Custom Iterators

Python makes a class behave like a list with magic methods such as __len__, __getitem__, __contains__ and __iter__. Godot has virtual methods for only one of these jobs, the for loop. Three methods make for item in my_object: work:

• _iter_init(iter) runs once, before the first round. It sets the starting position and returns true if there is a first element.
• _iter_get(position) returns the element at the current position, and the loop variable gets that value.
• _iter_next(iter) runs before each further round. It moves to the next position and returns false when there are no more elements, which ends the loop.

# team.gd
class_name Team
extends RefCounted

var _members: Array[String] = []

func add(member: String) -> void:
	_members.append(member)

func _iter_init(iter: Array) -> bool:
	iter[0] = 0
	return iter[0] < _members.size()

func _iter_next(iter: Array) -> bool:
	iter[0] += 1
	return iter[0] < _members.size()

func _iter_get(position):
	return _members[position]

iter is an Array with one slot that Godot creates for each loop. iter[0] holds that loop's position, and Godot passes its value to _iter_get(). A Team now works in a for loop:

var team := Team.new()
team.add("Ada")
team.add("Bo")
for member in team:
	print(member) # Ada, then Bo

Keep the position in iter[0], not in a member variable such as _position. Every loop gets its own iter, so two loops over the same team, even one inside the other, do not disturb each other. With a member variable, the inner loop would move the outer loop's position.

The other list jobs have no virtual method: len(team) and team[0] are errors. Write ordinary methods named like Array's, such as size(), get_at(index) and has(member). Do not test membership with in: "Ada" in team checks whether Team has a property or method named Ada, so it is false.