Swift delegation - when to use weak pointer on delegate

Question

Can someone explain when and when not to use a \'weak\' assignment to a delegate pointer in Swift, and why?

My understanding is that if you use a protocol that is n

Answer 1

As Rob said:

It's really a question of "ownership"

That's very true. 'Strong reference cycle' is all about getting that ownership right.

In the following example, we're not using weak var. Yet both objects will deallocate. Why?

protocol UserViewDelegate: class {
    func userDidTap()
}

class Container {
    let userView = UserView()
    let delegate = Delegate()
    init() {
        userView.delegate = delegate
    }

    deinit {
        print("container deallocated")
    }
}

class UserView {
    var delegate: UserViewDelegate?

    func mockDelegatecall() {
        delegate?.userDidTap()
    }

    deinit {
        print("UserView deallocated")
    }
}

class Delegate: UserViewDelegate {
    func userDidTap() {
        print("userDidTap Delegate callback in separate delegate object")
    }
}

Usage:

var container: Container? = Container()
container?.userView.mockDelegatecall()
container = nil // will deallocate both objects

Memory ownership graph (doesn't have cycle)

    +---------+container +--------+
    |                             |
    |                             |
    |                             |
    |                             |
    |                             |
    |                             |
    v                             v
userView +------------------> delegate

In order to create a strong reference cycle, the cycle needs be complete. delegate needs to point back to container but it doesn't. So this isn't an issue. But purely for ownership reasons and as Rob has said here:

In an object hierarchy, a child object should not maintain strong references to the parent object. That is a red flag, indicating a strong reference cycle

So regardless of leaking, still use weak for your delegate objects.

---

In the following example, we're not using weak var. As a result neither of the classes will deallocate.

protocol UserViewDelegate: class {
    func userDidTap()
}

class Container: UserViewDelegate {
    let userView = UserView()

    init() {
        userView.delegate = self
    }

    func userDidTap() {
        print("userDidTap Delegate callback by Container itself")
    }
    deinit {
        print("container deallocated")
    }
}

class UserView {
    var delegate: UserViewDelegate?

    func mockDelegatecall() {
        delegate?.userDidTap()
    }

    deinit {
        print("UserView deallocated")
    }
}

Usage:

var container: Container? = Container()
container?.userView.mockDelegatecall()
container = nil // will NOT deallocate either objects

Memory ownership graph (has cycle)

     +--------------------------------------------------+
     |                                                  |
     |                                                  |
     +                                                  v
 container                                           userview
     ^                                                  |
     |                                                  |
     |                                                  |
     +------+userView.delegate = self //container+------+

using weak var will avoid the strong reference cycle