Oh, I think we already have something you ask for, though it’s not a class 
Namely, you can use global routine RegisterUrlProtocol to register your own URL protocol, with custom callbacks for reading and writing. This way all the engine reading/writing routines will recognize the new URL protocol.
In effect, a virtual filesystem is defined by the callbacks (and you can naturally implement them all by a single class, which is what TCastleZip does). There are 3 callbacks provided to RegisterUrlProtocol now, all optional:
- asynchronous reader
- synchronous reader (just return readable TStream for the resource)
- writer (just return writeable TStream for the resource)
I want to keep extending this API, add additional callbacks to
- check file/directory existence (so that UriExists can handle the custom protocol). This should happen soon, it’s necessary for one top-secret project I do to showcase our web support
- enumerate directory contents (so that you can search arbitrary filesystems with FindFiles ). This is also on TODO, though not immediate.
Possibly, at some point RegisterUrlProtocol will take a record/class with these callbacks, to avoid having a procedure with zillion optional parameters
I have just reworked our documentation about URLs, loading (downloading) and saving resources so hopefully it nicely documents / links to our capabilities in this regard.
See and run castle-engine/examples/network/custom_url_handler at master · castle-engine/castle-engine · GitHub (without the USE_ZIP_URL_HANDLER symbol defined) for a demo how RegisterUrlProtocol can be used.