iOS - UIButton 部分圆角

按钮左上角及左下角圆角：

    UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:self.signBtn.bounds   byRoundingCorners:UIRectCornerBottomLeft |    UIRectCornerTopLeft    cornerRadii:CGSizeMake(11, 11)];
    CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
    maskLayer.frame = self.signBtn.bounds;
    maskLayer.path = maskPath.CGPath;
    self.signBtn.layer.mask = maskLayer;