tutorial_scene_main_loop

SceneMainLoop

Introduction

This is where things start getting abstract, but don't panic, as there's not really more depth than this. In previous tutorials, everything revolves around the concept of Nodes, scenes are made of them, and they become active once they enter the active scene.

This deserves going a little more into depth. In fact, the scene system is not even a core component of Godot, as it is possible to skip it and make a script (or C++ code) that talks directly to the Servers. But making a game that way would be a lot of work and is reserved for other uses.

MainLoop

The way Godot works internally is as follows. There is the the OS class, which is the only instance that runs at the beginning. Afterwards, all drivers, servers, scripting languages, scene system, etc are loaded. When initialization is complete, OS needs to be supplied a MainLoop to run. Up to this point, all this is internals working (you can check main/main.cpp file in the source code if you are ever interested to see how this works internally).

The user program, or game, starts in the MainLoop. This class has a few methods, for initialization, idle (frame-syncronized callback), fixed (physics-synchronized callback), and input. Again, this is really low level and when making games in Godot, writing your own MainLoop does not even make sense.

SceneMainLoop

One of the ways to explain how Godot works, is that it's a high level game engine over a low level middleware. The scene system is the game engine, while the OS and servers are the low level API.

In any case, the scene system provides it's own main loop to OS, SceneMainLoop. This is automatically instanced and set when running a scene, no need to do any extra work.

It's important to know that this class exists because it has a few important uses:

• It contains the root Viewport, when a scene is first opened, it's added as a child of it to become part of the active scene (more on that next)

• It contains information about the groups, and has means to call all nodes in a group, or get a list of them.

• It contains some global state functionality, such as setting pause mode, or quitting the process.

When a node is part of the active scene, the SceneMainLoop can be obtained by simply calling Node.get_scene().

Root Viewport

The root Viewport is always a top of the scene. From a node, it can be obtained in two different ways:

    get_scene().get_root() # access via scenemainloop
    get_node("/root") # access via absolute path

This node contains the main viewport, anything that is a child of a Viewport is drawn inside of it by default, so it makes sense that the top of all nodes is always a node of this type, otherwise nothing would be seen!

While other viewports can be created in the scene (for split-screen effects and such), this one is the only one that is never created by the user. It's created automatically inside SceneMainLoop.

Active Scene

When a node is connected, directly or indirectly, to the root viewport, it becomes part of the active scene. This means that, as explained in previous tutorials, will get the _enter_scene() and _ready() callbacks (as well as _exit_scene()).

When nodes become active (inside the scene), they get access to everything they need to process, get input, display 2D and 3D, notifications, play sound, groups, etc. When they are removed from the active scene, they lose it.

Tree Order

Most node operations in Godot, such as drawing 2D, processing or getting notifications are done in tree order. This means that parents and siblings with less order will get notified before the current node.

"Becoming Active" In Detail

1. A scene is loaded from disk or created by scripting.
2. The root node of that scene (only one root, remember?) is added as either a child of the “root” Viewport (from SceneMainLoop), or to any child or grand-child of it.
3. Every node of the newly added scene, will receive the “enter_scene” notification ( _enter_scene() callback in GDScript) in top-to-bottom order.
4. An extra notification, “ready” ( _ready() callback in GDScript) is provided for convenience, when a node and all it’s children are inside the active scene.
5. When a scene (or part of it) is removed, they receive the “exit scene” rotification ( _exit_scene() callback in GDScript) in bottom-to-top order