Set UIPopover arrow direction and position manually

I'm using UIBarButtonItem. In my scenario I want to show a popover from every bar button when I double-click that bar button. So I use a UITapGesture from that UIBarButtonItem. But, the popover arrow always appear in the middle of all UIBarButtonItem.

I can't make the arrow appear in the middle of each bar button. My friend was telling me to set the arrow direction using point, but I don't know how to do that.

How can I set both the position and direction of the popover's arrow?