Question

以下是我在ViewController中初始化UIView然后将其约束到ViewController的主视图后得到的错误消息。

Unable to activate constraint with anchors because they have no common ancestor. Does the constraint or its anchors reference items in different view hierarchies? That's illegal.

如果你看一下我提供的图片，你会看到在故事板中我可以在主视图中删除另一个UIView然后我可以将它约束到主视图的边缘。这非常好用，但我想知道如何在代码中做同样的事情。只使用NSLayoutConstraint并将其固定到顶部，底部，前导和尾随似乎对我不起作用。对此的任何见解都会很棒。

    let webView: WKWebView = WKWebView()

    override func viewDidLoad() {
        super.viewDidLoad()

        webView.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint(item: webView,
                       attribute: .leading,
                       relatedBy: .equal,
                       toItem: view,
                       attribute: .leading,
                       multiplier: 1.0,
                       constant: 0.0).isActive = true

        NSLayoutConstraint(item: webView,
                       attribute: .trailing,
                       relatedBy: .equal,
                       toItem: view,
                       attribute: .trailing,
                       multiplier: 1.0,
                       constant: 0.0).isActive = true

        NSLayoutConstraint(item: webView,
                       attribute: .top,
                       relatedBy: .equal,
                       toItem: view,
                       attribute: .top,
                       multiplier: 1.0,
                       constant: 0.0).isActive = true

        NSLayoutConstraint(item: webView,
                       attribute: .bottom,
                       relatedBy: .equal,
                       toItem: view,
                       attribute: .bottom,
                       multiplier: 1.0,
                       constant: 0.0).isActive = true

        let url = NSURL (string: someURL);
        let requestObj = NSURLRequest(url: url! as URL);
        webView.load(requestObj as URLRequest);
    }

Answer 1

您似乎忘记在添加约束之前将webView添加到view：

view.addSubview(webView)

最终代码：

webView.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(webView)

NSLayoutConstraint(item: webView,
                   attribute: .leading,
                   relatedBy: .equal,
                   toItem: view,
                   attribute: .leading,
                   multiplier: 1.0,
                   constant: 0.0).isActive = true

NSLayoutConstraint(item: webView,
                   attribute: .trailing,
                   relatedBy: .equal,
                   toItem: view,
                   attribute: .trailing,
                   multiplier: 1.0,
                   constant: 0.0).isActive = true

NSLayoutConstraint(item: webView,
                   attribute: .top,
                   relatedBy: .equal,
                   toItem: view,
                   attribute: .top,
                   multiplier: 1.0,
                   constant: 0.0).isActive = true

NSLayoutConstraint(item: webView,
                   attribute: .bottom,
                   relatedBy: .equal,
                   toItem: view,
                   attribute: .bottom,
                   multiplier: 1.0,
                   constant: 0.0).isActive = true

let url = NSURL (string: "");
let requestObj = NSURLRequest(url: url! as URL);
webView.load(requestObj as URLRequest);

为什么我不能将viewDidLoad方法中创建的视图约束到ViewControllers视图？

1 个答案: