Add callbacks to Main::setup() / Main::start() / Main::cleanup(), so to allow much freedom to modules.

[AndreaCatania]

Describe the project you are working on:
I'm working on a game, and I'm creating various C++ modules. Couple of those modules, need to know when the engine setup is over, and so the SceneTree or other singletons are ready to be used.

Describe the problem or limitation you are having in your project:
Those modules try to connect to some events like process and so on, but since the engine is not ready, there is no way to connects to those events. Right now, from the register_MODULE_types() it's not possible to know when the engine starts.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
The idea is to add 3 callbacks to the Main class:

• on_setup_done
• on_start
• on_cleanup

From the register_MODULE_types() will be possible to add callbacks in the following way:

void register_MODULE_types(){
    Main::add_callback(&my_setup_function, Main::SETUP_CALLBACK);
}

All the registered callbacks will be called when the setup/start terminates or the cleanup is started.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
Right before the Main::setup2 is done https://github.com/godotengine/godot/blob/master/main/main.cpp#L1719-L1720 the following code is executed:

for(int i = 0; i < on_setup_done_callbacks.size(); i+=1){
    on_setup_done_callbacks[i]();
}

So the modules, will be able to interact with the engine extensively.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
Low level feature.

Is there a reason why this should be core and not an add-on in the asset library?:
Low level feature.

[Xrayez]

This is needed because of issues like Zylann/godot_voxel#189.

Godot 4.0 adds ability to preregister_types(), but it has no corresponding preunregister_types(). Of course having Main callbacks as proposed here could also provide an extensive solution, I believe. 🙂

[Zylann]

Would this also allow my module to register a "process" callback without needing a node in the tree? I have some sort of server which needs to run an update loop like some other servers do (Physics Server, Visual Server) but I could not find a way to insert this within the main loop.

[AndreaCatania]

Yes, you can obtain the SceneTree singleton (into the setup_done callback) and register the function to the process event. In this way your singleton is naturally called each frame or physics frame without the need to use a node.

[Xrayez]

I've noticed there's SceneTree::add_idle_callback() which can be used in modules, but this is only useful for flushing custom deferred calls since that's how MessageQueue flushing is currently used throughout the engine (those idle callbacks are called on _call_idle_callbacks()), you can actually see it in action in goostengine/goost#51.

I suspect that it might not be always thread-safe, so that's where we'd also like to have ability to register process callbacks as well. Due to this, I think that it may be also required to check whether you're inside physics frame with Engine.is_in_physics_frame() too if you do use SceneTree::add_idle_callback(), but I'm not 100% sure about it.

Yes, you can obtain the SceneTree singleton (into the setup_done callback) and register the function to the process event.

@AndreaCatania not sure whether I understand this correctly, do you imply that there's some other/existing mechanism already available for registering process callbacks within the engine (just like SceneTree::add_idle_callback(), or perhaps we're talking about the same thing already)?

[AndreaCatania]

I've implemented this feature here: godotengine/godot#50179 @Xrayez check it, maybe it's cleaner what I meant by checking the code.