如何在自定义键盘按钮swift中实现功能齐全的退格键

我为此挣扎了一段时间，以为我会回答这个问题。以下代码响应在作为UITextfield的第一响应者的uikeyboard上按下的自定义退格键：

// the tag for the backspace button
if tag == "<"{
        //the range of the selected text (handles multiple highlighted characters of a uitextfield)
        let range = textField.selectedTextRange?;

        // if the range is empty indicating the cursor position is 
        // singular (no multiple characters are highlighted) then delete the character 
        // in the position before the cursor
        if range!.empty {

            let start = textField.positionFromPosition(range!.start, offset: -1);
            let end = axiomBuilderTV.positionFromPosition(range!.end, offset: 0);
            axiomBuilderTV.replaceRange(axiomBuilderTV.textRangeFromPosition(start, toPosition: end), withText: "");
        }
        // multiple characters have been highlighted so remove them all
        else {
            textField.replaceRange(range!, withText: "");
        }

    }