Write a migration guide for Godot 4

[dploeger]

Issue description:

Because I remember the switch from Godot 2 to 3, I'm just putting this here to request a full migration guide for Godot 4.

I'd suggest that all developers, that make a breaking change on an object's properties or behavior or GDScript or C# language changes, comment a short info about that change and maybe some additional context to this issue and at the end I'll write a migration guide and add a PR for it.

[Calinou]

I have a preliminary 4.0 changelog here: https://gist.github.com/Calinou/49aefe52ce8f67ffa3f743932123d14f

Node renames are handled automatically by the editor, and it's planned to do the same for node properties as well. However, scripts will likely not be edited automatically.

See also godotengine/godot-proposals#387.

[qarmin]

Most renames in GDScript files(like classes or function names) should be handled by godotengine/godot#51950 (this isn't merged yet).

I think that with its help upgrading should be quite easy(not yet, since there is a lot of GDScript 2.0 bugs which prevents sometimes from using basic functionalities).

I'm still trying to get list of all C# classes and methods, but for now I'm unable to do this, so converting C# project probably will be more difficult due lack of support by this tool.

[and-rad]

For now, animations that contain transform tracks have to be altered manually by replacing those with individual tracks for position_3d, rotation_3d, and scale_3d. See this issue for an example of the problem.

EDIT: Nevermind, didn't look at the preliminary changelog first. It's already mentioned there.

[WolfgangSenff]

I've started the upgrade for Crystal Climber to 4.0 and have begun creating a gist with all the things I've had to do so far. There will be quite a bit more to it as I'm able to work on it, but see https://gist.github.com/WolfgangSenff/168cb0cbd486c8c9cd507f232165b976 for a good start on a GDScript 4.0 upgrade cheat sheet.

[Toasterson]

I started an upgrade experiment aswell for Fleets of Sol. But I hit a blockade. Node3D has no more get_world() function. Where has that landed? Does it still exist?

[qarmin]

get_world -> get_world_3d
You can look at the code inside file editor/converter.cpp to see for function renames

[Cerno-b]

Just as a note: It is probably worthwhile to have a section in the manual for all the code changes in a systematic and complete table.

There are a lot of Godot 3 tutorials and Q&As out there that will cause some frustration if the answers don't work anymore in Godot 4 because OS.get_screen_size() is now DisplayServer.screen_get_size() etc. So having a handy table of all the functions that have been renamed or moved would be really neat.

[deralmas]

@Cerno-b perhaps godotengine/godot#51950 might be used to build one?

[Cerno-b]

@Riteo Sounds like a good way to semi-automatically generate the list and make sure it is kept up to date in case the autoconverter gets extended later.

[deralmas]

@Cerno-b Thinking about it, I wonder if it'd be useful to have such a list if there's already a converter.

[dploeger]

@Riteo I guess it makes sense to see it documented how the converter works. Just as a reference or if something goes wrong.