Clear UITextField when standard number pad is pressed

Question

Hi all,

I'm sorry if I didn't explain my problem well enough in the title.

In my application, I'm using a small subview to create a very basic calculator on the top side of the screen (I just have an UITextField to show the operations and two buttons). Instead of using a custom keyboard for it, I want to use the standard iPhone number pad.

My problem is that after doing an operation (e.g. adding two numbers) and showing the result, I cannot figure out how to clear the screen when the user enters a new number.

So, for example:

• User enters "6" -> Number 6 is shown in UITextField
• User selects "+" -> UITextField is cleared to make room for the next number
• User enters "10" -> Number 10 is shown in UITextField

• User selects "+" -> 16 is shown as the result of the previous operation, and should stay there until another number is pressed (he wants to continue adding more numbers)

• User enters "5"

At this point, if I was using a custom keyboard, I could clear the UITextField as soon as the button "5" is pressed by the user, but I cannot figure out how to do so when using the standard number pad. So, the result I get at the moment is "165".

Is there a way to detect when a key is pressed in the standard number pad so that I can clear the UITextField before the new number appears? I thought there may be a NSNotification for that, but I couldn't find it.

I'm aware that I could solve my problem if I created a custom keyboard or if I used two separated UITextFields (one for the operations and another one for the total), but I would like to use the standard number pad if it's possible.

Thanks very much!

Answer 1

Set a delegate for the UITextField and use shouldChangeCharactersInRange:replacementString:

This will trap every individual keystroke enabling you to do what you want. See

http://developer.apple.com/iphone/library/documentation/uikit/reference/UITextFieldDelegate_Protocol/UITextFieldDelegate/UITextFieldDelegate.html#//apple_ref/occ/intfm/UITextFieldDelegate/textField:shouldChangeCharactersInRange:replacementString:

If I've understood you correctly that should cover it.