godot-docs/learning/step_by_step/scene_tree.rst

.. _doc_scene_tree:

SceneTree
=========

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 *scene tree*.

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

MainLoop
--------

The way Godot works internally is as follows. There is the
:ref:`OS <class_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, :ref:`OS <class_OS>` needs to be
supplied a :ref:`MainLoop <class_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.

SceneTree
---------

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 :ref:`OS <class_OS>`
and servers are the low level API.

In any case, the scene system provides its own main loop to OS,
:ref:`SceneTree <class_SceneTree>`.
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 :ref:`Viewport <class_Viewport>`, to which a
   scene is added as a child when it's first opened, to become
   part of the *Scene Tree* (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 Scene Tree, the
:ref:`SceneTree <class_SceneTree>`
singleton can be obtained by simply calling
:ref:`Node.get_tree() <class_Node_get_tree>`.

Root viewport
-------------

The root :ref:`Viewport <class_Viewport>`
is always at the top of the scene. From a node, it can be obtained in
two different ways:

::

        get_tree().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
:ref:`Viewport <class_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 SceneTree.

Scene tree
----------

When a node is connected, directly or indirectly, to the root
viewport, it becomes part of the *scene tree*.

This means that, as explained in previous tutorials, it will get the
_enter_tree() and _ready() callbacks (as well as _exit_tree()).

.. image:: /img/activescene.png

When nodes enter the *Scene Tree*, they become active. 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
*scene tree*, they lose access.

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 a smaller rank in the tree order will get notified before
the current node.

.. image:: /img/toptobottom.png

"Becoming active" by entering the *Scene Tree*
----------------------------------------------

#. A scene is loaded from disk or created by scripting.
#. The root node of that scene (only one root, remember?) is added as
   either a child of the "root" Viewport (from SceneTree), or to any
   child or grand-child of it.
#. Every node of the newly added scene, will receive the "enter_tree"
   notification ( _enter_tree() callback in GDScript) in top-to-bottom
   order.
#. An extra notification, "ready" ( _ready() callback in GDScript) is
   provided for convenience, when a node and all its children are
   inside the active scene.
#. When a scene (or part of it) is removed, they receive the "exit
   scene" notification ( _exit_tree() callback in GDScript) in
   bottom-to-top order

Changing current scene
----------------------

After a scene is loaded, it is often desired to change this scene for
another one. The simple way to do this is to use the
:ref:`SceneTree.change_scene() <class_SceneTree_change_scene>`
function:

::

    func _my_level_was_completed():
        get_tree().change_scene("res://levels/level2.scn")

This is a quick and useful way to switch scenes, but has the drawback
that the game will stall until the new scene is loaded and running. At
some point in your game, it may be desired to create proper loading
screens with progress bar, animated indicators or thread (background)
loading. This must be done manually using autoloads (see next chapter!)
and :ref:`doc_background_loading`.