Combine: Sink/Assign not called when not assigned to a variable

[hfhbd]

Hey,

I want to request https://swift.org. If I do not assign the result of the publisher/subject sink(receiveCompletion:receiveValue:) of type AnyCancellable, the request will not executed.

Do you know if this behaviour is correct? Or does the compiler removes this code?

import SwiftUI
import Combine

struct ContentView: View {
    @ObservedObject private var viewModel = ViewModel()
    
    var body: some View {
        Form {
            Text(self.viewModel.content)
            Text(self.viewModel.output)
            Button("noVar") {
                self.viewModel.noVar()
            }
            Button("variable") {
                self.viewModel.variable()
            }
        }
    }
}
final class ViewModel: ObservableObject {
    @Published var content = "Loading swift.org"
    @Published var output = "no output"
    private var cancellable: AnyCancellable! = nil
    
    func noVar() {
        self.output = "noVar start"
        _ = URLSession.shared.dataTaskPublisher(for: URL(string: "https://swift.org")!) //never called
            .receive(on: RunLoop.main)
            .tryMap { (data, response) in
                self.output = "_gotRespond"
                let result = String(data: data, encoding: .utf8)!
                print(result) // no print
                self.output = "_isOnline \(result)"
                return result
            }
            .mapError { error -> Error in
                self.output = error.localizedDescription
                return error
            }
            .sink { completion in
                switch completion {
                case .failure(let error):
                    self.output = error.localizedDescription
                case .finished:
                    self.output = "noVar ends"
                }
            } receiveValue: {
                self.content = $0
            }
    }
    
    func variable() {
        self.output = "variable"
        self.cancellable = URLSession.shared.dataTaskPublisher(for: URL(string: "https://swift.org")!)
            .receive(on: RunLoop.main)
            .tryMap { (data, response) in
                self.output = "v_gotRespond"
                let result = String(data: data, encoding: .utf8)!
                print(result)
                self.output = "v_isOnline \(result)"
                return result
            }
            .mapError { error -> Error in
                self.output = error.localizedDescription
                return error
            }
            .replaceError(with: "v_Error printed")
            .assign(to: \.content, on: self)
    }
    
    deinit {
        print("canceling")
        self.cancellable.cancel()
    }
}

[Jon_Shier]

This is expected behavior. The lifetime of the cancellation token controls the lifetime of the observation and underlying work, like the network request.

[hfhbd]

Thank you! I think, maybe this behaviour was changed, because my old code, created 2019 with Combine (iOS 13) does not execute the publisher anymore... But now, the reason why is clear and logically.

[Jon_Shier]

This behavior is racy between the cancellation of the token once it's deinitd and the completion of the network request, so I would expect differing behavior through a variety of variables.