Rotating a small rect that traces the strokeEnd point of a circle's path.

C4WorkSpace.m

#import "C4WorkSpace.h"
@implementation C4WorkSpace {
    C4Shape *s, *sq;
}

-(void)setup {
    //create a shape and style it
    s = [C4Shape ellipse:CGRectMake(0, 0, 200, 200)];
    s.lineWidth = 10.0f;
    s.center = self.canvas.center;
    [self.canvas addShape:s];

    //create a small square and style it
    sq = [C4Shape rect:CGRectMake(0, 0, s.lineWidth, s.lineWidth)];
    sq.fillColor = C4RED;
    sq.lineWidth = 0.0f;
    
    //offset it's anchor point
    //add 0.5 to offset the square by half its width
    sq.anchorPoint = CGPointMake(-1 * (s.width / 2) / sq.width + 0.5,0.5);
    
    //center the square, based on its anchor point, to the center of the shape
    sq.center = CGPointMake(s.width / 2,s.height / 2);
    [s addShape:sq];
    
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    //figure out how far along the screen the user has touched
    CGPoint p = [[touches anyObject] locationInView:self.canvas];
    //normalize it to 1.0
    CGFloat normalized = p.x / self.canvas.width;
    //change the end of the stroke
    s.strokeEnd = normalized;
    //rotate the square to match the end of the stroke
    sq.rotation = TWO_PI * normalized;
}

@end