swift UITextFieldDelegate extension

Question

I am fairly new with swift and I can't understand the following situation. I am trying to extends the UIViewController class with with a couple of UITextFieldDelegate functions...

class ViewController: UIViewController{
    @IBOutlet weak var textField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()
        textField.delegate = self

In another file, if I define the function with the parameter unwrapped, the function is not called...

extension UIViewController: UITextFieldDelegate{

    internal func textFieldShouldBeginEditing(_ textField: UITextField) -> Bool {
        print("Text Field Should Begin Editing called")
        return true
    }

but if I unwrap the parameter, it WORKS.

    internal func textFieldShouldBeginEditing(_ textField: UITextField!) -> Bool 

can you please help me to understand why? thank you

swift 4.2

Answer 1

Because Objective-C does not make any guarantees that an object is non-nil, Swift makes all classes in argument types and return types optional in imported Objective-C APIs. Before you use an Objective-C object, you should check to ensure that it is not missing.

If you use _ textField: UITextField the parameter can be null, hence the method is not invoked by the delegate as its looking for a non null value within method signature i.e internal func textFieldShouldBeginEditing(_ textField: UITextField!) -> Bool