使用CGContextRef中的Transform来辅助作图

//开始绘画
    UIGraphicsBeginImageContext(self.view.bounds.size);
    CGContextRef gc = UIGraphicsGetCurrentContext();

    //设置颜色
    [[UIColor grayColor] setFill];

    //设置中心点
    CGFloat cenX = CGRectGetMidX(self.view.bounds);
    CGFloat cenY = CGRectGetMidY(self.view.bounds);
    CGContextTranslateCTM(gc, cenX, cenY);



    //不断绘图并设置旋转
    for (int i = 0; i < 12; i ++) {
//        CGContextAddRect(gc, CGRectMake(-5, 0, 10, 100));
        CGContextAddRect(gc, CGRectMake(-5, 50, 10, 50));
//        CGContextAddRect(gc, CGRectMake(-5, 10, 2, 50));

        CGContextFillPath(gc);
        CGContextRotateCTM(gc, 30 * M_PI / 180);
    }

    //结束绘画
    UIImage *destImg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    //创建UIImageView并显示在界面上
    UIImageView *imageView = [[UIImageView alloc] initWithImage:destImg];
    [self.view addSubview:imageView];