给指定角度设计成圆角，方法步骤如下:
步骤：
1、新建UIBezierPath对象maskPath
2、新建CAShapeLayer对象maskLayer
3、将maskPath的CGPath赋值给maskLayer的path，即maskLayer.path = maskPath.CGPath
4、把caShapeLayer添加到某个显示该图形的layer中
UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:button.bounds byRoundingCorners:UIRectCornerBottomLeft | UIRectCornerBottomRight cornerRadii:CGSizeMake(10, 10)];
CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
maskLayer.frame = button.bounds;
maskLayer.path = maskPath.CGPath;
button.layer.mask = maskLayer;
圆角可选为四种
UIRectCornerBottomLeft
UIRectCornerBottomRight //UI底部两个角为圆角
UIRectCornerTopRight
UIRectCornerTopRight //UI顶部角为圆角
