Question

I have an overlay view (Which has self-drawn shapes) that I display over an ImageView. I want the view to be movable, resizable, and rotatable. I could allow the user to move the overlay by dragging it from the middle, or resize it by dragging it from one of two sides (right or bottom). What I still could not do is to allow the user to rotate it by moving the left-top edge.

myView.transform = CGAffineTransformMakeRotation(angle * M_PI / 180);

But how can I calculate the angle based on user touches? Any ideas?

Answer 1

The easiest way is to use a UIRotationGestureRecognizer which gives you the rotation value as a property.

If you can’t use a gesture recognizer, try something like this (untested):

// Assuming centerPoint is the center point of the object you want to rotate (the rotation axis),
// currentTouchLocation and initialTouchLocation are the coordinates of the points between 
// which you want to calculate the angle.
CGPoint centerPoint = ...
CGPoint currentTouchLocation = ...
CGPoint initialTouchLocation = ...

// Convert to polar coordinates with the centerPoint being (0,0)
CGPoint currentTouchLocationNormalized = CGPointMake(currentTouchLocation.x - centerPoint.x, currentTouchLocation.y - centerPoint.y);
CGPoint initialTouchLocationNormalized = CGPointMake(initialTouchLocation.x - centerPoint.x, initialTouchLocation.y - centerPoint.y);

CGFloat angleBetweenInitialTouchAndCenter = atan2(initialTouchLocationNormalized.y, initialTouchLocationNormalized.x);
CGFloat angleBetweenCurrentTouchAndCenter = atan2(currentTouchLocationNormalized.y, currentTouchLocationNormalized.x);

CGFloat rotationAngle = angleBetweenCurrentTouchAndCenter - angleBetweenInitialTouchAndCenter;

See Wikipedia or do a Google search to learn more about polar coordinates and how to convert between cartesian and polar coordinate systems.