CGPathAddArcDemo.m

static inline double radians (double degrees) { return degrees * M_PI/180; }
@implementation MyLayerDelegate
- (void)drawLayer:(CALayer *)theLayer inContext:(CGContextRef)theContext {
  CGContextSetStrokeColorWithColor(theContext, [UIColor greenColor].CGColor);
  CGMutablePathRef path = createArcPathFromBottomOfRect(CGRectMake(0, 0, 100, 100), 50.0f);
  CGContextBeginPath(theContext);
  CGContextAddPath(theContext, path);
  CGContextSetLineWidth(theContext, 3);
  CGContextStrokePath(theContext);
  CFRelease(path);
}

CGMutablePathRef createArcPathFromBottomOfRect(CGRect rect, CGFloat arcHeight) {
  CGRect arcRect = CGRectMake(rect.origin.x,
  rect.origin.y + rect.size.height - arcHeight,
  rect.size.width, arcHeight);
  CGFloat arcRadius = (arcRect.size.height/2) +
  (pow(arcRect.size.width, 2) / (8*arcRect.size.height));
  CGPoint arcCenter = CGPointMake(arcRect.origin.x + arcRect.size.width/2,
  arcRect.origin.y + arcRadius);
  CGFloat angle = acos(arcRect.size.width / (2*arcRadius));
  CGFloat startAngle = radians(180) + angle;
  CGFloat endAngle = radians(360) - angle;
  CGMutablePathRef path = CGPathCreateMutable();
  CGPathAddArc(path, NULL, arcCenter.x, arcCenter.y, arcRadius,
  startAngle, endAngle, 0);
  CGPathAddLineToPoint(path, NULL, CGRectGetMaxX(rect), CGRectGetMinY(rect));
  CGPathAddLineToPoint(path, NULL, CGRectGetMinX(rect), CGRectGetMinY(rect));
  CGPathAddLineToPoint(path, NULL, CGRectGetMinX(rect), CGRectGetMaxY(rect));
  
  return path;
}