Custom keyboard iphone , having problem with backspace button in UITextView

Check this code (My Custom Keyboard):

-(IBAction) updateTextBackSpace:(id)sender
{
    if([txtview.text length]>0)
    {
        NSString *deletedLastCharString = [txtview.text substringToIndex:([txtview.text length]-1)];
        [txtview setText:deletedLastCharString];
    }
    else
    {
        return nil;
    }
}  

The thing is that I can't figure out h开发者_如何学运维ow to change this code so that. I can erase any text in any give line at the cursor, the backspace starts to erase from end of the line. I should be able to erase(backspace) from the cursor location.

---

replace this

NSString *deletedLastCharString = [txtview.text substringToIndex:([txtview.text length]-1)];

with

NSRange range  = [txtview selectedRange];
NSString *deletedLastCharString = [txtview.text substringToIndex:([range.location]-1)];