draw a dial like line pointing at user's touch point - ios

Question

Assuming I want to draw a line which resemble a clock dial (blue line), starting from the center of the screen (center) and ending at the user's touch position (A,B or C)
it does not matter how far the finger is, the dial will always have the same length size.
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [[event allTouches] anyObject];
CGPoint touchLocation = [touch locationInView:touch.view];
NSLog(#"Center point = %f %f",self.view.center.x,self.view.center.y);
NSLog(#"finter at point = %f %f",touchLocation.x,touchLocation.y);
// line re drawing itself ...
NSLog(#"end point = %f %f",?,?);
}

Answer 1

You need to know the length of your line. It's unrelated to the touch point correct?
First find the coordinates of the touch relative to the center point
x = Touch.x - center.x
y = Touch.y - center.y
Now we need to get the angle
angle = arctan(y / x)
If the x is negative, adjust by 180 degrees (pi) - This restores what is lost in the division.
Now multiply sin(angle) and cos(angle) by your desired length to get the new point
newX = cos(angle) * length
newY = sin(angle) * length
Here is some Swift code that gets you mostly there. Try it in a playground to verify different touch values and lengths.
let lineLength = 13.0
// Touch points
let x = -5.0
let y = -5.0
// Calculate angle
var angle = atan(y / x)
if x < 0 {
angle += 3.14159;
}
// Get new X and Y
var newX = cos(angle) * lineLength
var newY = sin(angle) * lineLength