下载进度条

//1.获取上下文
    CGContextRef ref = UIGraphicsGetCurrentContext();
    //2.描述路径
    CGPoint center = CGPointMake(self.frame.size.width *0.5, self.frame.size.height *0.5);
    CGFloat radius = center.x- 10;
    CGFloat startAngle = - M_PI_2;
    CGFloat angele = self.progressValue * M_PI * 2;
    CGFloat endAngel = startAngle + angele;
    UIBezierPath *path = [UIBezierPath bezierPathWithArcCenter:center radius:radius startAngle:startAngle endAngle:endAngel clockwise:YES];
    //3.把路径添加到上下文当中
    CGContextAddPath(ref, path.CGPath);
  //3.把路径添加到上下文当中
    CGContextStrokePath(ref);