Putting negative sign on iphone numberpad

I need to have a negative sign on my iphone numberpad. I figured out how to a button to a toolbar over the numberpad. Here's a code snippet:

UIToolbar* numberToolbar = [[UIToolbar alloc]initWithFrame:CGRectMake(0, 0, 320, 50)];
numberToolbar.barStyle = UIBarStyleBlackTranslucent;
numberToolbar.items = [NSArray arrayWithObjects:
                       [[UIBarButtonItem alloc]initWithImage:[UIImage imageNamed:@"pm.png"] style:UIBarButtonItemStylePlain target:self action:@selector(pm)],nil];

pm.png is a +/- graphic. Now, i have multiple UItextfields that could call this numberpad, say textfield1 textfield2 textfield3, etc.

How do I write the method pm such that it will add the negative sign to the right textfield. I'm missing something basic here. I want something like

-(void) pm {

}

but i don't know how to tell this method is the textfield that is currently being edited.

Thanks

Solution

You need to keep track of which text field is the first responder. One way to do this is to register for the UITextFieldTextDidBeginEditingNotification notification. This notification will include a reference to the text field that now has the focus. Keep track of this text field in an instance variable. Then your pm method can reference that instance variable.