Entity-Component game programming using JRuby and libGDX – part 8

Introduction

Our Lunar Lander game is somewhat playable by this point but it still lacks some key features. After all, it would be nice if we could detect collisions and determine if the lander has safely landed on the pad. Let’s see how our flexible Entity-Component system permits us to expand our game with minimal fuss.

Collision Detection

First, a frank disclaimer: the following collision detection algorithm is entirely inefficient. It’s kept simple for our basic teaching purposes here but is probably undesirable in a game of any scale. But that’s OK: E-C will permit you to swap in a much more advanced collision detection system when you’re ready.

Entity-Component game programming using JRuby and libGDX – part 7

Introduction

Entity-Component systems, we’ve learned, are easy to implement and maintain; the elegance is basically “baked in” due to the way components and entities are married in the Entity Manager.

One particularly tidy aspect of an entity-component system is how well it lends itself to data persistence, or in practical terms: saving game state. Let’s take a look.

Where Is State?

In a conventional object oriented design, state is scattered all over the place, embedded in your far-flung object instances. But in E-C everything is neatly gathered together under one roof: the Entity Manager. This manager knows every entity “instance” along with every entity’s components, which are where the entity state data are stored.

Therefore, persist the entity manager to disk and you’ve saved the game in its entirety. Load from disk to memory and you’ve just loaded the game. It really is that easy.

Entity-Component game programming using JRuby and libGDX – part 6

Introduction

Now that we have laid the entity-component foundation and introduced some necessary libGDX concepts, we can finally get around to putting together a little game. Let’s make a “Lunar Lander” type game to illustrate all the concepts we’ve learned so far.

Remember that the source code for this Entity-Component Framework and the game we’re writing is all available at Github. The Github version is, of course, the “final version” that includes features I might not have addressed so far in the blog series, but if you’re eager to jump ahead…

Entities and Components

For this exercise let’s begin by defining our entities and some of their relevant components.

What are we going to need for this game?

• The lunar lander module
• A platform for it to try to land on
• Ground to collide with

That seems like a fair assessment of our initial entity needs. Now, moving on to components. What are some of the aspects / behaviors / features that we should provide to our entities?

Entity-Component game programming using JRuby and libGDX – part 5

Introduction

In previous installments we learned about Entity-Component theory, and we are almost ready to employ this in a real game. Before we do, however, we need to cover some basic game engine concepts.

The Java world is a fertile place for gaming, and there are numerous game engines available. JOGL and LWJGL are low-level frameworks that ease and abstract certain game-construction duties, whereas high-level frameworks like libGDX and Slick2D provide you with all the tools you need to easily author your game — often for multiple platforms at the same time. Both of those are fine choices, but for this series we’ll leverage libGDX. It has as robust community, excellent documentation, and can be used to target the Windows, Android, Mac and Linux platforms.

This post will not attempt to be a tutorial for installing libGDX nor a configuration guide for its many powerful features. Rather, we’ll walk through basic libGDX usage to get a high-level overview of how it works — often a helpful starting point for a newcomer to the technology. The canonical source for libGDX is its official wiki, with supplemental information found at the libGDX-users wiki.

Note that here we are now embracing one of the key features of JRuby: leveraging Java libraries. libGDX is, after all, a Java framework. But why write your game code in ugly, verbose, old-fashioned Java when you can use your favorite language to do it? The code will certainly be tidier, more readable, and a lot more enjoyable to write and maintain.

Now is an appropriate time to point you to the official Github repository for this blog series’ source code. This repository not only includes all the source code that has been (and will be) discussed in this blog series, but also includes a simplistic but runnable “lunar lander” type game to help cement the concepts with real, working code. I believe you’ll find it very helpful as a learning tool.

Entity-Component game programming using JRuby and libGDX – part 4

Introduction

In previous parts we established nomenclature; entites and entity management; and the components that supply entities their data. Now we turn our attention to “systems.” Systems are where we store all logic and algorithms. We have many systems, each responsible for a particular task, and these systems collect data from one or more components in order to act on a given entity.

The System

Here’s a very simple System base class from which all systems will derive:

class System
  def process_one_game_tick
    raise RuntimeError, "systems must override process_one_game_tick()"
  end
end

Let’s agree that our game runs at a certain number of frames-per-second, and for simplicity’s sake we’ll also agree to update our logic every frame. We don’t know at compile-time what the FPS will be; could be 20 FPS, could be 60 FPS. Regardless, we’ll say that every frame-to-frame transition is a “game tick”. That’s what we’re referring to with “process_one_game_tick”, which is the method every system must implement.

Entity-Component game programming using JRuby and libGDX – part 3

Introduction

In part 1 and part 2 we introduced nomenclature and explored the construction of Entities using the EntityManager. Now it’s time to breathe life into your entities with components (for data) and systems (to act upon the data). In this post we’ll discuss components and in the subsequent post, systems.

The Component

Recall that we never store any data or features of any kind inside an entity. (Actually the point is moot: we cannot store anything in an entity since entities aren’t even object instances, they are merely numeric IDs managed in the EntityManager.) This is where we truly depart from OO as a design paradigm. Attributes, aspects, data, behaviors are no longer embedded in the objects but are “bolted on” by attaching component instances to the entities.

Here’s a very basic start to a Component base class from which all components will derive:

class Component
  attr_reader :id

  def initialize
    @id = java.util.UUID.randomUUID().to_s
  end

  def to_s
    "Component {#{id}: #{self.class.name}}"
  end
end

Every component gets a unique ID for tracking and logging purposes; it is a very basic class.

Entity-Component game programming using JRuby and libGDX – part 2

Introduction

In Part 1 we explored EC nomenclature to set the stage for this part: learning how to work with entities, components and systems using an Entity Manager. (And if you are finding this post via Google or another entry, you can find the index to the full series in Part 1.)

As you’ll recall, your game-world “things” (Entities) are not traditional OO classes. EC Entities are skinny beings that exist merely to have a unique ID. Only after the Components are “attached” to an entity does it attain any personality.

Entity-Component game programming using JRuby and libGDX – part 1

Introduction

In this series of posts I’ll show you how to use JRuby and Entity-Component architecture to build a working game. Entity-Component Systems are a fairly new alternative programming methodology. They are prominent in the game-programming community and solve many of the problems that OOP cannot solve.

If you’re a Rails programmer you’re already familiar with the mantra “composition over inheritance.” Entity-Component Systems are probably the highest form of composition over inheritance that you will encounter.

“Pure” EC is what I’ll work with in this blog series. There are also hybrid systems and alternative methodologies. EC is a young enough architecture that even in the “pure” space there are still disagreements about certain implementation details. I’ll give you the solid foundation; the mechanism I’ll discuss in these posts will be adaptable by you to embrace these subtle shifts as you see fit.

Series Index

• Part 1 – introduction and nomenclature
• Part 2 – entities and the entity manager
• Part 3 – components
• Part 4 – systems
• Part 5 – libGDX concepts
• Part 6 – the Lunar Lander game
• Part 7 – serialization
• Part 8 – collision detection and more

MacRuby notes series: code snippets in Xcode 4

If you use TextMate’s “code snippets” feature to insert commonly-used templates of Ruby code, you’ll be pleased to see that Xcode has a similar facility for MacRuby. It comes pre-populated with a variety of Objective-C snippets, but it’s easy to add your own Ruby snippets.

If you read my article on Beautiful standardized RDoc comments for your Ruby / Rails methods you’ll know that I’ve standardized on this method commenting pattern:

  # Provides a human-readable size of the attachment
  #
  # * *Args*    :
  #   - +precision+ -> the precision of the returned value, defaults to 1
  # * *Returns* :
  #   - a size value such as "5.1 KB"
  # * *Raises* :
  #   - +RuntimeError+ -> if the body is nil
  def human_size(precision=1)
    #...

In TextMate I use “rd <TAB>” to insert the comment snippet, and “defc ” to insert a commented blank method. In Xcode I do the same; here’s how.

MacRuby notes series: taking the pain out of Core Data (part 2)

In Part 1 we addressed the current state of Core Data programming in MacRuby, and discussed some of the problems and challenges facing Rails programmers accustomed to ActiveRecord. Now let’s address some solutions.

First, some necessary terminology.

These are Core Data concepts that you must be familiar with before you’ll be effective at Core Data persistence in MacRuby (or Objective-C, for that matter).

The Managed Object Model (or MOM) keeps track of objects and their relationships. It doesn’t care about actual persistence; you might think of it as just a picture of the data structures, their attributes and their relationships.

In a Core Data-enabled project there is an .xcdatamodeld file.  It contains the model definitions. When you compile your app, this is compiled into a mom file and stored in a folder with a .momd extension. This is how the application knows about the data structures and relationships to use.

The Managed Object Context (or MOC) acts as a bridge between the MOM and the actual persistence mechanism. You can think of the MOC as an in-memory scratch pad. When you fetch objects from a persistent store, you bring temporary copies into the scratch pad where they form an object graph. You can then modify those objects however you like. Unless you actually save those changes, however, the persistent store remains unaltered.

The Persistent Store Coordinator (or PSC) This is the actual persistence mechanism, and is connected to a MOC.  The PSC is analogous to the actual data storage layer, whether it’s SQLite, XML, or whatever. When you fetch objects, the MOC asks the PSC to return those objects that match the fetch request.