Asynchronous processing executed in Future in Swift Combine is sinking, but it is canceled in the middle

I was addicted to using Combine, so I will leave it as a memorandum.

problem

Set the closure (subscriber) by sink to the instance of Future, and when the asynchronous process is completed, perform the closure set by sink. I want to, but Subscribe is canceled before it completes.

func start() {
        Future<Void, Error>{ promise in
            DispatchQueue.main.asyncAfter(deadline: .now() + 5.0){
                promise(.success(()))
            }
        }
        .handleEvents(receiveSubscription: { _ in
            print("receiveSubscription")
        }, receiveOutput: {
            print("receiveOutput")
        }, receiveCompletion: { _ in
            print("receiveCompletion")
        }, receiveCancel: {
            print("receiveCancel")
        }, receiveRequest: { _ in
            print("receiveRequest")
        })
        //Closure does not run
        .sink(receiveCompletion: { completion in
            print("completion:\(completion)")
        }, receiveValue: {
            
        })
    }

Debug console

receiveSubscription
receiveRequest
receiveCancel

Solutions

You need to keep the return value of the sink method in a variable or use the [store](https://developer.apple.com/documentation/combine/anycancellable/store(in:) -6cr9i) method.

func start() {
        cancellable = Future<Void, Error>{ promise in
            DispatchQueue.main.asyncAfter(deadline: .now() + 5.0){
                promise(.success(()))
            }
        }
        .handleEvents(receiveSubscription: { _ in
            print("receiveSubscription")
        }, receiveOutput: {
            print("receiveoutput")
        }, receiveCompletion: { _ in
            print("receiveCompletion")
        }, receiveCancel: {
            print("receiveCancel")
        }, receiveRequest: { _ in
            print("receiveRequest")
        })
        .sink(receiveCompletion: { completion in
            print("completion:\(completion)")
        }, receiveValue: {
            
        })
    }

Debug console

receiveSubscription
receiveRequest
receiveoutput
receiveCompletion
completion:finished

Cause

It seems that it will be canceled if AnyCancellable returned from sink is destroyed before the process implemented in Future is completed (promise () is called). When I read the official reference, it was quietly written in the "Return Value" section.

https://developer.apple.com/documentation/combine/future/sink(receivecompletion:receivevalue:)