ios9和ios8中的弱引用

Question

[Problem soluted!Just want to know why there is such a difference in ios8 and ios9] I was making a register view controller these days and face with some problem about weak reference.

and below is some part of the code(swift)

problem come when I use an iphone6 ios8.1 it crashed. And then I noticed that the weak reference is not proper here. But the code runs well in my ios9 iphone6s. I ran this code on an iphone6 ios8 simulator, app crashed. So I think there is some thing different in processing weak reference in ios8 and ios9, But who can explain why..?

class VC: UIViewController {
    weak var verifyTextField: UITextField?
    override func viewdidload() {
       //....
       verifyTextField = newTextField();
       view.addSubview(verifyTextField!);
    }
    func newTextField() -> UITextField {
        let ntf = UITextField();
        //do some settings to ntf;
        return ntf;
    }
}

Answer 1

You set your new UITextField instance to the weak var verifyTextField but before you add it as a subview (which increments the retain count) it is deallocated (the count is 0 since the var is weak) so verifyTextField! crashes, the crash you're getting is most likely the famous

Unexpectedly found nil while unwrapping an Optional

It's easy to fix it

1. Don't use a weak var
2. Don't force unwrap (use if let instead)

The code should be as follows:

class VC: UIViewController {
    var verifyTextField: UITextField? //should not be weak
    override func viewdidload() {
       //....
       verifyTextField = newTextField()
       if let verifyTextField = verifyTextField {
          view.addSubview(verifyTextField!)
       }
    }
    func newTextField() -> UITextField {
        let ntf = UITextField()
        //do some settings to ntf
        return ntf
    }
}

Answer 2

Looks like your object is deallocated instantly after initialization because you don't store any strong reference for it.

Try this code:

override func viewdidload() {
   //....
   let verifyTextField = newTextField();
   view.addSubview(verifyTextField);
   self.verifyTextField = verifyTextField;
}

Also no need to use weak reference here, because verifyTextField doesn't have reference to your VC, so you won't get a retain cycle.