Nodes and resources¶
Up to this tutorial, we focused on the Node class in Godot as that's the one you use to code behavior and most of the engine's features rely on it. There is another datatype that is just as important: Resource.
Nodes give you functionality: they draw sprites, 3D models, simulate physics, arrange user interfaces, etc. Resources are data containers. They don't do anything on their own: instead, nodes use the data contained in resources.
Anything Godot saves or loads from disk is a resource. Be it a scene (a
.scn file), an image, a script... Here are some
Texture, Script, Mesh, Animation, AudioStream, Font, Translation.
When the engine loads a resource from disk, it only loads it once. If a copy of that resource is already in memory, trying to load the resource again will return the same copy every time. As resources only contain data, there is no need to duplicate them.
Every object, be it a Node or a Resource, can export properties. There are many types of Properties, like String, integer, Vector2, etc., and any of these types can become a resource. This means that both nodes and resources can contain resources as properties:
External vs built-in¶
There are two ways to save resources. They can be:
- External to a scene, saved on the disk as individual files.
- Built-in, saved inside the
.scnfile they're attached to.
Clicking the resource preview allows us to view and edit the resource's properties.
The path property tells us where the resource comes from. In this case, it comes
from a PNG image called
robi.png. When the resource comes from a file like
this, it is an external resource. If you erase the path or this path is empty,
it becomes a built-in resource.
The switch between built-in and external resources happens when you save the
scene. In the example above, if you erase the path
save, Godot will save the image inside the
.tscn scene file.
Even if you save a built-in resource, when you instance a scene multiple times, the engine will only load one copy of it.
Loading resources from code¶
There are two ways to load resources from code. First, you can use the
load() function anytime:
You can also
preload resources. Unlike
load, this function will read the
file from disk and load it at compile-time. As a result, you cannot call preload
with a variable path: you need to use a constant string.
Scenes are also resources, but there is a catch. Scenes saved to disk are resources of type PackedScene. The scene is packed inside a resource.
To get an instance of the scene, you have to use the PackedScene.instance() method.
This method creates the nodes in the scene's hierarchy, configures them, and returns the root node of the scene. You can then add it as a child of any other node.
The approach has several advantages. As the PackedScene.instance() function is fast, you can create new enemies, bullets, effects, etc. without having to load them again from disk each time. Remember that, as always, images, meshes, etc. are all shared between the scene instances.
Resource is no longer in use, it will automatically free itself.
Since, in most cases, Resources are contained in Nodes, when you free a node,
the engine frees all the resources it owns as well if no other node uses them.
Creating your own resources¶
Like any Object in Godot, users can also script Resources. Resource scripts inherit the ability to freely translate between object properties and serialized text or binary data (*.tres, *.res). They also inherit the reference-counting memory management from the Reference type.
This comes with many distinct advantages over alternative data structures, such as JSON, CSV, or custom TXT files. Users can only import these assets as a Dictionary (JSON) or as a File to parse. What sets Resources apart is their inheritance of Object, Reference, and Resource features:
- They can define constants, so constants from other data fields or objects are not needed.
- They can define methods, including setter/getter methods for properties. This allows for abstraction and encapsulation of the underlying data. If the Resource script's structure needs to change, the game using the Resource need not also change.
- They can define signals, so Resources can trigger responses to changes in the data they manage.
- They have defined properties, so users know 100% that their data will exist.
- Resource auto-serialization and deserialization is a built-in Godot Engine feature. Users do not need to implement custom logic to import/export a resource file's data.
- Resources can even serialize sub-Resources recursively, meaning users can design even more sophisticated data structures.
- Users can save Resources as version-control-friendly text files (*.tres). Upon exporting a game, Godot serializes resource files as binary files (*.res) for increased speed and compression.
- Godot Engine's Inspector renders and edits Resource files out-of-the-box. As such, users often do not need to implement custom logic to visualize or edit their data. To do so, double-click the resource file in the FileSystem dock or click the folder icon in the Inspector and open the file in the dialog.
- They can extend other resource types besides just the base Resource.
Godot makes it easy to create custom Resources in the Inspector.
- Create a plain Resource object in the Inspector. This can even be a type that derives Resource, so long as your script is extending that type.
- Set the
scriptproperty in the Inspector to be your script.
The Inspector will now display your Resource script's custom properties. If one edits those values and saves the resource, the Inspector serializes the custom properties too! To save a resource from the Inspector, click the Inspector's tools menu (top right), and select "Save" or "Save As...".
If the script's language supports script classes, then it streamlines the process. Defining a name for your script alone will add it to the Inspector's creation dialog. This will auto-add your script to the Resource object you create.
Let's see some examples.
Resource scripts are similar to Unity's ScriptableObjects. The Inspector provides built-in support for custom resources. If desired though, users can even design their own Control-based tool scripts and combine them with an EditorPlugin to create custom visualizations and editors for their data.
Unreal Engine 4's DataTables and CurveTables are also easy to recreate with Resource scripts. DataTables are a String mapped to a custom struct, similar to a Dictionary mapping a String to a secondary custom Resource script.
Instead of just inlining the Dictionary values, one could also, alternatively...
- Import a table of values from a spreadsheet and generate these key-value pairs, or...
- Design a visualization within the editor and create a simple plugin that adds it to the Inspector when you open these types of Resources.
Beware that resource files (*.tres/*.res) will store the path of the script
they use in the file. When loaded, they will fetch and load this script as an
extension of their type. This means that trying to assign a subclass, i.e. an
inner class of a script (such as using the
class keyword in GDScript) won't
work. Godot will not serialize the custom properties on the script subclass properly.
In the example below, Godot would load the
Node script, see that it doesn't
Resource, and then determine that the script failed to load for the
Resource object since the types are incompatible.