あなたの場合Swift4例MyCustomView.swiftとMyCustomView.xib枠組みインチ これをMyCustomViewのinitに配置します。
let bundle = Bundle(for: type(of: self))
let nib = UINib(nibName: "MyCustomView", bundle: bundle)
if let nibView = nib.instantiate(withOwner: self, options: nil).first as? UIView {
self.aViewInMyCustomView = nibView
self.aViewInMyCustomView.frame = self.frame
self.addSubview(self.aViewInMyCustomView)
// set constraints
self.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint(item: self.aViewInMyCustomView, attribute: .leading, relatedBy: .equal, toItem: self, attribute: .leading, multiplier: 1.0, constant: 0).isActive = true
NSLayoutConstraint(item: self.aViewInMyCustomView, attribute: .trailing, relatedBy: .equal, toItem: self, attribute: .trailing, multiplier: 1.0, constant: 0).isActive = true
NSLayoutConstraint(item: self.aViewInMyCustomView, attribute: .top, relatedBy: .equal, toItem: self, attribute: .top, multiplier: 1.0, constant: 0).isActive = true
NSLayoutConstraint(item: self.aViewInMyCustomView, attribute: .bottom, relatedBy: .equal, toItem: self, attribute: .bottom, multiplier: 1.0, constant: 0).isActive = true
}