For WKWebview there is also wknavigationdelegate didfinish, but not do the trick as the SO question WKWebView didn't finish loading, when didFinishNavigation is called - Bug in WKWebView? and this answer show.
I also found when the page to load is very complicate and dinamic, UIWebview's webViewDidFinishLoad(- webView: UIWebView) also not works.
And I think use native swift or objective-c to detect when the page is loaded is a bad choice. The more flexable and effective way is to use javascript to call the native swift code when page finishing loading. What's more this also work for a specific dom finishing loading event and very dynamic content.
For how to call swift from javascript refer to this post.
Here is a sample code for dynamic content with anjularjs.
js,
var homeApp = angular.module('home', ['ui.bootstrap', 'ngAnimate']);
homeApp
    .directive("printerinfo",
    function() {
        return {
            restrict: "E",
            link: function() {       //call navite swift when page finishing loading
                try {
                    window.webkit.messageHandlers.testHandler.postMessage("Hello from JavaScript");
                } catch(err) {
                    console.log('The native context does not exist yet');
                }
            },
            templateUrl: "/static/tpls/cloud/app/printerinfo.php",
        }
    })
swift3,
func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
    print(message.name)
    if(message.name == "testHandler") {
        //webpage content loaded
        print(Date())
        print("javascript test call swift")
    }
Here I use angularjs to check element ready, you can also refter how-to-call-a-function-after-a-div-is-ready  or check-if-a-given-dom-element-is-ready or jquery-ready-equivalent-event-listener-on-elements for more.