Can I detect the delete key even if the UITextField is empty?

When UITextField contains nothing, pressing the delete key of keyboard won't call any of UITextFieldDelegate's methods.

How can I detect it?

I'm adding this answer for a more complete example in Swift 3. Basically, I needed a pincode-type view where I have multiple text fields that allow one character in each cell.

like this Start by creating a subclass of UITextField and a protocol that defines a new func.

protocol MYDeleteActionTextFieldDelegate {
    func textFieldDidSelectDeleteButton(_ textField: UITextField) -> Void
}
class MYDeleteActionTextField: UITextField {
    var deleteDelegate: MYDeleteActionTextFieldDelegate?
    override func deleteBackward() {
        // Need to call this before super or the textfield edit may already be in place
        self.deleteDelegate?.textFieldDidSelectDeleteButton(self)
        super.deleteBackward()
    }
}

Then you create the text fields with the new subclass and implement the delegate in your view controller. In my case, I manage the textfields in an array for ease of use and layout the cells with PureLayout. I store them like this

var pinFields = [UITextField]()

Then in viewDidLoad(), I add all the pin fields into the array like so:

for _ in 1...6 {
            let field = EDFDeleteActionTextField.init(forAutoLayout: ())
            field.addTarget(self, action: #selector(textFieldDidChange(textField:)), for: UIControlEvents.editingChanged)
            field.delegate = self
            field.deleteDelegate = self
            field.textAlignment = .center
            field.font = UIFont.newBigTitle()
            field.textColor = UIColor.edfBlue()
            field.backgroundColor = UIColor.edfWhite()
            self.pinFields.append(field)
            self.pinView.addSubview(field)
        }

Now you just need to respond to all the appropriate delegate methods and the textFieldDidChange target that was added above.

// MARK: UITextFieldDelegate
    func textFieldDidChange(textField: UITextField) {
        // If the user typed one character, move to the next cell.
        if (textField.text?.characters.count == 1) {
            let index = pinFields.index(of: textField)
            textField.resignFirstResponder()
            if (pinFields.count > index! + 1) {
                pinFields[index! + 1].becomeFirstResponder()
            }
        } // If they deleted the character move to previous cell
        else if (textField.text?.characters.count == 0) {
            let index = pinFields.index(of: textField)
            if (index! - 1 >= 0) {
                pinFields[index! - 1].becomeFirstResponder()
            }
        }
    }
    
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        if range.location > 0 {
            let index = pinFields.index(of: textField)
            // If there is already text in the text field and the next cell is empty - move the newly typed character to that cell.
            if (pinFields.count > index! + 1) {
                let nextField = pinFields[index! + 1]
                if (nextField.text?.characters.count == 0) {
                    textField.resignFirstResponder()
                    nextField.becomeFirstResponder()
                    nextField.text = string
                }
            }
            return false
        }
        return true
    }
    
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        textField.resignFirstResponder()
        return false
    }
    
    // MARK: EDFDeleteActionTextFieldDelegate
    func textFieldDidSelectDeleteButton(_ textField: UITextField) {
        // If user clicked delete, and there are no characters, move to previous cell if available.
        // If there are characters, it is handled in UITextFieldDelegate
        if (textField.text?.characters.count == 0) {
            let index = pinFields.index(of: textField)
            if (index! - 1 >= 0) {
                pinFields[index! - 1].becomeFirstResponder()
            }
            else {
                textField.resignFirstResponder()
            }
        }
    }

I'll leave out the boring parts (like laying out text fields etc), since this general functionality is useful in more cases than this pincode view, but implementing this child class and protocol should give all the functionality that you would need for similar type views and solve for the question at hand (which is probably needing something similar).

Happy coding.

This seems doable by subclassing UITextField. UITextField conforms to a protocol called UITextInput which inturn conforms to another protocol called UIKeyInput. UIKeyInput protocol has a method deleteBackward which fires every time the backspace key in keyboard is pressed.

Here is how it looks

Header

#import <UIKit/UIKit.h>

@interface EmptyDeleteTextField : UITextField

@end

Implementation

- (void)deleteBackward
{
    NSLog_SM(@"Length: %d", (int)self.text.length);
    [super deleteBackward];
}

PS: Don't forget to call super because you don't want to mess with the default behaviour of UITextField.