如何使UIButton在所选的UITextField中插入否定符号？

I am creating an app for iOS8 in SWIFT that requires the user to input a negative symbol by pressing a UIButton. The keyboard I am using is the DecimalPad which does not have that option. I have multiple textfields that the button needs to work with. For example if a UITextField is selected and the user presses the "-" button then a "-" will be inserted into that textfield. I am having trouble with the UIButton knowing which UITextField is being selected. I am creating an app for iOS8 in SWIFT that req