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

It's open source - https://developers.google.com/cast/

But unfortunately is only available for "Android, Chrome or iOS app"

So this means other applications like Firefox, VLC etc... would have to reverse engineer the protocol.

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.

reverse engineering != porting to an other platform

edit: but is it really open source as said by previous comment or is it just an SDK ?

It's closed source. This project uses an open source implementation he wrote himself: https://github.com/insidegui/ChromeCastCore

> 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.