swift - How to prevent strong reference cycles when using Apple's new Combine framework (.assign is causing problems)

Question

I don't quite understand how to properly store subscribers inside a class so that they persist but don't prevent the object from being deinitialized. Here's an example where the object won't deinit:

import UIKit
import Combine

class Test {
    public var name: String = ""

    private var disposeBag: Set<AnyCancellable> = Set()

    deinit {
        print("deinit")
    }

    init(publisher: CurrentValueSubject<String, Never>) {
        publisher.assign(to: .name, on: self).store(in: &disposeBag)
    }
}

let publisher = CurrentValueSubject<String, Never>("Test")

var test: Test? = Test(publisher: publisher)
test = nil

When I replace the assign with a sink (in which I properly declare [weak self]) it actually does deinit properly (probably because the assign accesses self in a way that causes problems).

How can I prevent strong reference cycles when using .assign for instance?

Thanks

Answer 1

you can replace .asign(to:) with sink where [weak self] in its closure brake the memory cycle. Try it in Playground to see the difference

final class Bar: ObservableObject {
    @Published var input: String = ""
    @Published var output: String = ""

    private var subscription: AnyCancellable?

    init() {
        subscription = $input
            .filter { $0.count > 0 }
            .map { "($0) World!" }
            //.assignNoRetain(to: .output, on: self)
            .sink { [weak self] (value) in
                self?.output = value
        }

    }

    deinit {
        subscription?.cancel()
        print("(self): (#function)")
    }
}

// test it!!
var bar: Bar? = Bar()
let foo = bar?.$output.sink { print($0) }
bar?.input = "Hello"
bar?.input = "Goodby,"
bar = nil

it prints

Hello World!
Goodby, World!
__lldb_expr_4.Bar: deinit

so we don't have the memory leak !

finally at forums.swift.org someone make a nice little

extension Publisher where Self.Failure == Never {
    public func assignNoRetain<Root>(to keyPath: ReferenceWritableKeyPath<Root, Self.Output>, on object: Root) -> AnyCancellable where Root: AnyObject {
        sink { [weak object] (value) in
        object?[keyPath: keyPath] = value
    }
  }
}