/* Add an arc of a circle to the context's path, possibly preceded by a
straight line segment. `(x, y)' is the center of the arc; `radius' is its
radius; `startAngle' is the angle to the first endpoint of the arc;
`endAngle' is the angle to the second endpoint of the arc; and
`clockwise' is 1 if the arc is to be drawn clockwise, 0 otherwise.
`startAngle' and `endAngle' are measured in radians. */
”(x,y)”是弧的中心,“radius”是它的半径;“startAngle”是第一个的角弧的端点;“endAngle”的第二个端点弧角;和
“clockwise” ＝＝ 1电弧是顺时针,否则为0。
“startAngle”和“endAngle测量的弧度。
CG_EXTERN void CGContextAddArc(CGContextRef c, CGFloat x, CGFloat y,
CGFloat radius, CGFloat startAngle, CGFloat endAngle, int clockwise)
CG_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);
CGContextAddArc(context, x, y, 3, 0, 2*PI, 0); //添加一个圆 圆心为（x, y）半径为3 圆周为2*PI的小圆，