利用CGContextClip、CGPathAddEllipseInRect裁剪视图

以下可讲视图裁剪成一个圆形或椭圆：

- (void) drawRect: (CGRect) aRect
{
    UIImage *logo = [UIImageimageNamed:@"logo1.png"];
    CGRect bounds = CGRectMake(0.0f, 0.0f,rect.origin.x + aRect.size.width, rect.origin.y + aRect.size.height);
    // Create a new path
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGMutablePathRef path = CGPathCreateMutable();
   
    // Add circle to path
    CGPathAddEllipseInRect(path, NULL, bounds);//这句话就是剪辑作用
    CGContextAddPath(context, path);

    // Clip to the circle and draw the logo
    CGContextClip(context);
    [logo drawInRect:bounds];
    CFRelease(path);
}