Is there any particular use of closure in swift? and what's the benefit?

Question

I have learnt Swift for a while and I have read the Swift language guide.

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_

Answer 1

Closures are a concept you use mainly on asynchronous function calls. (at least I do so)

A good example where Apple uses closures is URLSession:

func downloadTask(with url: URL, completionHandler: @escaping (URL?, URLResponse?, Error?) -> Void) -> URLSessionDownloadTask

This method creates an URLSessionDownloadTask that runs in a background thread. When you call this method, you are passing a closure as completionHandler. This closure is executed, when the task has finished.

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There are some other cases, but I haven't used them this much yet, because I love the delegate pattern. I found an article that compares Closures and Delegates in Swift.

Delegates in Swift have a big problem: Unless you are using @objc, you have to implement every method of a protocol, which could lead to lots of unnecessary code and would look confusing. That's why I guess it would be much better to use closures in many cases.

Answer 2

The question might be a little broad, but I will try to recap.

Referring to the Documentation:

Closures are self-contained blocks of functionality that can be passed around and used in your code.

You should use closure(s) when you want to pass a chunk of code as a parameter to a method that you want to execute it asynchronously.

To make it simpler -by giving a real-world example-, imagine that there is a method responsible for scanning the user's photos, so the method should returns an array of photos and another array of videos:

Pesudo-code:

// the method should scan the the user's photos and return them after fetching is finished

// the 'completion' (success) closure should returns two arrays
// what if there is something wrong happened? another closure should returns an error

// for the example purposes they are arrays of strings, the error is also a string
func scanPhotos( completion: @escaping (_ photos: [String], _ videos: [String]) -> (), failure: @escaping (String) -> ()) {
    // imagine that the scanning has been successfully done and you filled the two arrays:

    //if scanningSuccess {
        let fetchedPhotos = [String]()
        let fetchedVideos = [String]()

        completion(fetchedPhotos, fetchedVideos)
    //} else {
        // if something goes wrong
        failure("the error!!")
    //}
}

Calling the method:

scanPhotos(completion: { (photos, videos) in
    // now you can use the fetched photos and videos
}, failure: { error in
    // display an alert -for example- based on the error is saying...
})

Note the the scanning proccess should be executed asynchronously, when it finished, one of the two blocks (success or failure) should be executed.

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Some of the popular methods that work with clousres:

• animateWithDuration:animations:completion:
• urlSession(_:dataTask:didReceive:completionHandler:)
• sort(by:)

AGAIN: it is just a simple usage of the closures; You need to check the documentation for more details.

I hope it helped.