Interface Design

Part of the Object Oriented Programming section of Coddy's GDScript journey. Lesson 24 of 65.

An interface is a list of methods that a class promises to have, with no code behind them. Java and C# have an interface keyword; GDScript does not. You build the same thing from an abstract class whose methods are all abstract:

# drawable.gd
@abstract
class_name Drawable
extends RefCounted

@abstract func draw() -> String
@abstract func resize(factor: int) -> void

It has no variables and no method bodies. It describes what a drawable thing can do, and each class decides how:

# banner.gd
class_name Banner
extends Drawable

var text: String
var size := 1

func _init(t: String) -> void:
	text = t

func draw() -> String:
	return "Banner '%s' at size %d" % [text, size]

func resize(factor: int) -> void:
	size *= factor

A Circle can extend Drawable too, with a radius and its own draw() and resize(). Code that uses the interface depends only on it, so it works with every class that extends Drawable, including ones written later:

func render_all(items: Array[Drawable]) -> void:
	for item in items:
		item.resize(2)
		print(item.draw())

var items: Array[Drawable] = [Banner.new("Sale"), Circle.new(3)]
render_all(items)

Banner 'Sale' at size 2
Circle with radius 6

A class has one parent, so it can extend only one interface. When a class plays a second role, such as being saved to a file, extend the main interface and check the second role with has_method():

if item.has_method("save"):
	item.save()

Keep each interface small: a few methods that belong to one role. A class that extends it then writes only methods it needs.

Challenge

Easy

Build a media player around an interface.

In playable.gd, make Playable an interface: an abstract class with the abstract methods play() -> String and duration() -> int (in seconds).
Song extends Playable. play() returns Playing song: TITLE by ARTIST, and duration() returns its seconds.
Video extends Playable. play() returns Playing video: TITLE in RESOLUTIONp (for example in 1080p), and duration() returns its seconds.
Playlist knows only Playable. play_all() prints play() of every item, and total_time() returns the summed duration as M:SS (seconds with two digits, for example 4:05).

The constructors of Song and Video are written. The supplied code reads how many items follow, then one line per item: song TITLE ARTIST SECONDS or video TITLE RESOLUTION SECONDS. It calls play_all() and prints the total time. For song Imagine Lennon 183 and video Trailer 1080 95 it prints:

Playing song: Imagine by Lennon
Playing video: Trailer in 1080p
Total time: 4:38

Your code goes in playable.gd, song.gd, video.gd and playlist.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 count := int(OS.read_string_from_stdin())
	var playlist := Playlist.new()
	for i in count:
		var p := OS.read_string_from_stdin().split(" ")
		if p[0] == "song":
			playlist.add(Song.new(p[1], p[2], int(p[3])))
		else:
			playlist.add(Video.new(p[1], int(p[2]), int(p[3])))
	playlist.play_all()
	print("Total time: " + playlist.total_time())

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Object Oriented Programming

3Inheritance

Basic Inheritance The super Keyword Method Overriding Inheritance Chains Type Checks with is Recap - Employee Hierarchy

6Special Methods

Virtual Methods Introduction The _to_string Method Custom Iterators Recap - Custom List

9Design Patterns Part 1

Intro to Design Patterns Singleton Pattern Factory Pattern Observer Pattern Strategy Pattern

12Final Challenges

E-learning Platform Banking System Game Character Development Vehicle Rental Service

Practice on your own: Online GDScript compiler