Closure use of non-escaping parameter may allow it to escape

Question

I have a protocol:

enum DataFetchResult {
    case success(data: Data)
    case failure
}

protocol DataServiceType {
    func fetchData(location: String, completion: (DataFetchResult) -> (Void))
    func cachedData(location: String) -> Data?
}

With an example implementation:

    /// An implementation of DataServiceType protocol returning predefined results using arbitrary queue for asynchronyous mechanisms.
    /// Dedicated to be used in various tests (Unit Tests).
    class DataMockService: DataServiceType {

        var result      : DataFetchResult
        var async       : Bool = true
        var queue       : DispatchQueue = DispatchQueue.global(qos: .background)
        var cachedData  : Data? = nil

        init(result : DataFetchResult) {
            self.result = result
        }

        func cachedData(location: String) -> Data? {
            switch self.result {
            case .success(let data):
                return data
            default:
                return nil
            }
        }

        func fetchData(location: String, completion: (DataFetchResult) -> (Void)) {

            // Returning result on arbitrary queue should be tested,
            // so we can check if client can work with any (even worse) implementation:

            if async == true {
                queue.async { [weak self ] in
                    guard let weakSelf = self else { return }

                    // This line produces compiler error: 
                    // "Closure use of non-escaping parameter 'completion' may allow it to escape"
                    completion(weakSelf.result)
                }
            } else {
               completion(self.result)
            }
        }
    }

The code above compiled and worked in Swift3 (Xcode8-beta5) but does not work with beta 6 anymore. Can you point me to the underlying cause?

This is a very great article on why it's done that way in Swift 3 — mfaani
– mfaani, Commented Jan 26, 2017 at 19:07
It makes no sense that we have to do this. No other language requires it. — Andrew Koster
– Andrew Koster, Commented Oct 2, 2019 at 1:44

Hamish · Accepted Answer · 2019-04-08 13:46:20Z

324

This is due to a change in the default behaviour for parameters of function type. Prior to Swift 3 (specifically the build that ships with Xcode 8 beta 6), they would default to being escaping – you would have to mark them @noescape in order to prevent them from being stored or captured, which guarantees they won't outlive the duration of the function call.

However, now @noescape is the default for function-typed parameters. If you want to store or capture such functions, you now need to mark them @escaping:

protocol DataServiceType {
  func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void)
  func cachedData(location: String) -> Data?
}

func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void) {
  // ...
}

See the Swift Evolution proposal for more info about this change.

edited Apr 8, 2019 at 13:46

answered Aug 17, 2016 at 7:50

Hamish

81.2k19 gold badges196 silver badges298 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Eneko Alonso Over a year ago

But, how do you use a closure so it does not allow to escape?

Hamish Over a year ago

@EnekoAlonso Not entirely sure what you're asking – you can either call a non-escaping function parameter directly in the function itself, or you can call it when captured in a non-escaping closure. In this case, as we're dealing with asynchronous code, there is no guarantee that the async function parameter (and therefore the completion function) will be called before fetchData exits – and therefore must be @escaping.

Sajjon Over a year ago

Feels ugly that we have to specify @escaping as method signature for protocols... is that what we should do? The proposal does not say! :S

Hamish Over a year ago

@Sajjon Currently, you do need to match an @escaping parameter in a protocol requirement with an @escaping parameter in the implementation of that requirement (and vice versa for non-escaping parameters). It was the same in Swift 2 for @noescape.

Peter Schorn Over a year ago

@EnekoAlonso See developer.apple.com/documentation/swift/…

Vitalii · Accepted Answer · 2017-10-12 16:30:47Z

Since @noescape is the default, there 2 options to fix the error:

1) as @Hamish pointed out in his answer, just mark the completion as @escaping if you do care about the result and really want it to escape (that probably is the case in @Lukasz's question with Unit Tests as example and possibility of async completion)

func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void)

OR

2) keep the default @noescape behavior by making the completion optional discarding the results altogether in cases when you don't care about the result. For example when user has already "walked away" and the calling view controller doesn't have to hang in memory just because there was some careless network call. Just as it was in my case when I came here looking for answer and the sample code wasn't very relevant for me, so marking @noescape was not the best option, event though it sounded as the only one from the first glance.

func fetchData(location: String, completion: ((DataFetchResult) -> Void)?) {
   ...
   completion?(self.result)
}

GDeep · Accepted Answer · 2023-03-16 18:42:44Z

0

Making completion block asoptional variable helped me to solve the issue in Swift 5.

 private func updateBreakTime(for id: String, to time: Time, onSucess: EmptyAction?) {
    dataRepository.updateBreak(
        id: id,
        to: time.seconds,
        onSuccess: { _ in
            onSucess?()
        },
        onError: { [weak self] error in
            self?.screen.showError(error)
        }
    )
}

Where the onSuccess of dataRepository was @escaping

answered Mar 16, 2023 at 18:42

GDeep

497 bronze badges

Collectives™ on Stack Overflow

Closure use of non-escaping parameter may allow it to escape

3 Answers 3

5 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

5 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related