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What is closure? Why we use closure instead of function sometime?

Ans : 

The two most used cases are completion blocks and higher order functions in Swift. 

Completion blocks: for example, when you have some time consuming task, you want to be notified when that task is finished. You can use closures for that, instead of a delegate (or many other things)


func longAction(completion: () -> ()) {
    for index in veryLargeArray {
        // do something with veryLargeArray, which is extremely time-consuming
    }
    completion() // notify the caller that the longAction is finished
}

//Or asynch version
func longAction(completion: () -> ()) {
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        
        for elem in veryLargeArray {
            // do something with veryLargeArray, which is extremely time-consuming
        }
        dispatch_async(dispatch_get_main_queue(), {
            completion() // notify the caller that the longAction is finished
        })
    }
}

longAction { print("work done") }

In the example above, when you have a time consuming task, you want to know when the for loop finishes iterating through the very large array. You put the closure { println("work done") } as an input parameter for the function which will be executed after the for loop finishes its work, and print "work done". And what happened is that you gave a function (closure) to longAction and name it to completion, and that function will be executed when you call completion in longAction.

Sorted method works using closure.

About how sorted (probably) works: So the idea is, that sorted will go through the array, and compare two consecutive elements (i, i + 1) with each other, and swap them, if needed. What does it mean "if needed"? You provided the closure { (s1: String, s2: String) -> Bool in return s1 > s2 }, which will return true if s1 is greater than s2. And if that closure returned true, the sorted algorithm will swap those two elements, and continues this with the next two elements (i + 1, i + 2, if the end of the array is not reached). So basically you have to provide a closure for sorted which will tell "when" to swap to elements.

Understand Closure


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