为什么在情节提要中看不到我的自定义视图的预览？

TL; DR; 调用prepareForInterfaceBuilder

// Call The Custom Setup Here
override func prepareForInterfaceBuilder() {
    setupView()
}

在layoutSubviews中调用也可以，但是在运行时被多次调用，prepareForInterfaceBuilder仅针对Designables Changes进行调用，并且仅出于此目的。

长码：

@IBDesignable
class CustomView: UIView {

    override init(frame: CGRect) {
        super.init(frame: frame)
        setupView()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupView()
    }

    override func prepareForInterfaceBuilder() {
        setupView()
    }

    func InitChildPosition() {
        var i = 1
        for _view in self.subviews {
            if _view is UIButton {
                _view.center.x = (_view.bounds.width / 2)
                _view.center.y = (_view.bounds.height / 2)
            }

            if _view is UIButton && i == 2 {
                _view.center.x = self.bounds.width - (_view.bounds.width / 2)
                _view.center.y = self.bounds.height - (_view.bounds.height / 2)
                _view.backgroundColor = UIColor.darkGray
            }
            i += 1
        }
    }

    func setupView() {
        self.backgroundColor = UIColor.black

        InitChildPosition()
    }
}