把UIView切成任意形状

- (void)dwMakeBottomRoundCornerWithRadius:(CGFloat)radius
{
    CGSize size = self.frame.size;
    CAShapeLayer *shapeLayer = [CAShapeLayer layer];
    [shapeLayer setFillColor:[[UIColor whiteColor] CGColor]];
    
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, NULL, size.width - radius, size.height);
    CGPathAddArc(path, NULL, size.width-radius, size.height-radius, radius, M_PI/2, 0.0, YES);
    CGPathAddLineToPoint(path, NULL, size.width, 0.0);
    CGPathAddLineToPoint(path, NULL, 0.0, 0.0);
    CGPathAddLineToPoint(path, NULL, 0.0, size.height - radius);
    CGPathAddArc(path, NULL, radius, size.height - radius, radius, M_PI, M_PI/2, YES);
    CGPathCloseSubpath(path);
    [shapeLayer setPath:path];
    CFRelease(path);
    self.layer.mask = shapeLayer;//layer的mask，顾名思义，是种位掩蔽，在shapeLayer的填充区域中，alpha值不为零的部分，self会被绘制；alpha值为零的部分，self不会被绘制，甚至不会响应touch
}

- (id)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        [self dwMakeBottomRoundCornerWithRadius:3.0];
    }
}

这样就能切出一个只有下半部圆角的View，更厉害的是，切去的部分不响应用户点击！

这种方法，只要稍作修改就能实现很多效果，比如一个多边形的按钮，把一张图片切成邮票。

UIView:

layer.mask:

最终绘制：