Add support for cross-language mixins using MixinScript (aka MultiScript)

[Xrayez]

This is a resurrection of multiscript PR in Godot, see godotengine/godot#8718. The feature got re-added in development versions of Godot in 2017, but got quickly removed again, so most users have never seen MultiScript in Godot.

See also godotengine/godot-proposals#1346, godotengine/godot-proposals#758, godotengine/godot#23101.

The reason why it was removed is because of various limitations inherent to cross-language implementation of having multiple scripts per instance, and the usability of the feature was really lacking. I've took some time to improve upon the previous implementation in Godot and even made an editor for it, and now it's an actual language which is recognized via ms extension:

Mixin script create dialog

Mixin script editor

The reason why I've rebranded the MultiScript to MixinScript is mainly because it better reflects the logic behind the inner workings, and potentially allows to avoid ambiguity with the notion of having "multiple" scripts per node: https://stackoverflow.com/questions/925609/mixins-vs-traits.

With this feature, it's possible to combine the methods, properties, signals of GDScript, VisualScript, NativeScript etc. into a single instance. Notice that I've emphasized instance because MixinScript only extends run-time functionality of objects/nodes. For example, here's how editing a Mixin looks like:

extends Mixin
#extends KinematicBody2D # The real object.

const GRAVITY = 980
var velocity = Vector2()

func _ready():
	print("Character is ready (character_main.gd)")

func _physics_process(_delta):
	velocity.y += GRAVITY * _delta
	velocity = owner.move_and_slide(velocity, Vector2.UP)

func jump():
	if owner.is_on_floor():
		velocity.y -= GRAVITY * 0.5

So, the real limitation is that all objects must inherit Mixin class (at least in GDScript), but at the end of the day, you can code the real object via owner property (owner has MixinScript attached). If other mixins define new methods, those will be available in owner as well, regardless of whether they are written in GDScript, VisualScript, C# etc. This allows you to avoid depending on node hierarchy.

If you're dealing with the MixinScript instance from outside of the class (say when you instantiate a scene), then you won't have to use owner property; all properties and methods should be made available as if they were defined in the same script for real.

To-do

Depending on whether the feature will be useful for other developers, there are still quite some work to do on the editor side to make the development experience more smooth:

• Drag-n-drop.
• Undo-redo.
• Update list on filesystem changes.

Test project

https://github.com/goostengine/goost-examples/tree/8e634b4a30404a2a55fb48dfb7382b4eabbbace5/scripting/mixins

[Xrayez]

YOLO.

[ghost]

This is such an amazing feature!