How to add a 'Done' button to numpad keyboard in iOS

The much safer approach is to use a UIToolBar with Done Button as inputAccessoryView.

Sample Code :

UIToolbar *keyboardDoneButtonView = [[UIToolbar alloc] init];
[keyboardDoneButtonView sizeToFit];
UIBarButtonItem *doneButton = [[UIBarButtonItem alloc] initWithTitle:@"Done"
                                                               style:UIBarButtonItemStyleBordered target:self
                                                              action:@selector(doneClicked:)];
[keyboardDoneButtonView setItems:[NSArray arrayWithObjects:doneButton, nil]];
txtField.inputAccessoryView = keyboardDoneButtonView;

Your -doneClicked method should look like this :

- (IBAction)doneClicked:(id)sender
{
    NSLog(@"Done Clicked.");
    [self.view endEditing:YES];
}

Sample Code Swift:

let keyboardDoneButtonView = UIToolbar.init()
keyboardDoneButtonView.sizeToFit()
let doneButton = UIBarButtonItem.init(barButtonSystemItem: UIBarButtonSystemItem.Done, 
                                                   target: self, 
                                                   action: Selector("doneClicked:")))    

keyboardDoneButtonView.items = [doneButton]
textFieldInput.inputAccessoryView = keyboardDoneButtonView

Your -doneClicked method should look like this :

func doneClicked(sender: AnyObject) {
  self.view.endEditing(true)
}

Even easier way:

Swift 3.0 and above:

func addDoneButton() {
    let keyboardToolbar = UIToolbar()
    keyboardToolbar.sizeToFit()
    let flexBarButton = UIBarButtonItem(barButtonSystemItem: .flexibleSpace,
        target: nil, action: nil)
    let doneBarButton = UIBarButtonItem(barButtonSystemItem: .done,
        target: view, action: #selector(UIView.endEditing(_:)))
    keyboardToolbar.items = [flexBarButton, doneBarButton]
    textField.inputAccessoryView = keyboardToolbar
}

Swift 2.3 and below:

func addDoneButton() {
    let keyboardToolbar = UIToolbar()
    keyboardToolbar.sizeToFit()
    let flexBarButton = UIBarButtonItem(barButtonSystemItem: .FlexibleSpace,
        target: nil, action: nil)
    let doneBarButton = UIBarButtonItem(barButtonSystemItem: .Done,
        target: view, action: #selector(UIView.endEditing(_:)))
    keyboardToolbar.items = [flexBarButton, doneBarButton]
    textField.inputAccessoryView = keyboardToolbar
}

Objective C:

- (void)addDoneButton {
    UIToolbar* keyboardToolbar = [[UIToolbar alloc] init];
    [keyboardToolbar sizeToFit];
    UIBarButtonItem *flexBarButton = [[UIBarButtonItem alloc]
    initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace
    target:nil action:nil];
    UIBarButtonItem *doneBarButton = [[UIBarButtonItem alloc]
    initWithBarButtonSystemItem:UIBarButtonSystemItemDone
    target:self.view action:@selector(endEditing:)];
    keyboardToolbar.items = @[flexBarButton, doneBarButton];
    self.textField.inputAccessoryView = keyboardToolbar;
}

EDIT:

I've created a useful library called DCKit, which already have the toolbar out of the box:

It also has many other cool features.

This is a simple way of projecting a done button in iOS7 num-keypad. In the below delegate method of UITextField, add a notification for keyboard show.

-(void)textFieldDidBeginEditing:(UITextField *)textField {
    
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];
}

Now implement the method keyboardWillShow as below. Here we need to take extra care for iOS7.

- (void)keyboardWillShow:(NSNotification *)note {
    // create custom button
    UIButton *doneButton = [UIButton buttonWithType:UIButtonTypeCustom];
    doneButton.frame = CGRectMake(0, 163, 106, 53);
    doneButton.adjustsImageWhenHighlighted = NO;
    [doneButton setImage:[UIImage imageNamed:@"doneButtonNormal.png"] forState:UIControlStateNormal];
    [doneButton setImage:[UIImage imageNamed:@"doneButtonPressed.png"] forState:UIControlStateHighlighted];
    [doneButton addTarget:self action:@selector(doneButton:) forControlEvents:UIControlEventTouchUpInside];
    
    if (SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"7.0")) {
        dispatch_async(dispatch_get_main_queue(), ^{
            UIView *keyboardView = [[[[[UIApplication sharedApplication] windows] lastObject] subviews] firstObject];
            [doneButton setFrame:CGRectMake(0, keyboardView.frame.size.height - 53, 106, 53)];
            [keyboardView addSubview:doneButton];
            [keyboardView bringSubviewToFront:doneButton];
            
            [UIView animateWithDuration:[[note.userInfo objectForKey:UIKeyboardAnimationDurationUserInfoKey] floatValue]-.02
                                  delay:.0
                                options:[[note.userInfo objectForKey:UIKeyboardAnimationCurveUserInfoKey] intValue]
                             animations:^{
                                 self.view.frame = CGRectOffset(self.view.frame, 0, 0);
                             } completion:nil];
        });
    } else {
        // locate keyboard view
        dispatch_async(dispatch_get_main_queue(), ^{
            UIWindow* tempWindow = [[[UIApplication sharedApplication] windows] objectAtIndex:1];
            UIView* keyboard;
            for(int i=0; i<[tempWindow.subviews count]; i++) {
                keyboard = [tempWindow.subviews objectAtIndex:i];
                // keyboard view found; add the custom button to it
                if([[keyboard description] hasPrefix:@"UIKeyboard"] == YES)
                    [keyboard addSubview:doneButton];
            }
        });
    }
}

Now add this macro to suitable header to detect the SYSTEM_VERSION

#define SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(v)  ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] != NSOrderedAscending)