Detect backspace in empty UITextField

Question

Is there any way to detect when the Backspace/Delete key is pressed in the iPhone keyboard on a UITextField that is empty? I want to know when Backspace is pressed only if the UITextField is empty.

---

Based on the suggestion from @Alex Reynolds in a comment, I've added the following code while creating my text field:

[[NSNotificationCenter defaultCenter] addObserver:self
          selector:@selector(handleTextFieldChanged:)
              name:UITextFieldTextDidChangeNotification
            object:searchTextField];

This notification is received (handleTextFieldChanged function is called), but still not when I press the Backspace key in an empty field. Any ideas?

---

There seems to be some confusion around this question. I want to receive a notification when the Backspace key is pressed. That's it. But the solution must also work when the UITextField is already empty.

Answer 1

Code like following:

@interface MyTextField : UITextField
@end

@implementation MyTextField

- (void)deleteBackward
{
    [super deleteBackward];

    //At here, you can handle backspace key pressed event even the text field is empty
}

@end

At last, do forget to change the Custom Class property of the Text Field to "MyTextField"

Answer 2

Niklas Alvaeus's answer helped me out with a similar issue

I was limiting entry to a specific character set, but it was ignoring backspaces. So I had it check range.length == 1 before trimming the NSString. If it is true, I just return the string and don't trim it. See below

 - (BOOL) textField:(UITextField *)textField 
          shouldChangeCharactersInRange:(NSRange)range 
          replacementString:(NSString *)string
 {
     NSCharacterSet *nonNumberSet = 
      [[NSCharacterSet characterSetWithCharactersInString:@"0123456789."] 
         invertedSet];

    if (range.length == 1) {
       return string;
    }
    else {
       return ([string stringByTrimmingCharactersInSet:nonNumberSet].length > 0);
    }   
 }

Answer 3

Update: See JacobCaraballo's answer for an example that overrides -[UITextField deleteBackward].

Check out UITextInput, specifically UIKeyInput has a deleteBackward delegate method that always gets called when the delete key is pressed. If you're doing something simple, then you might consider just subclassing UILabel and making it conform to the UIKeyInput protocol, as done by SimpleTextInput and this iPhone UIKeyInput Example. Note: UITextInput and its relatives (including UIKeyInput) are only available in iOS 3.2 and later.

Answer 4

I have implemented the similar solution with minor improvements that will tell me that if the text field has any value while the user has tapped the backspace. This is useful for my case when I should only focus on another text field if the text field is empty when backspace pressed.

protocol MyTextFieldDelegate : UITextFieldDelegate {
    func textFieldDidDelete(textField: MyTextField, hasValue: Bool)
}

override func deleteBackward() {
    let currentText = self.text ?? ""
    super.deleteBackward()
    let hasValue = currentText.isEmpty ? false : true
    if let delegate = self.delegate as? MyTextFieldDelegate {
        delegate.textFieldDidDelete(textField: self, hasValue: hasValue)
    }
}

Answer 5

Swift 4:

---

Subclass UITextField:

// MyTextField.swift
import UIKit

protocol MyTextFieldDelegate {
    func textFieldDidDelete()
}

class MyTextField: UITextField {

    var myDelegate: MyTextFieldDelegate?

    override func deleteBackward() {
        super.deleteBackward()
        myDelegate?.textFieldDidDelete()
    }

}

Implementation:

// ViewController.swift

import UIKit

class ViewController: UIViewController, MyTextFieldDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

        // initialize textField
        let input = MyTextField(frame: CGRect(x: 50, y: 50, width: 150, height: 40))

        // set viewController as "myDelegate"
        input.myDelegate = self

        // add textField to view
        view.addSubview(input)

        // focus the text field
        input.becomeFirstResponder()
    }

    func textFieldDidDelete() {
        print("delete")
    }

}

---

Objective-C:

---

Subclass UITextField:

//Header
//MyTextField.h

//create delegate protocol
@protocol MyTextFieldDelegate <NSObject>
@optional
- (void)textFieldDidDelete;
@end

@interface MyTextField : UITextField<UIKeyInput>

//create "myDelegate"
@property (nonatomic, assign) id<MyTextFieldDelegate> myDelegate;
@end

//Implementation
#import "MyTextField.h"

@implementation MyTextField

- (void)deleteBackward {
    [super deleteBackward];

    if ([_myDelegate respondsToSelector:@selector(textFieldDidDelete)]){
        [_myDelegate textFieldDidDelete];
    }
}

@end

Now simply add MyTextFieldDelegate to your UIViewController and set your UITextFields myDelegate to self:

//View Controller Header
#import "MyTextField.h"

//add "MyTextFieldDelegate" to you view controller
@interface ViewController : UIViewController <MyTextFieldDelegate>
@end

//View Controller Implementation
- (void)viewDidLoad {
    //initialize your text field
    MyTextField *input = 
     [[MyTextField alloc] initWithFrame:CGRectMake(0, 0, 70, 30)];

    //set your view controller as "myDelegate"
    input.myDelegate = self;

    //add your text field to the view
    [self.view addSubview:input];
}

//MyTextField Delegate
- (void)textFieldDidDelete {
    NSLog(@"delete");
}