向WebView - Swift添加约束时出错

Question

我在swift中练习Webkit。我遵循本教程也是出于同样的目的。我在代码的前几行遇到了一个错误。

这是我目前在viewcontroller。swift中的内容:

import UIKit
import WebKit
class ViewController: UIViewController {
var webView : WKWebView
required init(coder aDecoder: NSCoder) {
    self.webView = WKWebView(frame: CGRectZero)
    super.init(coder: aDecoder)!
}
override func viewDidLoad() {
    super.viewDidLoad()
    self.view.addSubview(webView)
    webView.translatesAutoresizingMaskIntoConstraints = false
    let height = NSLayoutConstraint(item: webView, attribute: .Height, relatedBy: .Equal, toItem: view, attribute: .Height, multiplier: 1, constant: 0)
    let width = NSLayoutConstraint(item: webView, attribute: .Width, relatedBy: .Equal, toItem: view, attribute: .Width, multiplier: 1, constant: 0)
    webView.addConstraints([height,width])
    let url = NSURL(string: "http://www.google.com")
    let urlRequest = NSURLRequest(URL: url!)
    webView.loadRequest(urlRequest)
}
override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}

}

，下面是控制台错误输出的样子:

2016-10-27 11:29:09.015 Browser[68308:1113663] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '***
+[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: Constraint items must each be an instance of UIView, or NSLayoutGuide.'
*** First throw call stack: (   0   CoreFoundation                      0x00000001060e0d85 __exceptionPreprocess + 165  1   libobjc.A.dylib    0x0000000108486deb objc_exception_throw + 48     2   CoreFoundation       0x00000001060e0cbd +[NSException raise:format:] + 205  3   Foundation  0x00000001064af0ac +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]
+ 277   4   Browser                             0x0000000105ef890b _TTOFCSo18NSLayoutConstraintCfT4itemPs9AnyObject_9attributeOSC17NSLayoutAttribute9relatedByOSC16NSLayoutRelation6toItemGSqPS0___9attributeS1_10multiplierV12CoreGraphics7CGFloat8constantS4__S_
+ 123   5   Browser                             0x0000000105ef78ee _TFC7Browser14ViewController11viewDidLoadfT_T_ + 494     6   Browser                             0x0000000105ef7c12 _TToFC7Browser14ViewController11viewDidLoadfT_T_ + 34    7   UIKit                               0x0000000106aa5984
-[UIViewController loadViewIfRequired] + 1198   8   UIKit                               0x0000000106aa5cd3 -[UIViewController view] + 27    9   UIKit            0x000000010697bfb4 -[UIWindow addRootViewControllerViewIfPossible] + 61    10  UIKit                               0x000000010697c69d
-[UIWindow _setHidden:forced:] + 282    11  UIKit                               0x000000010698e180 -[UIWindow makeKeyAndVisible] + 42   12  UIKit       0x0000000106902ed9 -[UIApplication
_callInitializationDelegatesForMainScene:transitionContext:] + 4131     13  UIKit                               0x0000000106909568
-[UIApplication _runWithMainScene:transitionContext:completion:] + 1769     14  UIKit                               0x0000000106906714
-[UIApplication workspaceDidEndTransaction:] + 188  15  FrontBoardServices                  0x000000010a3558c8
__FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK__ + 24   16  FrontBoardServices                  0x000000010a355741
-[FBSSerialQueue _performNext] + 178    17  FrontBoardServices                  0x000000010a355aca -[FBSSerialQueue _performNextFromRunLoopSource] + 45     18  CoreFoundation                      0x0000000106006301
__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17     19  CoreFoundation                      0x0000000105ffc22c
__CFRunLoopDoSources0 + 556     20  CoreFoundation                      0x0000000105ffb6e3 __CFRunLoopRun + 867     21  CoreFoundation            0x0000000105ffb0f8 CFRunLoopRunSpecific + 488     22  UIKit               0x0000000106905f21 -[UIApplication _run] + 402  23  UIKit              0x000000010690af09 UIApplicationMain + 171   24  Browser                0x0000000105ef92f2 main + 114    25  libdyld.dylib                       0x0000000108f5d92d start + 1 ) libc++abi.dylib: terminating with uncaught exception of type NSException

我是初学者，所以不用说，我不知道这是什么意思。此外，我得到的错误信息我通常得到Thread 1: signal SIGABRT，如果我忽略删除我之前创建的出口引用。在本例中，我没有创建任何输出。

我在Stack上看过一些类似问题的答案，但是我不能很好地理解它。

我知道解决方案可能相对简单，但任何帮助都会很感激。

Answer 1

每个视图需要4个约束，这样系统可以计算:x,y，宽度，高度(标签和imageViews已经有固有的尺寸，但你仍然可以约束他们，如果你想要)。以下是webview的4个约束条件:

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(webView)
        webView.translatesAutoresizingMaskIntoConstraints = false
        let attributes: [NSLayoutAttribute] = [.centerX, .centerY, .width, .height]
        NSLayoutConstraint.activate(attributes.map {NSLayoutConstraint(item: webView, attribute: $0, relatedBy: .equal, toItem: webView.superview, attribute: $0, multiplier: 1, constant: 0) })
        let url = URL(string: "http://www.google.com")!
        let urlRequest = URLRequest(url: url)
        webView.load(urlRequest)
    }

Answer 2

所以我终于修复了代码!这是更正后的代码。

import UIKit
import WebKit
class ViewController: UIViewController {    
    var webView : WKWebView
    required init(coder aDecoder: NSCoder) {
        self.webView = WKWebView(frame: CGRect(x: 0, y: 0, width: 414, height: 736))
        super.init(coder: aDecoder)!
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(webView)
        webView.translatesAutoresizingMaskIntoConstraints = false
        let views = ["webView" : webView]
        let h = NSLayoutConstraint.constraintsWithVisualFormat("H:|[webView]|", options: [], metrics: nil , views: views)
        let w = NSLayoutConstraint.constraintsWithVisualFormat("V:|[webView]|", options: [], metrics: nil, views: views)
        view.addConstraints(h)  //This is where I was going wrong
        view.addConstraints(w)  //This is where I was going wrong 
        let url = NSURL(string: "http://www.google.com")
        let urlRequest = NSURLRequest(URL: url!)
        webView.loadRequest(urlRequest)
    }
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

原来我是把约束添加到webView，我应该把它添加到view。