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

Question

以下是我在ViewController中初始化UIView后得到的错误消息，然后将其限制在ViewController的主视图中。

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.

如果你看看我提供你会看到，在故事板，我可以刚落，另一个UIView的主视角的图片，然后我可以将其约束到的边缘主视图。这工作得很好，但我想知道如何在代码中做同样的事情。只需使用NSLayoutConstraint并将其固定到顶部，底部，前导和尾随似乎不适合我。任何有关这方面的见解都会很棒。

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

你似乎忘了添加约束之前的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);