What are Godot classes really?¶
Godot offers two main means of creating types: scripts and scenes. Both of these represent a “class” since Godot revolves around Object-Oriented design. How they do this may not be clear to beginner or intermediate users though.
Godot Engine provides classes out-of-the-box (like Node), but user-created types are not actually classes. Instead they are resources that tell the engine a sequence of initializations to perform on an engine class.
Godot’s internal classes have methods that register a class’s data with a ClassDB. This database provides runtime access to class information (also called “reflection”). Things stored in the ClassDB include, among other things…
Furthermore, this ClassDB is what Objects actually check against when performing any operation. Access a property? Call a method? Emit a signal? It will check the database’s records (and the records of the Object’s base types) to see if the Object supports the operation. Every C++ Object defines a static _bind_methods() function that describes what C++ content it registers to the database and how.
So, if the engine provides all of this data at startup, then how does a user define their own data? It’d be nice if users could define a custom set of data to be appended to an object’s data. That way, users could inject their own properties and methods into the engine’s Object query requests.
This is what a Script is. Objects check their attached script before the database, so scripts can even override methods. If a script defines a _get_property_list() method, that data is appended to the list of properties the Object fetches from the ClassDB. The same holds true for other declarative code.
This can lead to some users’ confusion when they see a script as being a class unto itself. In reality, the engine just auto-instantiates the base engine class and then adds the script to that object. This then allows the Object to defer to the Script’s content where the engine logic deems appropriate.
A problem does present itself though. As the size of Objects increases, the scripts’ necessary size to create them grows much, much larger. Creating node hierarchies demonstrates this. Each individual Node’s logic could be several hundred lines of code in length.
let’s see a simple example of creating a single Node as a child.
Notice that only two pieces of declarative code are involved in the creation of this child node: the variable declaration and the constructor declaration. Everything else about the child must be setup using imperative code. However, script code is much slower than engine C++ code. Each change must make a separate call to the scripting API which means a lot of C++ “lookups” within data structures to find the corresponding logic to execute.
To help offload the work, it would be convenient if one could batch up all operations involved in creating and setting up node hierarchies. The engine could then handle the construction using its fast C++ code, and the script code would be free from the perils of imperative code.
This is what a scene (PackedScene) is: a resource that provides an advanced “constructor” serialization which is offloaded to the engine for batch processing.
Now, why is any of this important to scene organization? Because one must understand that scenes are objects. One often pairs a scene with a scripted root node that makes use of the sub-nodes. This means that the scene is often an extension of the script’s declarative code.
It helps to define…
- what objects are available to the script?
- how are they organized?
- how are they initialized?
- what connections to each other do they have, if any?
As such, many Object-Oriented principles which apply to “programming”, i.e. scripts, also apply to scenes. Some scripts are designed to only work in one scene (which are often bundled into the scene itself). Other scripts are meant to be re-used between scenes.
Regardless, the scene is always an extension of the root script, and can therefore be interpreted as a part of the class. Most of the points covered in this series will build on this point, so keep it in mind.