Find Frame Coordinates After UIView Transform is Applied (CGAffineTransform)

Question

I rotate my view with CGAffineTransform

[view setTransform:newTransform];

The frame values remain the same after transform is applied but how do I find "rotated" or transformed values of this frame?

_{(source: informit.com)}

I want the exact coordinates of rotated frame edges, that is a, b, c, d points.

Answer 1

One thing to keep in mind is that the transform changes the coordinate system, so you will need to be able to convert between the parent 'view' and the child (transformed) view. Also, transforms preserve the center of the transformed object but not any of the other coordinates. So you need to calculate things in terms of the center. And there are several helpers you will need. (I got most of the following approach from Erica Sadun's book Core iOS Developer's Cookbook).

I usually add these as a category on UIView.

In order to transform the child's coordinates to those of the parent you need something like:

// helper to get pre transform frame
-(CGRect)originalFrame {
   CGAffineTransform currentTransform = self.transform;
   self.transform = CGAffineTransformIdentity;
   CGRect originalFrame = self.frame;
   self.transform = currentTransform;

   return originalFrame;
}

// helper to get point offset from center
-(CGPoint)centerOffset:(CGPoint)thePoint {
    return CGPointMake(thePoint.x - self.center.x, thePoint.y - self.center.y);
}
// helper to get point back relative to center
-(CGPoint)pointRelativeToCenter:(CGPoint)thePoint {
  return CGPointMake(thePoint.x + self.center.x, thePoint.y + self.center.y);
}
// helper to get point relative to transformed coords
-(CGPoint)newPointInView:(CGPoint)thePoint {
    // get offset from center
    CGPoint offset = [self centerOffset:thePoint];
    // get transformed point
    CGPoint transformedPoint = CGPointApplyAffineTransform(offset, self.transform);
    // make relative to center
    return [self pointRelativeToCenter:transformedPoint];
}

// now get your corners
-(CGPoint)newTopLeft {
    CGRect frame = [self originalFrame];
    return [self newPointInView:frame.origin];
}
-(CGPoint)newTopRight {
    CGRect frame = [self originalFrame];
    CGPoint point = frame.origin;
    point.x += frame.size.width;
    return [self newPointInView:point];
}
-(CGPoint)newBottomLeft {
    CGRect frame = [self originalFrame];
    CGPoint point = frame.origin;
    point.y += frame.size.height;
    return [self newPointInView:point];
}
-(CGPoint)newBottomRight {
    CGRect frame = [self originalFrame];
    CGPoint point = frame.origin;
    point.x += frame.size.width;
    point.y += frame.size.height;
    return [self newPointInView:point];
}

Swift 4

extension UIView {
    /// Helper to get pre transform frame
    var originalFrame: CGRect {
        let currentTransform = transform
        transform = .identity
        let originalFrame = frame
        transform = currentTransform
        return originalFrame
    }

    /// Helper to get point offset from center
    func centerOffset(_ point: CGPoint) -> CGPoint {
        return CGPoint(x: point.x - center.x, y: point.y - center.y)
    }

    /// Helper to get point back relative to center
    func pointRelativeToCenter(_ point: CGPoint) -> CGPoint {
        return CGPoint(x: point.x + center.x, y: point.y + center.y)
    }

    /// Helper to get point relative to transformed coords
    func newPointInView(_ point: CGPoint) -> CGPoint {
        // get offset from center
        let offset = centerOffset(point)
        // get transformed point
        let transformedPoint = offset.applying(transform)
        // make relative to center
        return pointRelativeToCenter(transformedPoint)
    }

    var newTopLeft: CGPoint {
        return newPointInView(originalFrame.origin)
    }

    var newTopRight: CGPoint {
        var point = originalFrame.origin
        point.x += originalFrame.width
        return newPointInView(point)
    }

    var newBottomLeft: CGPoint {
        var point = originalFrame.origin
        point.y += originalFrame.height
        return newPointInView(point)
    }

    var newBottomRight: CGPoint {
        var point = originalFrame.origin
        point.x += originalFrame.width
        point.y += originalFrame.height
        return newPointInView(point)
    }
}