.decode(Tasks.self, from: data)
self.tasks = downloadedTasks.tasks
print(downloadedTasks.tasks.count) //4
} catch {
print("something went wrong after downloading")
}
completion() // This is moment when code which you write inside closure get executed
}.resume()
}
In your willActivate() use this function like this:
downloadJsonTask(url: taskUrl) {
print(tasks.count)
}
So that means when you get your data, your code inside curly braces will get executed.

Answer 2

You’re correct in your assumption that tasks has not yet been assigned a value when it’s first printed.
The thing is network requests are performed asynchronously. It means that iOS does not wait until downloadJsonTask(url:) is finished but continues executing the code right away (i.e. it calls print(tasks.count) immediately after the network request started, without waiting for it to produce any results).
The piece of code inside brackets after URLSession.shared.dataTask(with:) is called a completion handler. This code gets executed once the network request is competed (hence the name). The tasks variable is assigned a value only when the request is finished. You can make sure it works by adding print(self.tasks.count) after self.tasks = downloadedTasks.tasks:
self.tasks = downloadedTasks.tasks
print(self.tasks)
print(downloadedTasks.tasks.count)