The Chromecast protocol specification (CASTV2) itself is sort of open. To implement it on macOS I did a little bit of reverse engineering and used a description of the protocol I found here: https://github.com/thibauts/node-castv2
VLC has Chromecast support in the development version, and has had it for over a year now. They had to reverse-engineer the protocol. I think the UI is a bit cludgey but it works.
> This framework implements the Google Cast APIs so they can be used in macOS apps. Google provides an official SDK but it is only for iOS and closed source.
Yes, I went to the link quite happy because I thought it was related to AppleEvents and maybe Apple Scripts. Your app is definitely cool, congratulations on your work but the name got me fooled.
PS: Maybe the HN people haven't used MacOS 9 and AppleScript a lot and will not do the association I did.
I don't know which part you're most interested in, but in a nutshell: the events are fetched from the same API as the Apple TV Events app (found that using a proxy), the Chromecast streaming uses Bonjour to find Chromecast devices on the network and the CASTV2 protocol over TCP to send commands to them.