Showing image until web container is ready

I have placed a web container in the background and tried to show loading image during the contents of this web container is ready.
How can I do this?

I think preload doesn't work on this because the background web container will be loaded after preloading.

The issue names no file, entry point, or test. Start by locating the web-container loading and preload handling in the iOS project, then determine how readiness is exposed; done means a loading image remains visible until the background web container's contents are ready.

Written by the indexing model from the issue text.