WKWebView evaluateJavaScript not returning html

Question

I am trying to parse the html returned from a WKWebView load() with evaluateJavaScript but it never prints anything. Am I doing this right? Any other ways? didFinish does print.

import UIKit
import WebKit

class MyWebViewController: UIViewController, WKNavigationDelegate {

var webView: WKWebView!

override func viewDidLoad() {
    super.viewDidLoad()

    webView = WKWebView(frame:  self.view.frame)
    webView.navigationDelegate = self

    let url = NSURL (string: "https://google.com");
    let request = NSURLRequest(url: url! as URL)
    webView.load(request as URLRequest)

    self.view.addSubview(webView)

    self.view.sendSubview(toBack: webView)

}


func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {

    webView.evaluateJavaScript("document.documentElement.outerHTML.toString()", completionHandler: { (html: AnyObject?, error: NSError?) in
        print(html!)
        } as? (Any?, Error?) -> Void)

    print("didFinish")

}

}

Answer 1

Using evaluateJavaScript with a WKWebView is a bit tricky.

Since I think this answer would be useful to many people, rather than address your specific question with a short code snippet and a comment that you need to implement WKScriptMessageHandler, I'm going to post a full, complete example that you can use to see how everything works together.

To use this, create a "Single View Application" iOS project in Xcode and paste this over the default ViewController.swift file.

//
//  WebViewController.swift
//  WKWebViewExample
//
//  Created by par on 4/2/17.
//  Copyright © 2017 par. All rights reserved.  MIT License.
//

import UIKit
import WebKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let webViewController = WebViewController()

        // install the WebViewController as a child view controller
        addChildViewController(webViewController)

        let webViewControllerView = webViewController.view!

        view.addSubview(webViewControllerView)

        webViewControllerView.translatesAutoresizingMaskIntoConstraints = false
        webViewControllerView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        webViewControllerView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        webViewControllerView.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
        webViewControllerView.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true

        webViewController.didMove(toParentViewController: self)
    }
}

class WebViewController: UIViewController, WKNavigationDelegate, WKScriptMessageHandler {
    private var webView: WKWebView!
    private var webViewContentIsLoaded = false

    init() {
        super.init(nibName: nil, bundle: nil)

        self.webView = {
            let contentController = WKUserContentController()

            contentController.add(self, name: "WebViewControllerMessageHandler")

            let configuration = WKWebViewConfiguration()
            configuration.userContentController = contentController

            let webView = WKWebView(frame: .zero, configuration: configuration)
            webView.scrollView.bounces = false
            webView.navigationDelegate = self

            return webView
        }()
    }

    required init?(coder aDecoder: NSCoder) { fatalError("init(coder:) has not been implemented") }

    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(webView)

        webView.translatesAutoresizingMaskIntoConstraints = false
        webView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        webView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        webView.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
        webView.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        if !webViewContentIsLoaded {
            let url = URL(string: "https://stackoverflow.com")!
            let request = URLRequest(url: url)

            webView.load(request)

            webViewContentIsLoaded = true
        }
    }

    private func evaluateJavascript(_ javascript: String, sourceURL: String? = nil, completion: ((_ error: String?) -> Void)? = nil) {
        var javascript = javascript

        // Adding a sourceURL comment makes the javascript source visible when debugging the simulator via Safari in Mac OS
        if let sourceURL = sourceURL {
            javascript = "//# sourceURL=\(sourceURL).js\n" + javascript
        }

        webView.evaluateJavaScript(javascript) { _, error in
            completion?(error?.localizedDescription)
        }
    }

    // MARK: - WKNavigationDelegate

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        // This must be valid javascript!  Critically don't forget to terminate statements with either a newline or semicolon! 
        let javascript =
            "var outerHTML = document.documentElement.outerHTML.toString()\n" +
            "var message = {\"type\": \"outerHTML\", \"outerHTML\": outerHTML }\n" +
            "window.webkit.messageHandlers.WebViewControllerMessageHandler.postMessage(message)\n"

        evaluateJavascript(javascript, sourceURL: "getOuterHMTL")
    }

    // MARK: - WKScriptMessageHandler

    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        guard let body = message.body as? [String: Any] else {
            print("could not convert message body to dictionary: \(message.body)")
            return
        }

        guard let type = body["type"] as? String else {
            print("could not convert body[\"type\"] to string: \(body)")
            return
        }

        switch type {
        case "outerHTML":
            guard let outerHTML = body["outerHTML"] as? String else {
                print("could not convert body[\"outerHTML\"] to string: \(body)")
                return
            }
            print("outerHTML is \(outerHTML)")
        default:
            print("unknown message type \(type)")
            return
        }
    }
}