Allow only Numbers for UITextField input

This is how you might handle the problem on a SSN verification field, you can modify the max length and remove the if statement checking for keyboard type if you need to.

There is also logic to suppress the max length alerts when the user is typing as opposed to pasting data.

Within the context of this code, presentAlert()/presentAlert: is just some basic function that presents a UIAlertController (or a legacy UIAlertView) using the message string passed.

Swift 5

// NOTE: This code assumes you have set the UITextField(s)'s delegate property to the 
// object that will contain this code, because otherwise it would never be called.
//
// There are also some better stylistic approaches in Swift to avoid all the 
// nested statements, but I wanted to keep the styles similar to allow others 
// to contrast and compare between the two languages a little easier.

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

    // Handle backspace/delete
    guard !string.isEmpty else {

        // Backspace detected, allow text change, no need to process the text any further
        return true
    }

    // Input Validation
    // Prevent invalid character input, if keyboard is numberpad
    if textField.keyboardType == .numberPad {

        // Check for invalid input characters
        if CharacterSet(charactersIn: "0123456789").isSuperset(of: CharacterSet(charactersIn: string)) {

            // Present alert so the user knows what went wrong
            presentAlert("This field accepts only numeric entries.")

            // Invalid characters detected, disallow text change
            return false
        }
    }

    // Length Processing
    // Need to convert the NSRange to a Swift-appropriate type
    if let text = textField.text, let range = Range(range, in: text) {

        let proposedText = text.replacingCharacters(in: range, with: string)

        // Check proposed text length does not exceed max character count
        guard proposedText.count <= maxCharacters else {

            // Present alert if pasting text
            // easy: pasted data has a length greater than 1; who copy/pastes one character?
            if string.count > 1 {

                // Pasting text, present alert so the user knows what went wrong
                presentAlert("Paste failed: Maximum character count exceeded.")
            }

            // Character count exceeded, disallow text change
            return false
        }

        // Only enable the OK/submit button if they have entered all numbers for the last four
        // of their SSN (prevents early submissions/trips to authentication server, etc)
        answerButton.isEnabled = (proposedText.count == 4)
    }

    // Allow text change
    return true
}

Objective-C

// NOTE: This code assumes you have set the UITextField(s)'s delegate property to the 
// object that will contain this code, because otherwise it would never be called.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    // Handle backspace/delete
    if (!string.length)
    {
        // Backspace detected, allow text change, no need to process the text any further
        return YES;
    }

    // Input Validation
    // Prevent invalid character input, if keyboard is numberpad
    if (textField.keyboardType == UIKeyboardTypeNumberPad)
    {
        if ([string rangeOfCharacterFromSet:[NSCharacterSet decimalDigitCharacterSet].invertedSet].location != NSNotFound)
        {
            [self presentAlert: @"This field accepts only numeric entries."];
            return NO;
        }
    }

    // Length Validation
    NSString *proposedText = [textField.text stringByReplacingCharactersInRange:range withString:string];

    // Check proposed text length does not exceed max character count
    if (proposedText.length > maxCharacters)
    {
        // Present alert if pasting text
        // easy: pasted data has a length greater than 1; who copy/pastes one character?
        if (string.length > 1)
        {
            // Pasting text, present alert so the user knows what went wrong
            [self presentAlert: @"Paste failed: Maximum character count exceeded."];
        }

        // Character count exceeded, disallow text change
        return NO;
    }

    // Only enable the OK/submit button if they have entered all numbers for the last four
    // of their SSN (prevents early submissions/trips to authentication server, etc)
    self.answerButton.enabled = (proposedText.length == maxCharacters);

    // Allow text change
    return YES;
}

Very specific steps for Swift code

You can provide logic that restricts the text field's input in the func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool method by implementing the UITextFieldDelegate protocol.

For the sake of clarity, these steps assume that your storyboard contains a View Controller with a text field object that should only accept digits.

1. Create a custom class for the view controller that extends UIViewController. Make sure that the scene in your storyboard refers to the custom class by setting the custom class value in Xcode's Identity Inspector.

   import UIKit
   class YourCustomController: UIViewController {
       override func viewDidLoad() {        
           super.viewDidLoad()
       }
   }
   

2. Create an outlet from your scene's text field to your custom View Controller.

   class YourCustomController: UIViewController {
       @IBOutlet weak var numberField: UITextField!
       ...
   }
   

3. Apply the UITextFieldDelegate protocol in your custom view controller.

   class YourCustomController: UIViewController, UITextFieldDelegate {
       ...
   }
   

4. In your custom view controller's viewDidLoad method, assign your text field's delegate to your custom view controller class.

   override func viewDidLoad() {        
       super.viewDidLoad()
       numberField.delegate = self
   }
   

5. Add the UITextFieldDelegate's func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool method.

   As a result of making your custom view controller the numberField's delegate in the previous step, this method will be called each time a user enters a character into the text field. If your method returns true then the character will remain in the text field. If your method returns false then the character will not remain in the text field.

   The string parameter is the character being entered by the user. If the string character can be converted to an Int then it is between 0 and 9; otherwise, it is some non-number character.

   class YourCustomController: UIViewController, UITextFieldDelegate {
       ...
       func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
   
           return Int(string) != nil
       }
   }
   

(See below for the full view controller code.)

---

Example View Controller with digits only text field

import UIKit

class YourCustomController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var numberField: UITextField!

    override func viewDidLoad() {        
        super.viewDidLoad()       
        numberField.delegate = self
    }

    func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {        
        return Int(string) != nil
    }    
}

---

Example View Controller with a Decimal text field

If you want to support a decimal number then take advantage of NSNumberFormatter. See the code comments for the differences.

import UIKit

class YourCustomController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var numberField: UITextField!

    private var formatter: NSNumberFormatter!

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

        // Initialize the formatter; minimum value is set to zero; style is Decimal. 
        formatter = NSNumberFormatter()
        formatter.numberStyle = NSNumberFormatterStyle.DecimalStyle
        formatter.minimum = 0
    }

    func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
        // Combine the current text field value and the new string
        // character. If it conforms to the formatter's settings then
        // it is valid. If it doesn't then nil is returned and the
        // string character should not be allowed in the text field.         
        return formatter.numberFromString("\(textField.text)\(string)") != nil
    }    
}

You can use this code to allow only number in textField.

Before that set delegate for textField

      textFieldName.delegate=self;

or

      [textFieldName setDelegate:self];

Than use this code to allow only digit to textField

      - (BOOL) textField: (UITextField *)theTextField shouldChangeCharactersInRange:(NSRange)range replacementString: (NSString *)string {
//return yes or no after comparing the characters

      // allow backspace
      if (!string.length)
      {
           return YES;
      }

      ////for Decimal value start//////This code use use for allowing single decimal value
      //    if ([theTextField.text rangeOfString:@"."].location == NSNotFound)
      //    {
      //        if ([string isEqualToString:@"."]) {
      //            return YES;
      //        }
      //    }
      //    else
      //    {
      //        if ([[theTextField.text substringFromIndex:[theTextField.text rangeOfString:@"."].location] length]>2)   // this allow 2 digit after decimal 
      //        {
      //            return NO;
      //        }
      //    }
      ////for Decimal value End//////This code use use for allowing single decimal value

      // allow digit 0 to 9
      if ([string intValue])
      {
            return YES;
      }

      return NO;
    }

Try this to avoid textfield clearing issue

Swift 3.0

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
    guard NSCharacterSet(charactersInString: "0123456789").isSupersetOfSet(NSCharacterSet(charactersInString: string)) else {
        return false
    }
    return true
}

Swift 4.0

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    guard CharacterSet(charactersIn: "0123456789").isSuperset(of: CharacterSet(charactersIn: string)) else {
        return false
    }
    return true
}