Attention: Here be dragons

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Autoloads versus regular nodes

Redot offers a feature to automatically load nodes at the root of your project, allowing you to access them globally, that can fulfill the role of a Singleton: Singletons (Autoload). These autoloaded nodes are not freed when you change the scene from code with SceneTree.change_scene_to_file.

In this guide, you will learn when to use the Autoload feature, and techniques you can use to avoid it.

The cutting audio issue

Other engines can encourage the use of creating manager classes, singletons that organize a lot of functionality into a globally accessible object. Redot offers many ways to avoid global state thanks to the node tree and signals.

For example, let's say we are building a platformer and want to collect coins that play a sound effect. There's a node for that: the AudioStreamPlayer. But if we call the AudioStreamPlayer while it is already playing a sound, the new sound interrupts the first.

A solution is to code a global, autoloaded sound manager class. It generates a pool of AudioStreamPlayer nodes that cycle through as each new request for sound effects comes in. Say we call that class Sound, you can use it from anywhere in your project by calling Sound.play("coin_pickup.ogg"). This solves the problem in the short term but causes more problems:

  1. Global state: one object is now responsible for all objects' data. If the Sound class has errors or doesn't have an AudioStreamPlayer available, all the nodes calling it can break.

  2. Global access: now that any object can call Sound.play(sound_path) from anywhere, there's no longer an easy way to find the source of a bug.

  3. Global resource allocation: with a pool of AudioStreamPlayer nodes stored from the start, you can either have too few and face bugs, or too many and use more memory than you need.

Note

About global access, the problem is that any code anywhere could pass wrong data to the Sound autoload in our example. As a result, the domain to explore to fix the bug spans the entire project.

When you keep code inside a scene, only one or two scripts may be involved in audio.

Contrast this with each scene keeping as many AudioStreamPlayer nodes as it needs within itself and all these problems go away:

  1. Each scene manages its own state information. If there is a problem with the data, it will only cause issues in that one scene.

  2. Each scene accesses only its own nodes. Now, if there is a bug, it's easy to find which node is at fault.

  3. Each scene allocates exactly the amount of resources it needs.

Managing shared functionality or data

Another reason to use an Autoload can be that you want to reuse the same method or data across many scenes.

In the case of functions, you can create a new type of Node that provides that feature for an individual scene using the class_name keyword in GDScript.

When it comes to data, you can either:

  1. Create a new type of Resource to share the data.

  2. Store the data in an object to which each node has access, for example using the owner property to access the scene's root node.

When you should use an Autoload

GDScript supports the creation of static functions using static func. When combined with class_name, this makes it possible to create libraries of helper functions without having to create an instance to call them. The limitation of static functions is that they can't reference member variables, non-static functions or self.

Since Redot 4.1, GDScript also supports static variables using static var. This means you can now share variables across instances of a class without having to create a separate autoload.

Still, autoloaded nodes can simplify your code for systems with a wide scope. If the autoload is managing its own information and not invading the data of other objects, then it's a great way to create systems that handle broad-scoped tasks. For example, a quest or a dialogue system.

Note

An autoload is not necessarily a singleton. Nothing prevents you from instantiating copies of an autoloaded node. An autoload is only a tool that makes a node load automatically as a child of the root of your scene tree, regardless of your game's node structure or which scene you run, e.g. by pressing the F6 key.

As a result, you can get the autoloaded node, for example an autoload called Sound, by calling get_node("/root/Sound").