How can a closure be used to only complete when data is loaded? (swift) - swift

Question

I'm slowly coming to terms with closures.
This is taken from the following post:
https://medium.com/#stasost/ios-three-ways-to-pass-data-from-model-to-controller-b47cc72a4336
I understand the function definition requestData is taking a closure which is called with completion(data):
class DataModel {
func requestData(completion: ((data: String) -> Void)) {
// the data was received and parsed to String
let data = "Data from wherever"
completion(data)
}
}
class ViewController: UIViewController {
private let dataModel = DataModel()
override func viewDidLoad() {
super.viewDidLoad()
dataModel.requestData { [weak self] (data: String) in
self?.useData(data: data)
}
}
private func useData(data: String) {
print(data)
}
}
While I understand requestData is being called in viewDidLoad below and that (data:String) is being passed into requestData I don't quite get what is being done when completion(data) is being called.
Is completion(data) executing the code that is happening after the keyword "in"?
{ [weak self] (data: String) in
self?.useData(data: data)
}
And I had a question about the order in which things are happening. Is it:
a) let data = "Data from wherever"
b) completion(data)
c) self?.useData(data: data)
In the current app I'm working on when the user starts the app I make an api call to load data. But i'm still a tiny bit unsure about how to tell the ViewController that the data has finished loading.
Thanks.

Answer 1

You understand that a function can take, say, an Int as parameter?
func f(_ i:Int) {
// do something with i
}
Well, a function can also take a function as parameter:
func requestData(completion: ((data: String) -> Void)) {
// do something with completion
}
In that declaration, completion is a function — a function that takes one parameter, called data.
What can you do when you receive a function as a parameter? One obvious thing to do with it would be to call it:
func requestData(completion: ((data: String) -> Void)) {
completion(data: "well howdy there")
}
It remains only to talk about the syntax of passing the desired function to requestData as its completion parameter. Here is one way:
func output(data: String) {
print(data)
}
requestData(completion:output)
Here's a nicer way, one that avoids giving the passed function a name:
requestData(completion:{
data in
print(data)
})
Finally, since completion is the only parameter, we are allowed to use "trailing closure" syntax — we delete the completion: label and the parentheses of the call:
requestData {
data in
print(data)
}