Illustrate Memory leak in Publisher.replaceError

ContentView.swift

//
//  ContentView.swift
//  Bug-Combine-Leak
//
import Combine
import SwiftUI

final class Store: ObservableObject {

    struct User: Codable {
        let login: String
        let avatar_url: String
        let name: String?
    }

    @Published var user: User?
    @Published var userImage: UIImage = UIImage(systemName: "photo")!

    var cancellables = Set<AnyCancellable>()

    let baseURL = URL(string: "https://api.github.com/")!

    func fetchUserInfo(_ user: String) {
        let request = URLRequest(url: baseURL.appendingPathComponent("users/\(user)"))
        URLSession.shared
            .dataTaskPublisher(for: request)
            .map { $0.data }
            .decode(type: User?.self, decoder: JSONDecoder())
            .replaceError(with: nil)   // FIXME
            .receive(on: RunLoop.main)
            .sink(receiveCompletion: { (result) in
                print("fetchUserInfo result", result)
            }) { (user: User?) in
                self.user = user
            }
            .store(in: &cancellables)
    }

    func fetchAvatar() {
        if let avatarUrl = URL(string: user?.avatar_url ?? "") {
            URLSession.shared
                .dataTaskPublisher(for: URLRequest(url: avatarUrl))
                .map { $0.data }
                .tryMap { UIImage(data: $0)! }
                .replaceError(with: UIImage(systemName: "photo")!)   // FIXME
                .receive(on: DispatchQueue.main)
                .assign(to: \.userImage, on: self)
                .store(in: &cancellables)
        }
    }
}

struct ContentView: View {
    @ObservedObject var store: Store

    @State private var username = "pd95"
    @State private var loadAvatar = false

    var body: some View {
        VStack {
            Text("Who is?")
                .font(.largeTitle)
                .bold()

            TextField("Github username", text: $username, onCommit: fetchUser)
                .textContentType(.username)
                .font(.headline)
                .padding()
                .background(RoundedRectangle(cornerRadius: 8).stroke(Color.secondary))

            HStack {
                Button(action: fetchUser) {
                    Text("Check")
                        .font(.headline)
                        .padding()
                        .background(RoundedRectangle(cornerRadius: 8).fill(Color.blue))
                        .foregroundColor(Color.white)
                }
                .padding()

                if store.user != nil {
                    Button(action: fetchAvatar) {
                        Text("Load Avatar")
                            .font(.headline)
                            .padding()
                            .background(RoundedRectangle(cornerRadius: 8).fill(Color.yellow))
                    }
                    .padding()
                }
            }

            Divider()

            if store.user != nil {
                Text(store.user?.login ?? "n/a")
                Text(store.user?.name ?? "n/a")

                if loadAvatar {
                    Image(uiImage: store.userImage)
                        .resizable()
                        .scaledToFit()
                }
            }
        }
        .padding()
    }

    func fetchUser() {
        loadAvatar = false
        self.store.fetchUserInfo(self.username)
    }

    func fetchAvatar() {
        loadAvatar = true
        self.store.fetchAvatar()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(store: Store())
    }
}

This code illustrates a memory leak bug I've come across while using SwiftUI and Combine.

The example does the following:

1. Ask for a username (default pd95)
2. After pressing the button "Check", a query is run to get the user record from GitHub for the given username. Upon success, the button "Load Avatar" appears.
3. After pressing the button "Load Avatar", the avatar image for the given user is load.

When checking for memory leaks on each step, you will see that after "Check" there are 3 leaks and it's clear from the details that they are caused by Store.fetchUserInfo.

And after "Load Avatar" I see 6 memory issues caused by Store.fetchAvatar.

To fix those issues, we have to comment .replaceError(with: ...) (on line 29 and line 45) .

For me, this tastes like a real bug in Combine framework.

Any comments are welcome :)