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Every Object in Godot implements a
_notification method. Its purpose is to
allow the Object to respond to a variety of engine-level callbacks that may
relate to it. For example, if the engine tells a
CanvasItem to "draw", it will call
Some of these notifications, like draw, are useful to override in scripts. So much so that Godot exposes many of them with dedicated functions:
What users might not realize is that notifications exist for types other than Node alone, for example:
Object::NOTIFICATION_POSTINITIALIZE: a callback that triggers during object initialization. Not accessible to scripts.
Object::NOTIFICATION_PREDELETE: a callback that triggers before the engine deletes an Object, i.e. a "destructor".
And many of the callbacks that do exist in Nodes don't have any dedicated methods, but are still quite useful.
Node::NOTIFICATION_PARENTED: a callback that triggers anytime one adds a child node to another node.
Node::NOTIFICATION_UNPARENTED: a callback that triggers anytime one removes a child node from another node.
One can access all these custom notifications from the universal
Methods in the documentation labeled as "virtual" are also intended to be overridden by scripts.
A classic example is the
_init method in Object. While it has no
NOTIFICATION_* equivalent, the engine still calls the method. Most languages
(except C#) rely on it as a constructor.
So, in which situation should one use each of these notifications or virtual functions?
_process vs. _physics_process vs. *_input¶
_process() when one needs a framerate-dependent delta time between
frames. If code that updates object data needs to update as often as
possible, this is the right place. Recurring logic checks and data caching
often execute here, but it comes down to the frequency at which one needs
the evaluations to update. If they don't need to execute every frame, then
implementing a Timer-timeout loop is another option.
# Allows for recurring operations that don't trigger script logic # every frame (or