expose webview onWebResourceError callback

[lisuizhe]

try resolve #129 properly

Is there a way to catch the Exception and call reload() for the webview render properly?

The blank issue may related to flutter/flutter#61795

I think this issue is because of a more general issue with the design of the framework itself. When the state of a widget changes, flutter doesn't change the widget, but discards it and makes a new one. This is fine for lightweight text widgets and such, but for webviews it means creating and destroying a webview every time the page is scrolled potentially.

I am using flutter_echarts plugin in my project, and facing blank issue too. It happen only on ios in practice(but may due to ios webview is resource heavy; more webview/echarts in a screen may increase the error rate)
For plugin side, currently best work-around maybe expose a callback when error for users to reload.

[lisuizhe]

@entronad Could you please check this PR, thanks!

[entronad]

Thanks!

[entronad]

Published 2.2.0

[lisuizhe]

Thanks! @entronad
However, I am debugging the issue and found that:
sometimes the WebResourceError:webViewInvalidated is not triggering onWebResourceError but delayed until evaluateJavascript is called in webview_flutter.
So probably I need to raise another PR to expose an onEvaluateJavascriptError callback for flutter_echarts's users to handle

[entronad]

OK, you can integrate the two callbacks in one.