Swift addArrangedSubview代码有什么问题？

我试图了解addArrangedSubview是如何工作的，所以我尝试了一个虚拟项目。这是我的VC：

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var stackOutlet: UIStackView!
override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    let newView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
    newView.backgroundColor = UIColor.darkGray

    let newView2 = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
    newView2.backgroundColor = UIColor.blue
    stackOutlet.addArrangedSubview(newView)
    stackOutlet.addArrangedSubview(newView2)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}


}

---

当我启动应用程序时，没有显示任何内容。我想我必须在stackView中设置一些约束条件？

好吧，为了大家的缘故：您需要使用这个=

mapView.heightAnchor.constraint(equalToConstant: 200).isActive = true
mapView.widthAnchor.constraint(equalToConstant: 200).isActive = true