T=Machine

Hume just posted his Lessons Learned from the warmup for Ludum Dare 23 (48 hours to write a game from scratch – starts this weekend!) – and his positive experience using an Entity System.

In his epic comment (sparked by a different Adam – not me, honest), is this gem:

“Using the entity system for the first time was unreal to me. It’s like polymorphic code. I did really weird things on the fly. For example:

- In the health processor, if the enemy was just destroyed, set a flag in the lifecycle component.
- In the lifecycle processor, if the fresh kill flag is set, extract its loot component and put that into a new entity with a small randomized velocity component and a gravity component so that the loot drops; then, remove most of the other components from the entity and add an explosion component.

The “enemy” still has the same entity ID — any other components that are looking for that entity will still find it (e.g. missiles homing in on the wreckage, or score processors looking for slain entities) — but by swapping one set of data with another, its implementation has changed from an enemy to some kind of non-interactive effect object.”

(emphasis mine)

Identity. It’s important.

(Quick sidenote: for all the people asking questions like “but … which variables do I put in Component A as opposed to Component B? How do I manage Events in an Entity System? … etc” – Hume’s approach above is a good concrete example of the first-draft, easy-to-write way of doing things. Copy it.)

Identity in games

This is one of those things that newbie game programmers seem to underestimate, frequently.

And when I say “newbie” I include “experienced, skilled programmers with 10+ years of coding experience – but who haven’t yet shipped a game of their *own*”.

(e.g. I’ve seen a couple of studios that started as Digital Agencies, or as Animation Studios, etc – that then transitioned to writing their own games. This is the kind of thing that they often struggle with. Not for lack of skill or general programming experience, but for lack of the domain-specific experience of game coding)

Examples of Identity in games, off the top of my head – all of these are independent, and interact in complex ways with each other :

1. Game-role: e.g. … “enemy”, “powerup”, “start location”
2. Code ‘object’ (in OOP terms): e.g. … “the sprite you are drawing at position (4,5) is part of Object X. X is coloured THIS colour”
3. Gameplay ‘object’: e.g. … “the sprite at (4,5) represents a Tank. If a Tank sprite ever touches a Glass sprite, we need to play the Broken Glass noise”
4. Physics element: e.g. … “5 milliseconds ago, our Physics Engine thought this thing was THERE. Now it’s over HERE. Don’t confuse the Physics Engine! Make sure it ‘knows’ they are the same object – not two separate objects”
5. Network “master/clone”: e.g. … in multiplayer, there are N copies of my avatar: one per computer in the game. One of those N is the original – and changes to the original are constantly used to overwrite the clones; changes to clones are LOCAL ONLY and are discarded. Which is original? What do we do with incoming “changes” – which local Code Object do we apply them to? (your Code Object will be different from my Code Object – but they’ll both be the same identical Network Object, save mine is flagged “clone”)
6. Proper Noun object: e.g. … “The Player’s Tank” is a very specific tank out of all the Tanks in the game. Many lines of game code don’t care about anything except finding and operating on that specific tank.
7. Game-Over representation: e.g. … after the player has killed all the enemies, and they see a Game Over (won/lost/etc) screen, and you want to list all the enemies they killed … how do you do that? The enemies – by definition – no longer exist. They got killed, removed from the screen, removed from memory. You could store just the absolute numbers – but what if you want to draw them, or replay the death animations?
8. …etc

Identity in Entity Systems

ES’s traditionally give you a SINGLE concept of Identity: the Entity (usually implemented as a single Integer). Hmm. That sounds worryingly bad, given what I wrote above. One identity cannot – by definition – encompass multiple, independent, interrelated identities.

But we’re being a bit too literal here. ES’s give you one PRIMARY identity, but they also give you a bunch of SECONDARY identities. So, in practice…

Secondary Identities in an ES

In OOP, the Object is atomic, and the Class is atomic. You cannot “split” an Object, nor a Class, without re-defining it (usually: re-compile).

In ES, the Entity is atomic, and the Component is atomic. But the equivalent of an OOP Object – i.e. “an Entity plus zero or more Components” – is *not* atomic. It can be split.

And from there comes the secondary identities…

A Primary Identity: e.g. “The Player’s Tank” (specific)
A Primary Identity: e.g. “a Gun Component” (generic)

A Secondary Identity: e.g. “The Gun component … of the Player’s Tank Entity” (specific)

Revisiting my ad-hoc list of Game Identities above, I hope it’s clear that you can easily re-write most of those in terms of secondary identity.

And – bonus! – suddenly the relationships between them start to become (a little) clearer and cleaner. Easier for humans to reason about. Easier *for you to debug*. Easier *for you to design new features*.

Global Identity vs. Local Identity

Noticeably, the network-related Identities are still hard to deal with.

On *my* computer, I can’t reference entities on *your* computer. I cannot store: “The Gun component … of YOUR player’s tank”, because your “Player’s Tank” only exists in the memory of your computer – not mine.

There are (trivially) obvious solutions / approaches here, not least: make your Entity integers global. e.g. split the 64bit Integer into 2 32bit Integers: first Integer is the computer that an Entity lives on, the second is the local Entity Integer. Combined, they are a “global Entity ID”.

(I’m grossly over-simplifying there – if you’re interested in this, google for “globally unique identifiers” – the problems and solutions have been around for decades. Don’t re-invent the wheel)

But … at this point, they also offer you the chance to consider your game’s network architecture. Are you peer-to-peer, or client-server?

For instance, P2P architectures practically beg for unique Global entity numbers. But C/S architectures can happily live off non-global. For instance:

• On each client, there are ONLY local Entity numbers
• When the client receives data from the server, it generates new, local, Entities
• …and adds a “ServerGenerated” component to each one, so it’s easy to see that they are “special” in some ways. That component could hold info like “the time in milliseconds that we last received an update on this object” – which is very useful for doing dead-reckoning, to make your remote objects appear to move smoothly on the local screen
• The server *does* partition all entities from all machines. But none of the clients need to know that

Or, to take it further, if your network arch is any good at all for high-paced gaming:

• The server differentiates between:
  1. The entity that the game-rules are operating on
  2. The entity that client 1 *believes* is current
  3. …ditto for client 2, client 3 … etc (each has their own one)
  4. The entity that the game-rules accept (e.g. if a hacked client has injected false info, the game-rules may override / rewrite some data in the local object)
• The server also tags all the entities for a single in-game object as being “perspectives on the same thing”, so that it can keep them in synch with each other
• The server does Clever Stuff, e.g.:
  • Every 2 milliseconds, it looks at the “current entity”, and compares it to the “client’s belief of that entity”. If it finds any differences, it sends a network message to the client, telling it that “you’re wrong, my friend: that entity’s components have changed their data. They are now X, Y and Z”

… or something like that. Again, I’m grossly over-simplifying – if you want to write decent network code, Google is your friend. But the fastest / lowest latency multiplayer code tends to work something like that.

How?

Ah, well.

What do you think?

(hint: you can do wonders using Reflection/Introspection on your entity / components. By their nature, they’re easy to write generic code for.

But you WILL need some extra metadata – to the extent that you may wish to ‘upgrade’ your Entity System into a SuperEntity System – something with a bit more expressive power, to handle the concept of multiple simultaneous *different* versions of the same Entity. Ouch)

Yeah, I’m bailing on you here. Too little time to write much right now – and it’s been a *long* time since I’ve implemented this level of network code for an ES. So, I’m going to have to think hard about it, refresh my memory, re-think what I think I knew. Will take some time…

Filed under : entity systems, games design, network programming, networking, programming

2 Comments »

Thanks to Mike Leahy for spotting this:

http://blog.gemserk.com/2012/02/02/how-we-use-box2d-with-artemis/

…a short blog post (with code) on how a team is integrating Box2D (a very well known open source physics lib) with Artemis (a java implementation of Entity Systems which starts from the same point as my Entity Systems posts, but fleshes it out)

Filed under : MMOG development, entity systems, games design, programming

1 Comment »

In the ongoing, epic comments (300+ and counting!) for my Entity Systems posts, one of the recurring questions is:

What makes a good Component?
How should I split my conceptual model into Entities and Components?
How should I split my algorithms and methods into Systems?

It’s often difficult to answer these questions without concrete examples, which are thin on the ground.

Good news, then…

Paul Gestwicki runs a CS315: Game Programming course, and last year his students used an Entity System to implement their game – Morgan’s Raid. In a recent email conversation, he mentioned he’d been monitoring the actual number – and nature – of the Components and Systems that the teams developed and used on the project.

He’s now posted this list, along with some brief analysis.

All systems and Components

Read Paul’s post – there are some caveats he mentions, and there’s a useful diagram showing roughly how many systems were using each component.

I strongly recommend you play the game too (it’s free, and quick to play) so you can get an idea straight away – just from the names – what data and code some of these contain.

To recap, here’s the list:

“For your reading convenience, here’s a simple tabular view of the systems and components

Systems	Components
BackgroundTileSystem CityNameSystem DestinationSystem FadingSystem GPSToScreenSystem HoverableSystem ImageRenderingSystem IntInterpolationSystem MinimumSleepTimeSystem MorganLocationSystem NightRenderingSystem OnClickMoveHereSystem OnScreenBoundingBoxSystem RaidSoundSystem RailwaySystem ReputationSystem RevealingTextSystem SpeedSystem StepwisePositionInterpolationSystem SunSystem TimePassingSystem TimeTriggeredSystem TownArrivalSystem TownArrowSystem TownUnderSiegeSystem	AnimationRenderable ArrivesAtTownIndex BackgroundTile BeenRaided CentersOnGPS CityData CityImages CityName CityPopulation CityTargets CommandPoint Destination DoesCityHaveMilitia DoesCityLoseGame DoesCityWinGame GPSPosition GPSPositionList HasMorganGPS Hoverable ImageRenderable ImageRenderLayer InGameTime IntInterpolated	MinimumSleepTimeOverride Morgan MorganLocation MovesOnClick OnClickMoveHere OnScreenBoundingBox OnScreenBoundingBoxList PositionInterpolated Raidable Raider Railway Reputation ReputationValue RevealingText Road RoadsToCity RouteTaken Speed Sun Terrain TimeToRaid TimeTriggeredEvent TownAdjacency TownArrow TownUnderSeige

”

Things I noticed straight away:

1. There’s approximately 2:1 ratio of “components” to “systems”
2. In Paul’s post, all the Systems are accessing *something*
3. In Paul’s post, quite a few Components are NOT accessed
4. A couple of components are used by almost every System
5. The names of some Systems suggest they’re very trivial – perhaps only a dozen lines of effective code
6. The names of some Components suggest they’re being designed in an OOP hierarchy

NB: I haven’t had time to look at the source code, but it’s freely downloadable here, so I’d recommend having a look if you have time.

How many Components per System?

I’ve generally started people off with: “aim for 1:1 ratio”. This is mainly to kick them out of the traditional class-based OOP mindset. In practice, there’s really no need to stick to that religiously – once you get the hang of ES design, you should be freely adding and subtracting components all over the place.

In reality, the pressures on “number of systems” and “number of components” are independent. Ideally, you add a new system when you have a major new concept to add to your game – e.g. “previously I was using hand-made jumping, now I want to add a complete physics-driven approach. This will mean changing collision-detection, changing the core game-loop, etc”.

Ideally, you add a new component when you have a new “dimension” to the game objects. For instance, if you’re adding a physics System, you may not need to add any new Components – it might be that all you need is Location (containing x,y,x position and dx,dy,dz velocity) and RenderState (containing screen-pixels x,y) – and that you already have those components.

Zero systems per component

One of the advantages of an ES is that old code can just fall off the radar and disappear. So I’m not surprised at all to see some components that appear to be unused – and it’s MUCH easier to simply delete this code from your project than it would be on a traditional OOP project. Does anything reference that data? If so, it’s a set of particular systems. For each system, you can look at MERELY the system and the component, and make a very quick decision about whether you still need this access – or if you can refactor to move (some of) it somewhere else. The amount of code you need to read to make such decisions safely is typically very small – i.e. easy, quick, and less error-prone.

Many systems per component

This is fine. However, it can also be an early-warning sign of a design or code-architecture bug. Sometimes, there are components that – innately – are just needed all over the place. For instance, in a team-based game, the component saying which “team” a given object/player/item/building belongs to is likely to affect almost every piece of algorithm code across the board. It’ll be referenced by many systems.

On the flip-side, it may be a sign that you’ve put too much data into one component. There are two usual versions of this:

1. You have – say – 8 variables in the struct where you should instead have two structs (components), one with 5 variables, the other with 3.
2. You have – say – 4 variables in the struct, but different systems are using those variables to mean different things. It works OK for now, but it’s very fragile – as soon as the different meanings diverge even a little, your code is going to start breaking

Of course, you get this exact problem in traditional OOP setups, but with an ES it’s trivial to fix. Split – or duplicate – the Component, change a few references in the Systems, and you’re done. If it turns out a week later that the split wasn’t necessary – or worse, was a step backwards (e.g. you find yourself frequently synching the data between those components) – it’s extremely cheap to swap it back.

By contrast, with OOP, this is a nightmare scenario, because you have to worry about every method on the original class. Does that method:

1. Need to exist on both the new classes, or just one?
2. Work correctly for the new class it will be on – or does it currently rely on some of the data (and shoudln’t) and will need to be re-written?
3. Get used by other parts of the codebase in ways that will break if/when you split the class?

Thoughts, Suggestions?

…this is just a lightning quick analysis, but I strongly invite you to do you own digging into the classes – and the codebase – and come up with your own thoughts and feedback. We have here a convenient, real-life, list of components/systems – something to dig our teeth into, and debate the rights and wrongs of each decision. And I’m sure the students involved on the project would be interested in your feedback on their approaches :)

Filed under : entity systems, games design, programming

19 Comments »

I couldn’t enter this LudumDare competition on a technicality, but here’s my entry which plays by the spirit of the rules. I took a total of 24 hours (out of 48), of which only 12 were actual design + development (see below). Hopefully next time I’ll be able to do it properly, and actually compete. I’ve kept to every rule, except that I did my 48 hours time-shifted :) from everyone else (two successive Sundays, instead of a contiguous Saturday + Sunday).

Screenshot + Android APK

Download link (APK – you need to know how to install APK’s manually (google it if you’re not sure, it only takes 5 seconds)): Escape From the Pit

Aims

1. Make a LudumDare entry as an Android application – none of this easy “make it in Flash” or “make it in java” stuff – let’s go for a full mobile game, designed, developed, and launched in exactly 2 days flat!
2. Use an Entity System (c.f. my long-running series of articles, and also the public Wiki I created), and show that ES’s facilitate you writing new games VERY VERY QUICKLY
3. Make a game that was mildly challenging and (almost) enjoyable

Failed to officially enter the competition, but otherwise succeeded…

Background

LudumDare challenges you to write an entire game in under 2 days (technically: 48 hours – it’s up to you how much of that you sleep). You can’t even design it in advance – there’s a theme that’s only decided shortly before the 48 hours starts.

LudumDare was the weekend before last – but I had to work that weekend on urgent day-job stuff. Like: I had to work all day Saturday, and there was no way out of it. So I couldn’t do the same 48-hour stint as everyone else.

Also, I know from previous experience that the “48 hours in one stretch” is very different from – say – “12 hours for 4 days”. When you do a 24 or 48 hour game, you tend to only manage a certain percent of “productive” hours. The challenge of designing + building something from scratch forces you to keep taking “time off” to go think about what do next.

So, I kept a diary of hours worked, and hours taken “off” as well. I’m confident I’d have fitted all of this – development time AND down-time – into the 48 hours. But I had to spread it over 2 successive weekends :(.

Day 1

(3 hours) – install Eclipse and the Android plugin, and the Android SDK. Document what I’ve done (1 hour) and check I can re-do it at will. Google, please wise-up and fix your install process – it’s not changed in almost 2 years, and it SUCKS

(1 hour) – install some extra Android OS versions, get the emulator working correctly, get projects imported, get everything in source-control, get empty app running on hardware device. Ready to start project!

— NB: everything up to this line I should have done before the contest started. If I were the kind of person that had free time on weekdays. Which sadly I’m not —

(1 hour) – getting Android base classes up and running. Takes a while: Android is insanely badly designed for the “Core application” part. Needs hundreds of lines of code to make a Hello World app that *actually* works as an app (Google’s code example that does it in 4 lines is fake: no real app could do that).

(3 hours) – on the beach, not working

(4 hours) – upgrading the open source Entity System Libraries on http://entitysystems.wikidot.com to support a bunch of features I’ve been using for a while in my own projects. This required writing a lot of stuff from scratch (using my own old source as inspiration), and integrating some improvements from patches/forks that other people had submitted.

— NB: everything up to this line I could have done before the contest started. Interesting though that I thought this was going to be “about to start writing the actual game” and I’ve only finally got to the point where I can write my first line of game-code —

Day 2

(0.5 hours): trying to make textures in Photoshop. Photoshop really sucks. Half the online resources for making the kinds of textures I want require PSP’s unique filters/effects – useless :(.

(0.5 hours): get a single sprite to appear on screen. A couple of idiot errors in one of my libraries – plus Google’s Eclipse plugin being really really bad at understanding “the scroll bar” (bug in ADT: it implements the world’s only non-static scrollbar)

(1 hour): random maze generation (using: http://en.wikipedia.org/wiki/Maze_generation_algorithm#Randomized_Prim.27s_algorithm ) that makes nice mazes, printing out onto the screen, still with my default “starfield” background. Rotating the screen is causing the entire game-state to be regenerated – includkng the maze – which was an accident, but actually helped A LOT with testing the maze algorithm (just tilt to re-run the algorithm instantly)

(0.5 hours): learn how to do Android input-handling correctly; en-route, discover I’m missing the SDK docs, and set about downloading + installing them … + updating my blog instructions on how to install Android to include “SDK docs” as a section.

(2.5 hours): discovering MAJOR bugs in Google’s basic “touch handling” API for Android – including bugs on Google’s own website source code, and an API designer on crack who broke the core Java contract didn’t document it. Not Happy.

Day 3

(1 hour) – implementing a collision detection system that does proper swept-collisions, but works OK with the poor fine-grained control of touch input

(1 hour) – added filters to collision detection so I could have background images that the player will NOT collide with
(previously was colliding with every on-screen rendered sprite). Also added a very simple lighting system where squares that the player has walked close to or upon gradually light up, showing how much has been explored

(1 hour) – refined the user-controls so you can hold your finger still and character keeps moving in that direction. Added handling in collision-detection system to allow character to slide along walls and into side-passages without the player having to stop and be ultra-precise (pixel perfect!) in timing the change of direction.

(0.5 hours) – added an exit, fixed bugs in the maze-generation (if started on a right or bottom edge, it crashed)

(1 hour) – fix Android’s brain-dead handlig of Bitmaps, giving a big speed boost, and re-learning how to use DDBS memory-allocation-tracking. I’m now auto-caching each bitmap inside my Sprite object. Sigh. There’s no easy workaround: Google says “don’t use getter methods” but Google also says “don’t call our getDrawable method more than once”.

(1 hour) – added ghosts, made them move around the map nicely, and collide with player was *automatic* on first compile (freebie from using an Entity System!). Also made arrows float nicely in same place on screen even while scrolling.

(1 hour) merge code from laptop back to desktop. Finally add the “win” conditions that makes the app playable!

Source Code

To make this game, I improved the basic Java Entity System up on the ES Wiki, and added some usability improvements and features. I created a whole new page for it here:

http://entity-systems.wikidot.com/rdbms-beta

NB: It’s called “Beta” simply meaning “second generation (beta == second letter of greek alphabet)”. Not because it’s a beta-quality release :).

Source code to the game itself is also up on github right now – https://github.com/adamgit/Game–Escape-from-the-Pit – although that’s a closed repository at the moment. I want to double-check there’s nothing included that shouldn’t be before I set it to “public”.

Filed under : android, computer games, entity systems, games design

7 Comments »

I’ve just added a new, improved Entity System design to the wiki. I’ve also created a github project for it where I’m about to check-in working source.

The only source I’ve provided is Java – but it’s an evolution upon the old “RDBMS with Code in Systems” type, that already has full source for Java and Objective-C – so you could easily upgrade the obj-c version to this new type (hint, hint).

What’s new?

Three major changes:

1. Included a bunch of sensible java-specific improvements, based on the forks other people had done of the previous Java implementation
2. Added a bunch of FANTASTICALLY USEFUL methods to the EntityManager – things you definitely need when writing a game
3. Added a new concept: the Meta Entity

The first one is obvious, the second is obvious when you look at what’s added (e.g. a “removeComponent” method; why wasn’t it there originally? Solely because it wasn’t necessary to show “the simplest possible implementation”).

The interesting one is number three: the Meta Entity

What’s a MetaEntity?

A while back, in the first Java/Android ES I wrote about, I mentioned that I was using a fake Entity class.

I made a big fuss about how this was NOT being used to store any game data – but was instead an OOP trick to make it easier to write code.

Thinking about this later, I realised that there was really no need for that class to be named “Entity” – and calling it that ran the risk of tempting people into using OOP for their core data / components (Remember: Never do that!). Instead, looking back at a couple of example ES’s I’d written, I saw that every method – and all data – in this class was a “meta method”.

For the methods, you need to read the source AND the javadocs to see this.

For the data, it’s a lot more obvious: the only data that a MetaEntity has is:

• The Entity itself (remember: that’s just an integer – or a UUID object in Java, which is a cleaner Java-specific way of handling it)
• The EntityManager object which created this Entity, and which contains the Components for that Entity, and … basically provides all access to the data etc

i.e. if you pass around MetaEntity objects, you can feel safe that you know where they came from, where their components are, etc.

Because when you pass around raw Entity’s, they’re just an integer – which makes it easy to create an Entity in one EntityManager, then accidentally start using it in another EntityManager. Technically that’s illegal – but from a compiler perspective, it’s 100% legal … so you’ll get bizarre runtime bugs. Ouch.

Equally, the MetaEntity can contain a lot of single-argument and zero-argument versions of methods that exist in the EntityManager as 2-argument, or 1-argument methods. This greatly reduces typing, increases readability, and reduces opportunities for bugs. It may sound like a very small change (2 arguments dropping to 1), but I promise you: in practice, it makes a big difference.

Why not use MetaEntity’s all the time?

They’re very inefficient in how they use memory, and they throw away many (but not all) of the performance advantages of an ES.

For instance, because you’re moving from “integer + array of structs” to “linked-list of objects”, you’re making your game go from “cache friendly” to “cache poor”, and making your MINIMUM mem usage go from “tiny” to “small”.

In practice … if those differences matter to you, you’re probably writing a hardcore custom ES anyway.

More importantly: even in a “hardcore” ES, you don’t actually *need* that performance all the time. If you’re just pulling out a handful of Entities and their Components – e.g. a player Entity (of which there’s only 1 – or a few for multiplayer) – then the above performance differences are NON EXISTENT (vanishingly small).

…but the code is more readable, easier to maintain, and more robust.

So. I recommend using the MetaEntity for *all* your ES coding … until you reach a point where performance is low. Then look at re-coding *only* the affected loops / methods (remember: you can do this on a System-by-System basis; your code is nicely separated already!). That way, you get the best of both worlds.

However … the reason none of this was included in the first Java ES I posted onto the wiki – the ultra-simple “RDBMS-inspired with Code in Systems” – is that really this is all just gloss on top of an ES. You don’t need it. I believe it makes your ES easier to work with – but it distracts from the core theory.

I’d recommend you start with the simpler ES, and understand it, before moving onto something like this for your practical work.

For more info, check out the wikidot link at the top of this post – and try the github project linked from it (currently empty, until I do a check-in)

Filed under : entity systems

67 Comments »

If you’re interested in using an ES on indie projects, and you’re craving concrete examples, you might want to look at the comments (page 1 and page 2) on Mythruna:

“Since this is a developers forum, I’ll describe a little bit about what went on behind the scenes.

During the early development for this phase, I read about an architectural pattern called an entity system.
…
I had (pre Entity Systems) a plan for how I was going to store the placement of objects in the world but this past weekend when I actually got to implementing it, I couldn’t make that original plan work and came to the point where I needed to solve the general problem of world state storage. This is the kind of thing that Entity Systems make relatively straight forward… and by Saturday, Mythruna had an embedded SQL database that autogenerates tables based on the components stored. (HyperSQLdb for the win.)

So at some point I swapped out the in-memory version of the entity system with the SQL version… and suddenly objects were persistent. It was so easy I had to double check a few different ways that it was actually working. :)”

…but of course, also: use the Entity Systems wiki – put your questions there, put your ideas there, and (most of all) if you have an ES project or ES source code you want to share, please add it to the wiki!

Filed under : MMOG development, entity systems

10 Comments »

I’ve just done a round of fixes for the source-examples of ES’s. Github projects updated on this page:

http://entity-systems.wikidot.com/rdbms-with-code-in-systems

Changed:

1. Added a complete Java implementation of the most basic ES example
2. Fixed some minor bugs in the Objective-C basic ES example; added some missing classes
3. Added a missing class method from the documentation (System.

Filed under : MMOG development, design, entity systems, network programming, programming

6 Comments »

For this page:

http://entity-systems.wikidot.com/start

…I want a list of published / self-published games that were built on top of an Entity System.

I know a few off the top of my head (I think – I’m going to mail some of the authors and double-check), but mostly I have no idea.

So … if you know of any, please add them to the wiki on that page.

BONUS POINTS

…if you can provide *any* of the following info:

• Link to a public interview / post-mortem about the game that mentions the team’s experiences with ES
• Names of any programmers and designers that definitely worked on the ES part of the game
• *SUPER BONUS*: link to a description of how they designed/implemetned the ES part

Filed under : entity systems, programming

11 Comments »

(this is a simplified, but complete, version of the “Entity System 1: Java/Android”)

NB: this post is part of the community effort to make working, usable, free, source-examples of actual Entity Systems, found at http://entity-systems.wikidot.com/

Wiki link: http://entity-systems.wikidot.com/rdbms-with-code-in-systems

Github source: https://github.com/adamgit/Entity-System–RDBMS-Inspired–Objective-C-

I’m posting my original design notes here – also copy/pasted to the wiki, but the wiki version will probably evolve over time. Comments on this post will be disabled – please comment direct on the wiki / wiki discussion pages.

NB: the source code is in GitHub, that’s where you should download it from (will include any fixes and updates over time)

Concepts

(integer) Entity
(enum) ComponentType
(interface OR empty base-class) Component
(interface OR empty base-class) System
(class with implementation) EntityManager

Data Classes

Entity

This is a non-negative integer, and uniquely identifies an entity within the ES. If you serialize your ES, this SHOULD be the same after serialization. New entities have to be assigned a new, unique, integer value when they are created. When an entity is destroyed, its integer value is freed, available for re-use.

ComponentType

This is needed because this ES is so simplistic it has no other way of identifying which Component you’re talking about when you’re looking at a single Entity.

(richer EntitySystems MAY not need this type)

Component

This exists purely to make method-signatures type-safe. We need to know that certain objects are valid instances of a “component” (so we use this superclass to indicate that), but we also need a method that’s guaranteed to work on all those objects (see below).

(richer EntitySystems make more use of this, and require it to be a class, but for this simple ES, you can use an interface/header file)

System

This exists purely to contain a known method-signature that your game-loop can call once per game-tick.

EntitySystem Implementation Classes

Each subclass of Component

Internally, the class has the following functions:

1. (ComponentType) getComponentType();

EntityManager

This contains:

1. The master collections (arrays, hashmaps, whatever) that contain all the data for all the entities
2. Logic for creating, modifying, fetching, and deleting entities
3. Logic for fetching and modifying components-from-entities

Internally, the class has the following variables:

1. MAP: from “Entity + ComponentType” to “concrete Component subclass”
2. LIST: all entities in existence (so it never duplicates entity-IDs!)

3. Internally, the class has the following functions:

   1. (Component) getComponent( Entity, ComponentType );
   2. (List) getAllComponentsOfType( ComponentType );
   3. (List) getAllEntitiesPossessingComponent( ComponentType );
   4. (void) addComponent( Entity, Component );
   5. (int) createEntity;
   6. (void) killEntity( Entity );
   Filed under : entity systems, games design, programming

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