Cannot load local resources from WebView

[marcprux]

It is common to want to load a locally-bundled HTML resource and load it in a web view, like so:

WebView(url: Bundle.main.url(forResource: "index", withExtension: "html", subdirectory: "about"))

This works in iOS, but on Android the load fails with the error:

The webpage at jar:file:/data/app/~~id/base.apk!/pgk/Resources/about/index.html could not be loaded because: 
net::ERR_UNKOWN_URL_SCHEME

The problem is that the Android web view doesn't automatically handle the jar:file: scheme.

If Skip resources were bundles as Android resources (which they aren't: they are traditional Java resources), then we might be able to transform local file loads into file://android_assets/ or file://android_res/ schemes, but this is expressly recommended against at https://developer.android.com/develop/ui/views/layout/webapps/load-local-content#antipatterns

The correct solution will probably be to implement a WebViewAssetLoader, as per https://developer.android.com/develop/ui/views/layout/webapps/load-local-content#assetloader . This would enable the customization of resource loading, and we could use the same mechanism to implement bespoke content loading schemes (e.g., loading content from a zip archive). Parity with WKWebView could be achieved through the WKNavigationDelegate decidePolicyFor delegate.

For the time being, the only workaround is to manually load the contents of the file into memory and load the HTML, like so:

WebView(html: try String(contentsOf: Bundle.main.url(forResource: "index", withExtension: "html", subdirectory: "about")))

This is very limited, though, because peer resources (e.g., scripts, stylesheets, and images) will not be resolved, so the HTML must be completely self-contained.