Swift @escaping and Completion Handler

Question

I am trying to understand 'Closure' of Swift more precisely.

But @escaping and Completion Handler are too difficult to understand

I searched many Swift postings and official documents, but I felt it was still not enough.

This is the code example of official documents

var completionHandlers: [()->Void] = []

func someFunctionWithEscapingClosure(completionHandler: @escaping ()->Void){
    completionHandlers.append(completionHandler)
}

func someFunctionWithNoneescapingClosure(closure: ()->Void){
    closure()
}

class SomeClass{
    var x:Int = 10
    func doSomething(){
        someFunctionWithEscapingClosure {
            self.x = 100
            //not excute yet
        }
        someFunctionWithNoneescapingClosure {
            x = 200
        }
    }
}

let instance = SomeClass()
instance.doSomething()
print(instance.x)

completionHandlers.first?() 
print(instance.x)

I heard that there are two ways and reasons using @escaping

First is for storing a closure, second is for Async operating purposes.

The following are my questions:

First, if doSomething executes then someFunctionWithEscapingClosure will executing with closure parameter and that closure will be saved in global variable array.

I think that closure is {self.x = 100}

How self in {self.x = 100} that saved in global variable completionHandlers can connect to instance that object of SomeClass ?

Second, I understanding someFunctionWithEscapingClosure like this.

To store local variable closure completionHandler to global variable 'completionHandlerswe using@escaping` keyword!

without @escaping keyword someFunctionWithEscapingClosure returns, local variable completionHandler will remove from memory

@escaping is keep that closure in the memory

Is this right?

Lastly, I just wonder about the existence of this grammar.

Maybe this is a very rudimentary question.

If we want some function to execute after some specific function. Why don't we just call some function after a specific function call?

What are the differences between using the above pattern and using an escaping callback function?

Shobhakar Tiwari · Accepted Answer · 2022-11-11T21:19:14.163

Swift Completion Handler Escaping & Non-Escaping:

Assume the user is updating an app while using it. You definitely want to notify the user when it is done. You possibly want to pop up a box that says, “Congratulations, now, you may fully enjoy!”

So, how do you run a block of code only after the download has been completed? Further, how do you animate certain objects only after a view controller has been moved to the next? Well, we are going to find out how to design one like a boss.

Based on my expansive vocabulary list, completion handlers stand for

Do stuff when things have been done

Bob’s post provides clarity about completion handlers (from a developer point of view it exactly defines what we need to understand).

@escaping closures:

When one passes a closure in function arguments, using it after the function’s body gets executed and returns the compiler back. When the function ends, the scope of the passed closure exist and have existence in memory, till the closure gets executed.

There are several ways to escaping the closure in containing function:

Storage: When you need to store the closure in the global variable, property or any other storage that exist in the memory past of the calling function get executed and return the compiler back.
Asynchronous execution: When you are executing the closure asynchronously on despatch queue, the queue will hold the closure in memory for you, can be used in future. In this case you have no idea when the closure will get executed.

When you try to use the closure in these scenarios the Swift compiler will show the error:

error screenshot

For more clarity about this topic you can check out this post on Medium.

Adding one more points , which every ios developer needs to understand :

Escaping Closure : An escaping closure is a closure that’s called after the function it was passed to returns. In other words, it outlives the function it was passed to.
Non-escaping closure : A closure that’s called within the function it was passed into, i.e. before it returns.

@shabhakar, what if we store a closure but don't call it later. Or if method got called twice but we called closure only one time. Since we know that result is same. — user1101733, Jul 26 '18 at 20:33
@user1101733 I think you talking about escaping closure, Closure will not execute until you will not call. In above example if call doSomething method 2 times 2 completionHandler object will add in completionHandlers array. If you take first object from completionHandlers array and call it will execute but completionHandlers array count will remain same(2). — Dipak, Aug 29 '18 at 20:52
@Deepak, Yes about escape closure. suppose we don't use array and use normal variable to store closure reference since we what to execute most recent call. Will some memory occupied by previous closures which will never call? — user1101733, Aug 29 '18 at 21:00
@user1101733 Closure are reference type (like class), when you assign new closures to the variable then property/variable will point to new closure so ARC will deallocate memory for previous closures. — Dipak, Sep 01 '18 at 19:44
@shobhakar, it looks like that first link no longer works, but this one does: https://medium.com/ios-geek-community/completion-handlers-in-swift-with-bob-6a2a1a854dc4 — Gorm, Nov 06 '22 at 16:54

score 35 · Answer 2 · edited Mar 14 '19 at 13:18

Here's a small class of examples I use to remind myself how @escaping works.

class EscapingExamples: NSObject {

    var closure: (() -> Void)?

    func storageExample(with completion: (() -> Void)) {
        //This will produce a compile-time error because `closure` is outside the scope of this
        //function - it's a class-instance level variable - and so it could be called by any other method at
        //any time, even after this function has completed. We need to tell `completion` that it may remain in memory, i.e. `escape` the scope of this
        //function.
        closure = completion
        //Run some function that may call `closure` at some point, but not necessary for the error to show up.
        //runOperation()
    }

    func asyncExample(with completion: (() -> Void)) {
        //This will produce a compile-time error because the completion closure may be called at any time
        //due to the async nature of the call which precedes/encloses it.  We need to tell `completion` that it should
        //stay in memory, i.e.`escape` the scope of this function.
        DispatchQueue.global().async {
            completion()
        }
    }

    func asyncExample2(with completion: (() -> Void)) {
        //The same as the above method - the compiler sees the `@escaping` nature of the
        //closure required by `runAsyncTask()` and tells us we need to allow our own completion
        //closure to be @escaping too. `runAsyncTask`'s completion block will be retained in memory until
        //it is executed, so our completion closure must explicitly do the same.
        runAsyncTask {
            completion()
        }
    }





    func runAsyncTask(completion: @escaping (() -> Void)) {
        DispatchQueue.global().async {
            completion()
        }
    }

}

This code is not correct. It’s missing the `@escaping` qualifiers. — Rob, Oct 24 '19 at 17:02
I liked this the most `i.e. escape the scope of this function.` — Gal, Mar 24 '20 at 12:23

score 2 · Answer 3 · answered Feb 17 '21 at 18:02

/*the long story short is that @escaping means that don't terminate the function life time until the @escaping closure has finished execution in the opposite of nonEscaping closure the function can be terminated before the closure finishes execution Ex:
*/

 func fillData(completion: @escaping: () -> Void){ 
     /// toDo 
    completion()
  }

//___________________________

//The call for this function can be in either way's @escaping or nonEscaping :

    
fillData{
 /// toDo
}
    

/* again the deference between the two is that the function can be terminated before finish of execution nonEscaping closure in the other hand the @escaping closure guarantees that the function execution will not be terminated before the end of @escaping closure execution. Hope that helps ***#(NOTE THAT THE CLOSURE CAN BE OF ANY SWIFT DATA TYPE EVEN IT CAN BE TYPEALIAS)*/

"Expected parameter type following ':'" – dub Jun 09 '21 at 05:26 — dub, Jun 09 '21 at 05:26
@dub just remove the semicolon ":" – christostsang Nov 12 '22 at 21:29 — christostsang, Nov 12 '22 at 21:29

Swift @escaping and Completion Handler

3 Answers3

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