非可选代表的弱者与无者

Question

If my viewcontroller must be initialized with a delegate, is there any danger at all to doing using an unowned instead?

Using weak seems to introduce the probability of functions failing (see below), although it will not crash.

Would using unowned in this case be unsafe in anyway?

class MyViewController: UIViewController
  private weak var delegate: MyViewControllerDelegate?
  init(delegate: MyViewControllerDelegat) {
    self.delegate = delegate
  }
  func foobar {
    delegate??
  }

compared to

class MyViewController: UIViewController
  private unowned var delegate: MyViewControllerDelegate
  init(delegate: MyViewControllerDelegate) {
    self.delegate = delegate
  }
  func foobar {
    delegate.doAction()
  }

Answer 1

The Apple Documentation says

Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialization.

So in your case, if you know that the delegate will be there for the whole lifetime then use unowned .

The difference between weak and unowned is that weak can be nil and unowned cannot be nil. Both do not add to the retain cycle and there is no danger with unowned as long as you do not deallocate the delegate.

Answer 2

If your controller must be initialized with a delegate and the controller cannot work without it then unowned is the correct solution. However, you have to make sure that the delegate is never deallocated before your controller is deallocated. Typically, the delegate should be the owner of your controller.

However, using weak is not complicated either:

delegate?.doAction()