UIView with Png image - ignore touch in transparent areas - iphone

Question

I have a PNG of a circle with a transparent background added as a subview. I'm using this type of method to rotate it:
CGPoint location = [touch locationInView:self.view];
if(CGRectContainsPoint(wheelfrom.frame, location))
{
}
the problem is that the transparent area's of the image are registering as part of the UIView. Is there a way to ignore those area's when touched? Is there a better way for me to set up the UIView to recognize the transparency?
thanks!

Answer 1

you can check the rbga pixel colour of the image and see if a (=alpha value) is == 0 (or <= aLowValue)... as suggested by Igor Pchelko...
but in your case it may be easier...
you are using a 2D circle, so just check how the finger click is far from the circle center and see if it's out of its radius... just a Pitagora's theorem application...
EDIT:
ok, so, if you create a new class for your button subclassing UIButton:
in YourButton.h:
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#interface YourButton : UIButton {
}
#end
in YourButton.m just add this code:
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
UITouch *touch = [[event allTouches] anyObject];
CGPoint touchPoint = [touch locationInView:self];
NSLog(#"Touch x : %f y : %f", touchPoint.x, touchPoint.y);
float circleRadius = self.frame.size.height / 2; // considering a circle inscricted in a quadRect (width = height)
float deltaTouchOnCenterX = touchPoint.x - circleRadius;
float deltaTouchOnCenterY = touchPoint.y - circleRadius;
float touchDistanceFromCenter = sqrt((deltaTouchOnCenterX * deltaTouchOnCenterX) + (deltaTouchOnCenterY * deltaTouchOnCenterY) );
// or: float touchDistanceFromCenter = hypot(deltaTouchOnCenterX,deltaTouchOnCenterY);
NSLog(#"sqrt_val: %f", touchDistanceFromCenter);
NSLog(#"Touch on center x : %f y : %f", deltaTouchOnCenterX, deltaTouchOnCenterY);
if (touchDistanceFromCenter <= circleRadius) {
// your code here
NSLog(#"___ OK: You are inside the circle!!");
}
}

Answer 2

Swift solution:
class RadiusTouchButton: UIButton {
override func point(inside point: CGPoint, with event: UIEvent?) -> Bool {
let radius = frame.size * 0.5
let delta = point - CGPoint(x: radius.height, y: radius.width)
return hypot(delta.x, delta.y) <= radius.height
}
}