When you draw a circle by using core animation , you might use CAShapeLayer and UIBezierPath to draw it.
UIBezierPath(arcCenter: view.center, radius: view.bounds.width / 2, startAngle: startAngle, endAngle: endAngle, clockwise: true).CGPath
The code above there are
endAngle parameters which affect where your animation start and end. So if you want to draw the circle from a specific position to another position, I recommend this link.
If you want to start from the top of the circle , your
startAngle should be
M_PI + M_PI_2.