on_value_changed

Detects a value change and executes a callback. The callback takes the old and the new value:

@tool # required for method calls in attributes
extends Node3D

@export_custom(PROPERTY_HINT_NONE, "on_value_changed:print_value_change")
var strength: int = 10

@export_custom(PROPERTY_HINT_NONE, "on_value_changed:print_value_change")
var dexterity: int = 7

@export_custom(PROPERTY_HINT_NONE, "read_only")
var power: int:
    get:
        return strength * 2 + dexterity


func print_value_change(old_value, new_value):
    print(old_value, " -> ", new_value)

The argument is a method name, without parentheses. Exactly one signature is accepted, (old_value, new_value), and a property can carry more than one callback – they are called in written order.

Note

The callback is a method call, so the script needs @tool. See Writing Attributes.

Note

Keep in mind that the event fires only when the value is changed from the inspector. Undo and redo restore values silently, and so does a clamp applied when the inspector is first built.

Note

Callbacks run after Godot has committed its own undo action, so they see the new value, and a property a callback writes lands on screen at once.