内部功能保留周期

Question

I was wondering how to avoid retain cycle in the following scenario:

private func setupDismissCallbacks() {

  // inner func     
  func dismiss() {
     self.videoExporter?.cancel()
     self.rootViewController.dismiss(animated: true, completion: nil)
     self.delegate?.childCoordinatorDidFinish(self)
  }

  // first clousre       
  saveModalViewController.onButtonDismiss = {  [weak self] in
     // not really using `self` here
     guard let self = self else { return }
     dismiss()
  }

  // second clousre  
  saveModalViewController.onDimmedAreaDismiss = { [weak self] in
     // not really using `self` here
     guard let self = self else { return }
     dismiss()
  }

}

I have a function setupDismissCallbacks that listens to two callbacks from a saveModalViewController self property. dismiss() is an inner function inside setupDismissCallbacks that I'm using to access self values.

But inside the closures onButtonDismiss and onDimmedAreaDismiss I don't access self to call dismiss , and I can't add [weak self] into dismiss function because it's a function.

How can I verify that the calls inside dismiss won't cause retain cycle?

Answer 1

Just assign your closure to a local variable. At that point, extracting out dismiss is pointless, so just inline it:

private func setupDismissCallbacks() {     
    let dismissCallback: () -> Void = {  [weak self] in
        guard let self = self else { return }
        self.videoExporter?.cancel()
        self.rootViewController.dismiss(animated: true, completion: nil)
        self.delegate?.childCoordinatorDidFinish(self)
    }

    saveModalViewController.onButtonDismiss = dismissCallback
    saveModalViewController.onDimmedAreaDismiss = dismissCallback
}

Answer 2

@Alexander explained the issue in the comments:

Your inner function captures self . Your two closures capture that inner function (including the context it encloses over, which includes self ). Since your two closures are strongly referenced by saveModalViewController (which I assume is strongly referenced by self ), you have a retain cycle.

You can break this cycle by not having dismiss capture self . Pass the SaveCoordinator to dismiss :

private func setupDismissCallbacks() {     
  func dismiss(_ coordinator: SaveCoordinator) {
     coordinator.videoExporter?.cancel()
     coordinator(animated: true, completion: nil)
     coordinator.delegate?.childCoordinatorDidFinish(coordinator)
  }

  // first clousre       
  saveModalViewController.onButtonDismiss = {  [weak self] in
     guard let self = self else { return }
     dismiss(self)
  }

  // second clousre  
  saveModalViewController.onDimmedAreaDismiss = { [weak self] in
     guard let self = self else { return }
     dismiss(self)
  }

}