Singletons (AutoLoad)

Introduction

Godot’s scene system, while powerful and flexible, has a drawback: there is no method for storing information (e.g. a player’s score or inventory) that is needed by more than one scene.

It’s possible to address this with some workarounds, but they come with their own limitations:

  • You can use a “master” scene that loads and unloads other scenes as its children. However, this means you can no longer run those scenes individually and expect them to work correctly.
  • Information can be stored to disk in user:// and then loaded by scenes that require it, but frequently saving and loading data is cumbersome and may be slow.

The Singleton Pattern is a useful tool for solving the common use case where you need to store persistent information between scenes. In our case it is possible re-use the same scene or class for multiple singletons, so long as they have different names.

Using this concept, you can create objects that:

  • Are always loaded, no matter which scene is currently running
  • Can store global variables, such as player information
  • Can handle switching scenes and between-scene transitions
  • Act like a singleton, since GDScript does not support global variables by design

Autoloading nodes and scripts caters to this need.

AutoLoad

You can use AutoLoad to load a scene or a script that inherits from Node. Note: when autoloading a script, a Node will be created and the script will be attached to it. This node will be added to the root viewport before any other scenes are loaded.

../../_images/singleton.png

To autoload a scene or script, select Project -> Project Settings from the menu and switch to the “AutoLoad” tab.

../../_images/autoload_tab.png

Here you can add any number of scenes or scripts. Each entry in the list requires a name, which assigned as the node’s name property. The order of the entries as they added to the global scene tree can be manipulated using the up/down arrow keys.

../../_images/autoload_example.png

This means that any node can access a singleton named “PlayerVariables” with:

var player_vars = get_node("/root/PlayerVariables")
player_vars.health -= 10
var playerVariables = (PlayerVariables)GetNode("/root/PlayerVariables");
playerVariables.Health -= 10; // Instance field.

If the “Enable” column is checked (default true) then the singleton can simply be accessed directly:

PlayerVariables.health -= 10
// Static members can be accessed by using the class name.
PlayerVariables.Health -= 10;

Note that autoload objects (scripts and/or scenes) are accessed just like any other node in the scene tree. In fact, if you look at the running scene tree, you’ll see the autoloaded nodes appear:

../../_images/autoload_runtime.png

Custom scene switcher

This tutorial will demonstrate building a scene switcher using autoload. For basic scene switching, you can use the SceneTree.change_scene() method (see SceneTree for details). However, if you need more complex behavior when changing scenes, this method provides more functionality.

To begin, download the template from here: autoload.zip and open it in Godot.

The project contains two scenes: Scene1.tscn and Scene2.tscn. Each scene contains a label displaying the scene name and a button with its pressed() signal connected. When you run the project, it starts in Scene1.tscn. However, pressing the button does nothing.

Global.gd

Switch to the “Script” tab and create a new script called Global.gd. Make sure it inherits from Node:

../../_images/autoload_script.png

The next step is to add this script to the autoLoad list. Open Project > Project Settings from the menu, switch to the “AutoLoad” tab and select the script by clicking the .. button or typing its path: res://Global.gd. Press “Add” to add it to the autoload list:

../../_images/autoload_tutorial1.png

Now whenever we run any scene in the project, this script will always be loaded.

Returning to the script, it needs to fetch the current scene in the _ready() function. Both the current scene (the one with the button) and global.gd are children of root, but autoloaded nodes are always first. This means that the last child of root is always the loaded scene.

extends Node

var current_scene = null

func _ready():
    var root = get_tree().get_root()
    current_scene = root.get_child(root.get_child_count() - 1)
using Godot;
using System;

public class Global : Godot.Node
{
    public Node CurrentScene { get; set; }

    public override void _Ready()
    {
        Viewport root = GetTree().GetRoot();
        CurrentScene = root.GetChild(root.GetChildCount() - 1);
    }
}

Now we need a function for changing the scene. This function needs to free the current scene and replace it with the requested one.

func goto_scene(path):
    # This function will usually be called from a signal callback,
    # or some other function in the current scene.
    # Deleting the current scene at this point is
    # a bad idea, because it may still be executing code.
    # This will result in a crash or unexpected behavior.

    # The solution is to defer the load to a later time, when
    # we can be sure that no code from the current scene is running:

    call_deferred("_deferred_goto_scene", path)


func _deferred_goto_scene(path):
    # It is now safe to remove the current scene
    current_scene.free()

    # Load the new scene.
    var s = ResourceLoader.load(path)

    # Instance the new scene.
    current_scene = s.instance()

    # Add it to the active scene, as child of root.
    get_tree().get_root().add_child(current_scene)

    # Optionally, to make it compatible with the SceneTree.change_scene() API.
    get_tree().set_current_scene(current_scene)
public void GotoScene(string path)
{
    // This function will usually be called from a signal callback,
    // or some other function from the current scene.
    // Deleting the current scene at this point is
    // a bad idea, because it may still be executing code.
    // This will result in a crash or unexpected behavior.

    // The solution is to defer the load to a later time, when
    // we can be sure that no code from the current scene is running:

    CallDeferred(nameof(DeferredGotoScene), path);
}

public void DeferredGotoScene(string path)
{
    // It is now safe to remove the current scene
    CurrentScene.Free();

    // Load a new scene.
    var nextScene = (PackedScene)GD.Load(path);

    // Instance the new scene.
    CurrentScene = nextScene.Instance();

    // Add it to the active scene, as child of root.
    GetTree().GetRoot().AddChild(CurrentScene);

    // Optionally, to make it compatible with the SceneTree.change_scene() API.
    GetTree().SetCurrentScene(CurrentScene);
}

As mentioned in the comments above, we need to avoid the situation of deleting the current scene while it is still being used (i.e. its code is still running), so using Object.call_deferred() is required at this point. The result is that the second function will run at a later time when any code from the current scene has completed.

Finally, we need to fill the empty callback functions in the two scenes:

# Add to 'Scene1.gd'.

func _on_Button_pressed():
    Global.goto_scene("res://Scene1.tscn")
// Add to 'Scene1.cs'.

public void OnButtonPressed()
{
    var global = (Global)GetNode("/root/Global");
    global.GotoScene("res://Scene2.tscn");
}

and

# Add to 'Scene2.gd'.

func _on_Button_pressed():
    Global.goto_scene("res://Scene2.tscn")
// Add to 'Scene2.cs'.

public void OnButtonPressed()
{
    var global = (Global)GetNode("/root/Global");
    global.GotoScene("res://Scene1.tscn");
}

Run the project and test that you can switch between scenes by pressing the button!

Note: When scenes are small, the transition is instantaneous. However, if your scenes are more complex, they may take a noticeable amount of time to appear. To learn how to handle this, see the next tutorial: Background loading