在当前上下文中绘制弧(Drawing arc in current context)

 我试图在当前上下文中绘制自定义弧，但结果不是我期望看到的，我有简单的代码行，如果绘制方法。 
 
-(void) drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextBeginPath(context);
    CGContextSetGrayFillColor(context, 1.0, 0.7);
    CGContextMoveToPoint(context, 100,100);
    CGContextAddArc(context, 100, 100, 80, 0, M_PI_2, 0);
    CGContextClosePath(context);
    CGContextFillPath(context);
}
 
 这是结果。 
 
 但我预计会看到这样的结果。 为什么M_PI_2出现在圆圈的底部？ 应该有3 * M_PI_2，我做错了什么？ 

I trying to draw custom arc in current context, but the result is not what i expected to see, i have the simple lines of code if draw method.
 
-(void) drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextBeginPath(context);
    CGContextSetGrayFillColor(context, 1.0, 0.7);
    CGContextMoveToPoint(context, 100,100);
    CGContextAddArc(context, 100, 100, 80, 0, M_PI_2, 0);
    CGContextClosePath(context);
    CGContextFillPath(context);
}
 
and here is the result. 
 
But I'm was expected to see result like this. Why M_PI_2 appears on bottom side of the circle? there should be a 3*M_PI_2, what I'm doing wrong? 

原文：https://stackoverflow.com/questions/14285518