Uppercase characters in UItextfield

Set your textfield type autocapitalizationType to UITextAutocapitalizationTypeAllCharacters on the UITextField

self.yourTexField.autocapitalizationType = UITextAutocapitalizationTypeAllCharacters;

After call delegate

// delegate method

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSRange lowercaseCharRange = [string rangeOfCharacterFromSet:[NSCharacterSet lowercaseLetterCharacterSet]];

    if (lowercaseCharRange.location != NSNotFound) {
        textField.text = [textField.text stringByReplacingCharactersInRange:range
                                                                 withString:[string uppercaseString]];
        return NO;
    }

    return YES;
}

Swift 5.4.2

       self.yourTextField.autocapitalizationType = .allCharacters

One issue I have with some of the above answers is if you try and set textfield.text, you will lose the cursor position. So if a user tries to edit the middle of the text, the cursor will jump to the end.

Here is my Swift solution, still using UITextFieldDelegate:

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {

    if textField == textFieldToUppercase {
        if string == "" {
            // User presses backspace
            textField.deleteBackward()
        } else {
            // User presses a key or pastes
            textField.insertText(string.uppercaseString)
        }
        // Do not let specified text range to be changed
        return false
    }

    return true
}