Swift Async Programming @escaping closure, and Completion handler

When I use HTTP connection for login system , I should catch the end point of getting data.

As you know, when HTTP connection is done asychronously, you should not use return value for this case.

I have used Future function in Flutter, but I had struggle to find the similar function in swift.

Escape closure supports @escaping property. If you add @escaping property after the name of argument(:), escape closure recieves ‘Completion handler’ which supports sending async task completion state to other side and sending the return value as well.

If @escaping property is not exist after argument, then this is Nonescape Closure. If you do not add @escaping property, Nonescape property is default.

Nonescape property does not support completion handler.

Because we have used Nonescape closure so far, I just focus on showing escape closure, and completion handler.

I arranged 2 cases which I used to do.

Completion Handle with TrailingClosure
Completion Handle with DeclaredClosure

There are getFruitColor examples below.

If you send parameter on getFruitColor function with completionHanlder, then completionHanlder prints the color of given fruit.

FULL Code : https://github.com/dev-wd/simple_swift_example/blob/master/escape_closure/ViewController.swift

Case 1. Completion Handle with TrailingClosure

Implement case is implemented in ‘viewDidLoad()’.
‘completion’ is completionHandler which includes ‘@escaping’ property.
you don’t need to send ‘completion’ parameter, but add trailing closure instead.

// case 1 : Completion Handle with TrailingClosure
func getFruitColorwithTrailingClosure(_ fruitName : String, completion: @escaping (String)-> Void) {
        
    var color = "Not yet"
        
    if fruitName == "banana" {
        color = "yellow"
    }
        
    if fruitName == "apple" {
        color = "red"
    }
        
    if fruitName == "melon" {
           color = "green"
    }
        
    completion(color)
}
    
    
// Implement case 1
getFruitColorwithTrailingClosure("banana") {
    color in
    print(color)
}
        
        

Case 2. Completion Handle with DeclaredClosure

Declare closure for completionHandler.
Because you declared closure first, you don’t need to add trailing closure when you implement getFruitColor function.

// Declare closure for case 2.
let getColorClosure : (String) -> Void = {
    color in
    print(color)
}
    
// case 2 : Completion Handle with DeclaredClosure
func getFruitColorwithClosure(_ fruitName : String, completion: @escaping (String)-> Void) {
    var color = "Not yet"
        
    if fruitName == "banana" {
        color = "yellow"
    }
        
    if fruitName == "apple" {
        color = "red"
    }
        
    if fruitName == "melon" {
        color = "green"
    }
        
    completion(color)
        
}
    
// Implement case 2        
getFruitColorwithClosure("melon",completion: getColorClosure)
    

Conclusion

Asynchoronous function’s end state can be catched by EscapingClosure.

Do not return the value on your service API.

Let’s get completion state by completionHandler from now on!

Reference

Swift Programming 2 by YAGOM

Categories

Case 1. Completion Handle with TrailingClosure

Case 2. Completion Handle with DeclaredClosure

Conclusion

Reference