Provide means to implement global singleton earlier

[Enyium]

I'm using Rust.

When you call callbacks from a global singleton in a component's init callback, they aren't yet implemented, even if the first thing you do after AppWindow::new() is to implement the global singleton. This limits you.

Would it be possible to provide another constructor function with_...() or so on window components that maybe demands an FnOnce closure where you can call through to your function that implements the window's global singletons?

[ogoffart]

Indeed.
And we also need a way to share globals accross windows.

[seijikun]

Issue for shared globals: #6877