tabris-js: WebView Change Events Not Triggering

Problem description

When navigating using a WebView, the ChangeEvents associated with certain properties are not firing, even when those properties are indeed changing.

Environment

• Tabris.js version: 3.0.0-beta1
• iOS 11.1.1

Here is some code I wrote to illustrate the problem

/* webview */
let wv = new WebView({
	top: 0,
	bottom: '#toolbar',
	left: 0,
	right: 0,
	url: "https://www.duckduckgo.com"
})
.on('canGoBackChanged', () => console.log('canGoBackChanged triggered'))
.on('canGoForwardChanged', () => console.log('canGoForwardChanged triggered'))
.on('urlChanged', () => console.log('urlChanged triggered'))
.on('htmlChanged', () => console.log('htmlChanged triggered'))
.on('load', () => console.log('load triggered'))
.on('navigate', () => console.log('navigate triggered'))
.on('download', () => console.log('download triggered'))
.on('message', () => console.log('message triggered'))
.appendTo(ui.contentView);

/* log properties */
new Button({
	id: 'toolbar',
	left: 0,
	right: '#goback',
	bottom: 0,
	top: '90%',
	text: 'Log Properties'
}).on('select', () => {
	console.debug("canGoBack: " + wv.canGoBack);
	console.debug("canGoForward: " + wv.canGoForward);
	console.debug("url: " + wv.url);
	console.debug("html: " + wv.html);
})
.appendTo(ui.contentView);

/* go back */
new Button({
	id: 'goback',
	left: '#toolbar',
	right: 0,
	bottom: 0,
	top: '90%',
	text: 'Go Back'
}).on('select', () => {
	wv.goBack();
}).appendTo(ui.contentView);

After running the code above, here is the console output, with comments to explain when which actions were performed:

<!-- initial appending to contentView -->
navigate triggered
navigate triggered
navigate triggered
load triggered

<!-- properties logged -->
canGoBack: false
canGoForward: false
url: https://duckduckgo.com/
html: undefined

<!-- searching for "test" -->
navigate triggered
navigate triggered
load triggered

<!-- properties logged -->
canGoBack: true
canGoForward: false
url: https://duckduckgo.com/?q=test&t=h_&ia=news
html: undefined

<!-- calling .goBack() -->
navigate triggered
load triggered

<!-- logging properties -->
canGoBack: false
canGoForward: true
url: https://duckduckgo.com/
html: undefined

As far as I can tell, canGoBackChanged, canGoForwardChanged, and urlChanged should be firing, as it is clear that their values are changing. I understand why download, htmlChanged, and message aren’t being triggered, but I figured I’d include them for completeness.

In the above example, navigate and load are triggered on every navigation, but I’ve also run into cases where they don’t fire when they’re supposed to as well. Here is another annotated console log, showing what happens when the URL is initially set as this tweet:

<!-- initial appending -->
navigate triggered
navigate triggered
load triggered

<!-- log properties -->
canGoBack: false
canGoForward: false
url: https://mobile.twitter.com/tabrisjs/status/1089827821487312897
html: undefined

<!-- navigating to Tabris.js user page -->

<!-- log properties -->
canGoBack: true
canGoForward: false
url: https://mobile.twitter.com/tabrisjs
html: undefined

<!-- navigating to a different tweet -->

<!-- log properties -->
canGoBack: true
canGoForward: false
url: https://mobile.twitter.com/tabrisjs/status/1075320982909407232
html: undefined

<!-- calling .goBack() -->

In my particular use case, the goal is to darken a back button if the WebView cannot navigate back. As of now, I initialize the button as dark, but there is no way to tell when the WebView becomes able to navigate back without a reliable event to listen to.

Thanks in advance!