Error while adding constraints to WebView - Swift

Question

I'm practicing Webkit in swift. I'm following this tutorial for the same purpose. And I'm stuck with an error in the very first few lines of the code.

This is what I have in the ViewController.swift so far :

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()
}


}

and the following is how the console error output looks:

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

I'm a beginner so needless to say, I don't have any idea what this means. Additionally I'm getting the error message Thread 1: signal SIGABRT which I normally get if I ignore to remove an outlet reference that I created earlier. In this case I have not created any outlets.

I've gone through some other answers on Stack on similar problems in Objective C but I couldn't understand it well.

I know the solution might be relatively simple but any help will be appreciated.

score 1 · Answer 1 · answered Oct 27 '16 at 06:24

1

Every view needs 4 constraints such that the system can calculate: x,y, width, height (labels and imageViews already have intrinsic dimensions but you can still constrain them if you want). Here are 4 constraints for the webview:

    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)
    }

answered Oct 27 '16 at 06:24

Josh Homann

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I'm not sure I completely understand how this code is supposed to work. It's not running, I'm using Swift 2.1 so I'll have to modify the code a bit. I'll let you know if it works. But I didn't get what the `$0` does. – Ajil O. Oct 27 '16 at 06:35
1

map transforms the attributes into NSLayoutConstraints. $0 is the default parameter that stands in for each element in the attributes array. It works in swift 2. You will have to capitalize all of the NSLayoutAttribute enums for swift 2 and change URL and URLRequest back to NSURL and NSURLRequest. – Josh Homann Oct 27 '16 at 06:41
I modified the code like this. `NSLayoutConstraint.activateConstraints(attributes.map({NSLayoutConstraint(item: webView, attribute: $0, relatedBy: .Equal, toItem: webView, attribute: $0, multiplier: 1, constant: 0)}))` – Ajil O. Oct 27 '16 at 06:43
It compiles but displays just an empty view. What am I doing wrong? NSLayoutConstarint doesn't have an activate property, which is why I changed it to activateConstraints – Ajil O. Oct 27 '16 at 06:46
check the view debugger; the webview is there. My guess is that you did not make an app transport exception for http: loads which are blocked by default. Changing this to https: works for me. Otherwise see http://stackoverflow.com/questions/30731785/how-do-i-load-an-http-url-with-app-transport-security-enabled-in-ios-9 – Josh Homann Oct 27 '16 at 06:51
Ok. I have now made the app transport exception. But still no output. How do I check the view debugger? Did you mean the View hierarchy? Well, I have UIView in the hierarchy that shows a faded blue on the View when I click on it. Right inside UIView I have WKWebView which doesn't show any output when clicked – Ajil O. Oct 27 '16 at 07:08
1

That means the webview is there. Something is wrong with your load. put static text in it to test (use a much bigger string) webView.loadHTMLString("test", baseURL: nil) – Josh Homann Oct 27 '16 at 07:13
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/126794/discussion-between-ajil-o-and-josh-homann). – Ajil O. Oct 27 '16 at 07:21

score 1 · Accepted Answer · answered Oct 28 '16 at 02:44

So I finally fixed the code! Here's the corrected code.

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.
    }
}

Turns out I was adding the constraints to webView where I should have been adding it to view.

Error while adding constraints to WebView - Swift

2 Answers2