Feature Request: A way to pass data between scenes

[GameDev-One]

Hi there,

I'd like to request a feature since I have been having a great time using this addon for my project.

Passing data between scenes is useful when you want persistent data between scenes without using a global script. In my case, I am creating a multiplayer game and each player obtains information(data) about their lobby. Which is then passed to the next scene as the transition happens. This data is used to initialize other parts of that "next scene", and then never used again.

It's not that pretty but, I have done the following to pass data between the scenes and thought you might know a cleaner way to implement it.

• Modified all methods that call _change_scene to accept a Dictionary as a parameter. (Defaults to an empty Dictionary)
• Ensure any scene that I wanted to pass data to has a variable called Msg as a Dictionary
• Set the message for the new scene after the old scene has been freed

My Project

# my_main_scene.gd
# Lobby Data as a Dictionary
var Msg: Dictionary = {}

# Change to the new scene after it is done loading
func _OnLoadingFinished() -> void:
	SceneManager.change_scene_to_loaded_scene(
				FadeOutOptions, 
				FadeInOptions,
				GeneralOptions,
				{Msg}
                                )
	pass

Scene Manager Addon

# scene_manager.gd
# checks different states of the scene and makes actual transitions happen
func _change_scene(scene, add_to_back: bool, Msg: Dictionary = {}) -> bool:
    #...
    get_tree().get_current_scene().free()

    # Pass the message to the current scene if the scene has a "Msg" variable
    if "Msg" in scene_instance and not Msg.is_empty():
        scene_instance.Msg = Msg

    root.add_child(scene_instance)
    #...
    pass

[maktoobgar]

Hello,

First, thanks for the issue and I'm happy that this is actually useful.
About the feature, at first glance, I'm not suggesting this.
This addon is supposed to just manage scenes and transition between them but data transfer at the middle of changing scenes... I'm not sure.
I'm not gonna say it isn't possible, it is... but it is not quite suitable.
What I'm thinking is that we already have a signal called change_scene but it gives nothing and just informs you that the scene is now changed. We can change that, in the input of that signal, I'm gonna pass the current new scene and then you can pass the information between the scenes or whatever else you see suitable.
And I know, yes... this requires to write a Global script to listen to the events and have the states for the previous scene on the Global script before changing the scene to then go on the other scene when change_scene signal is called. I know about all that but this is the best approach I can think of right now to keep the addon clean and sole purpose.

Just for recap, what I mean is this:

signal load_percent_changed(value: int)
# This line
signal scene_changed
signal fade_in_started

Changes to this:

signal load_percent_changed(value: int)
# Changes to this
signal scene_changed(current_scene: Node)
signal fade_in_started

What do you think?

[GameDev-One]

Could see that implementation being better, I like it. 😄

It takes better advantage of Godot's ability to pass data around via signals and doesn't require the current scene to store a variable just for passing that data.

Giving it a go myself to see if I can work out a decent implementation for it.

[GameDev-One]

I think the approach you suggested came out quite clean while maintaining the purpose of the addon.

It works great for the Complex examples you provided on the README.md but won't work for the Simple examples. A good reason why it may not be a suitable feature request. (Explained below)

Notably, through some testing, I learned a limitation of Godot when it comes to changing scenes.

Simple
Methods that connect to the signal scene_changed signal won't be triggered if SceneManager.change_scene uses a String as the scene parameter. This is due to how Godot performs the underlying change scene operations to ensure that both scenes AREN'T running at the same time, while still freeing the previous scene in a safe way similar to Node.queue_free. See Godot Docs on SceneTree.change_scene_to_packed() and SceneTree.change_scene_to_file() for more info.

Complex
However, this is not the case if using either SceneManger.change_scene_to_loaded_scene or SceneManager.change_scene_to_existing_scene_in_scene_tree because the scenes are loaded as a PackedScene or Node then are added to the SceneTree prior to swapping them. This means there is a moment where both scenes ARE running before the previous scene is freed, which if I am reading the docs right, is normally considered unsafe.

Below are the small changes I made for my use case.

SceneManager Addon

# scene_manager.gd
signal scene_changed(current_scene: Node)

func change_scene(...)-> void:
    #...
    if _change_scene(scene, general_options.add_to_back, Msg):
        #...
        scene_changed.emit(scene)
    pass

# Optional change
func change_scene_to_existing_scene_in_scene_tree(...) -> void:
    #...    
    # inform godot which now this is the current scene
    get_tree().set_current_scene(scene)
    scene_changed.emit(scene)
    pass

My Project

# my_main_scene.gd
# Lobby Data as a Dictionary
var Msg: Dictionary = {}

func _ready() -> void:
    SceneManager.scene_changed.connect(_OnSceneChanged)
    pass

# Change to the new scene after it is done loading
func _OnLoadingFinished() -> void:
    SceneManager.add_loaded_scene_to_scene_tree()
    SceneManager.change_scene_to_existing_scene_in_scene_tree(
				FadeOutOptions, 
				FadeInOptions,
				GeneralOptions
                                )
    pass

func _OnSceneChanged(current_scene: Node) -> void:
    # Pass message from the main scene to the current scene for initialization
    current_scene.Msg = Msg
    
    # After this method main scene and the current scene will run independently of each other
    pass

All that said, I appreciate, you providing this addon to the public.
I think it is an easy include in anyone's project. ❤️

[maktoobgar]

This needs a little bit of thinking and a little extra time to spend on understanding and then taking an action, I'll come back to this but I hope I can hear your thoughts about this again later because it is needed. 🙌