Question

I hope it is possible to draw a line using center point and angle. I need to draw the line to move along with a circle.But I couldn’t make it work. I am not getting any idea how could do this!I tried the following code to rotate the line by specified angle, it didn’t work. As I am fairly new, I couldn’t understand where I am making mistake!! This is how it looks if I use the below code

- (void)drawThumbAtPoint:(CGPoint)sliderButtonCenterPoint inContext:(CGContextRef)context {

//Vertical line
CGPoint newPoint = [self convertPoint:sliderButtonCenterPoint toView:self];
CGFloat angleInRadians = (CGFloat)M_PI/180.0f * currentAngle;
CGFloat distance = 15;
CGPoint point = CGPointMake(newPoint.x + distance * sinf(angleInRadians), newPoint.y + distance * cosf(angleInRadians));

UIGraphicsPushContext(context);
CGContextBeginPath(context);
UIBezierPath *path = [UIBezierPath bezierPath];
path.lineWidth = 10.f;
[path moveToPoint:sliderButtonCenterPoint];
[path addLineToPoint:point];
[[UIColor redColor] set];
[path stroke];
//  CGAffineTransform rot = CGAffineTransformMakeRotation(angleInRadians);
//  [path applyTransform:rot];
UIGraphicsPopContext();
}

And,

CGPoint thumbCenterPoint = CGContextGetPathCurrentPoint(context);
[self drawThumbAtPoint:thumbCenterPoint inContext:context];

Answer 1

A unit vector with angle alpha to the x-axis is

(cos(alpha), sin(alpha))

Now you want to draw a line that is tangential to the circle line, so it is perpendicular to the line from the center of the circle to the point on the circle line.

To get a perpendicular vector, you add 90º = π/2 to the angle:

(cos(alpha + π/2), sin(alpha + π/2)) = (-sin(alpha), cos(alpha))

using basic trigonometric identities.

This (hopefully) explains why the endpoint of the line must be computed as

CGPointMake(newPoint.x - distance * sinf(angleInRadians), newPoint.y + distance * cosf(angleInRadians));