[webview_flutter_wkwebview] Implementation of PlatformNavigationDelegate for WebKit

[bparrishMines]

Part of flutter/flutter#94051

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[bparrishMines]

In defense of this pattern:

I wanted setters for the callbacks methods because I wanted a consistent way for a platform to indicate it doesn't support a feature.

For example, if Android doesn't support WebViewController.callSomeMethod(), it throws an UnimplementedError by default. By using setters for callback methods, the default response for a callback method is to throw an UnimplementedError. I didn't know a way to do this with a class that takes the callback methods as constructor parameters.

Also note that the app facing package will probably use fields:

class NavigationDelegate {
  final Function(String) onPageFinished;
}

But the platform classes will use setters so they can throw when a callback is not supported.

[stuartmorgan-g]

By using setters for callback methods, the default response for a callback method is to throw an UnimplementedError.

I think I'm missing something about the implementation; where does that happen? It looks in the code above this like the default would be a silent no-op for a null method.

[bparrishMines]

It's done in the platform interface class: https://github.com/flutter/plugins/blob/main/packages/webview_flutter/webview_flutter_platform_interface/lib/v4/src/platform_navigation_delegate.dart#L63

PlatformNavigationDelegate has to be extended, so when a new callback method is added (e.g. setOnPageRedirected) platform implementations would throw an UnimplementedError by default.

[stuartmorgan-g]

LGTM

[stuartmorgan-g]

Nit: can't this be weakThis.target?._onProgress != null?