原始想法是同时设置maskToBounds = true 和 cornerRadius，然而并不能看到阴影，因为阴影已经被mask掉了

大家的做法都是加一个层或者加一个view

如下

然而 只有当把下面的addButton添加到shadow这个view里面的时候才会正确显示阴影，如果把addButton和shadow添加到另外某一个view则无法正确显示，原因待查证。

效果如图

Paste_Image.png

        let addButtonRadius:CGFloat = 28
        
        let shadow = UIView()
        shadow.layer.shadowColor = AtColor.blue2().CGColor
        shadow.layer.shadowRadius = 5
        shadow.layer.shadowOffset = CGSize(width:0, height:0)
        shadow.layer.shadowOpacity = 1
        shadow.layer.cornerRadius = addButtonRadius
        shadow.clipsToBounds = false
        self.view.addSubview(shadow)
        shadow.snp.makeConstraints { (btn) in
            btn.right.equalToSuperview().offset(-34)
            btn.bottom.equalToSuperview().offset(-56)
            btn.width.height.equalTo(addButtonRadius * 2)
        }
        
        addButton = UIButton()
        addButton.titleEdgeInsets = UIEdgeInsets(top: 3, left: 0, bottom: -3, right: 0)
        addButton.layer.masksToBounds = true
        addButton.layer.cornerRadius = addButtonRadius
        addButton.setBackgroundImage(AtColor.blue1().getImage(), for: UIControlState.normal)
        addButton.setBackgroundImage(AtColor.blue2().getImage(), for: UIControlState.highlighted)
        addButton.titleLabel?.font = AtFontIcon.getFont(20)
        addButton.setTitle(FontIconType.HomePlus.rawValue, for: UIControlState.normal)
        addButton.setTitleColor(AtColor.white().color, for: UIControlState.normal)
        shadow.addSubview(addButton)
        addButton.snp.makeConstraints { (btn) in
            btn.edges.equalToSuperview()
        }
        addButton.addTarget(self, action: #selector(self.onAddGroupButtonClick), for: UIControlEvents.touchUpInside)