What is closure in Swift language?

Introduction

In this article, I’m going to briefly explain what closure is.

Closures is self-conitained blocks of funcionality that can be passed around and used in your code. — Apple

Expression:

{ (params) -> return value in
  statements
}

@escaping

When closure is marked as escaping, it will outlive or leave the scope you passed.

func response(_ completionHandler: @escaping(Result) -> Void) {
   completionHandler(.success)
}

@nonescaping

By default, closures are nonescaping, meaning closure will no longer exist in memory after complete execution in the scope you have passed it to.

func filterImage(_ completionHandler: (Image) -> Void) {
   completionHandler(UIImage.filtered)
}

@autoclosure

Autoclosures automatically create closure from the argument that you passed into a function.

func animate(_ animation: @autoclosure @escaping () -> Void,
             duration: TimeInterval = 0.25) {
    UIView.animate(withDuration: duration, animations: animation)
}

Closure conceptualy looks like this first pointer points to the code that implements closure the second pointer pointed to the reference counted object.

struct Closure {
    var functionPointer: UnsafeRawPointer
    var closureContext: AnyObject?
}