Nodes as Components

Part of the Object Oriented Programming section of Coddy's GDScript journey. Lesson 42 of 65.

A Node can have child nodes. Godot builds game objects this way: a hero node gets one child node per ability, such as health or movement. Each child is a component: a small node that does one job.

# health.gd
class_name Health
extends Node

var hp := 10

func _ready() -> void:
	print("Health ready")

# hero.gd
class_name Hero
extends Node

func _init() -> void:
	var health := Health.new()
	health.name = "Health"
	add_child(health)

func _ready() -> void:
	print("Hero ready, hp: ", $Health.hp)

add_child() attaches a node as a child. Give each component a name: get_node("Health"), or its short form $Health, finds the child by that name. A node without a name, or whose name is taken by a sibling, gets an automatic one such as @Node@4.

A node's _ready() runs when it enters the scene tree, the tree of nodes the game is running. main.gd is in the tree, so adding the hero to it starts both _ready() methods, the children's first:

# main.gd
var hero := Hero.new()
add_child(hero)            # Health ready, then Hero ready, hp: 10
print(hero.get_child_count())   # 1

A parent reaches its components with get_children() (an array of the children, in the order they were added), get_child_count() and has_node("Health"). Nodes in the tree are freed together with the tree when the game ends, so a component that was added with add_child() needs no cleanup.

Challenge

Easy

Build a robot from component nodes.

Part extends Node. Its constructor receives a part name and a power value: it stores the power and sets the node's name to the part name. When a part enters the scene tree, its _ready() prints Laser attached.
Robot extends Node. add_part(part_name, power) adds a new Part as a child. If the robot already has a child with that name, it prints Laser is already attached and creates nothing.
total_power() returns the sum of the power of all its parts.

The supplied code adds the robot to the scene tree, reads how many commands follow (add Laser 5, power), and prints the number of parts at the end. For add Laser 5, add Wheels 2, power, add Laser 9, add Arm 3, power it prints:

Laser attached
Wheels attached
Total power: 7
Laser is already attached
Arm attached
Total power: 10
Parts: 3

Your code goes in part.gd and robot.gd. main.gd holds the supplied input/output code and cannot be edited.

Try it yourself

extends Node

func _ready():
	# Supplied input/output code: keep it as it is
	var robot := Robot.new()
	add_child(robot)
	var count := int(OS.read_string_from_stdin())
	for i in count:
		var parts := OS.read_string_from_stdin().split(" ")
		if parts[0] == "add":
			robot.add_part(parts[1], int(parts[2]))
		else:
			print("Total power: ", robot.total_power())
	print("Parts: ", robot.get_child_count())