How do I detect a touch over a specific area

Question

Currently I see that a touch event will show me the UIView where the touch occured. But what if I need to detect a touch of some non rectangular shape, like a circle. How would I go about doing something like that ?

Basically I want to do something only if the user touches somewhere within a circular area that's not visible.

Any help/direction is appreciated, TIA!

Answer 1

You would do it like so. Note that 'locationInView' will return the coordinates of the touch with respect to the specified view, so a touch in the top-left corner of a view will return (0,0) regardless of where that view is onscreen.

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event 
{   
  UITouch *touch = [touches anyObject];

  // gets the coordinats of the touch with respect to the specified view. 
  CGPoint touchPoint = [touch locationInView:self];

  // test the coordinates however you wish, 
  ...
}

To test against a sphere you would calculate the distance from the touch point to the center of the sphere, then check whether this was less than the sphere radius.