Accept global classes for MainLoop type in project settings

[Xrayez]

Fixes #35822.
Helps godotengine/godot-proposals#1349.

The application/run/main_loop_type setting can handle custom global classes now (classes exposed with class_name keyword). For instance: MySceneTree.

The setting's default is changed from empty to SceneTree as to give some hint of what kind of input is accepted for the main loop type.

Tested on both editor and release builds.

Some custom scene tree class:

class_name MySceneTree extends SceneTree

func _initialize():
	print("MySceneTree startup!")

func _finalize():
	print("MySceneTree cleanup!")

func _iteration(_delta):
	return false

func _drop_files(files, from_screen):
	print(files, " ", from_screen)

Test project:

global_class_main_loop.zip

[dalexeev]

Note that you can do this:

get_tree().set_script(preload("res://my_scene_tree.gd"))

# my_scene_tree.gd
extends SceneTree

func change_scene(path):
    print("--- %s ---" % path)
    return .change_scene(path)

This will work as expected.

[Xrayez]

get_tree has to be already initialized to do this. You'd miss the ability to receive the _initialize callback:

godot/core/os/main_loop.cpp

Lines 63 to 71 in dc90b17

	void MainLoop::init() {
	if (init_script.is_valid()) {
	set_script(init_script);
	}
	if (get_script_instance()) {
	get_script_instance()->call("_initialize");
	}
	}

If your script inherits directly from MainLoop (not SceneTree), then get_tree() is likely to return null as well, so there would be hardly a way to override this via script.

[Xrayez]

Suggested change

	if (!editor && !ClassDB::class_exists(main_loop_type) && ScriptServer::is_global_class(main_loop_type)) {
	if (!ClassDB::class_exists(main_loop_type) && ScriptServer::is_global_class(main_loop_type)) {

Technically it's possible to extend/override the main loop of the editor itself upon restart, but if there are any errors (wrong base type, invalid script etc), then the editor will always crash with an alert (you'd have to modify project.godot to revert to previous behavior). 😅

Or make it so that it resets the value to SceneTree if we're running the editor automatically.

[akien-mga]

Thanks!

[eon-s]

Can this be added to 3.x too or this depends on GDScript changes?

[Xrayez]

@eon-s I've tried to apply the same patch to 3.2, patch fails, so needs a dedicated PR for 3.2. If @akien-mga is fine about these changes for 3.2, I could certainly make one. 🙂

I think nothing has changed fundamentally in terms of global classes in 4.0.

[RimaitosLab]

Hi @Xrayez,
I'm currently trying to run a piece of code before any Node entered the Scene Tree.
In search of a solution, I stumbled upon the possibility to set your own MainLoop. I wanted to set a MainLoop which extends SceneTree but overrides the _initialize() method.
But I receive the error "Error: MainLoop type doesn't exist: MySceneTree".

I'm running Godot v3.3.2.stable on Linux.

I tried to run your example which failed to parse the project.godot file because the '@' raised a "ConfigFile parse error at [omitted]/project.godot:6: Unexpected character.". After manually editing the project.godot file, the example throws the same error as my previous try.

I hope you can help me with that.
Thanks in advance!

[Xrayez]

I'm running Godot v3.3.2.stable on Linux.

@RimaitosLab this feature was not backported to Godot 3.x branch, that's why you receive the error, but this is currently implemented in Godot 4.x (which is in development, not released yet).

As I said earlier, it's possible to backport the feature, but I need an approval from the maintainers before doing the backport. 🙂

[RimaitosLab]

oh sorry, it seems like I misunderstood the conversation.
Well it seems like I'm gonna learn how to compile Godot 😃

[tommyZZM]

hello @Xrayez @RimaitosLab @eon-s ~

I've tried to backport this feature to 3.x, And it works fine successfully for me. 😄

I've created a pull request here (#52438)