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4.1 The Basics

In this section the basic concepts behind Crystal Entity Layer are explained. Crystal Entity Layer is a game layer that sits on top of Crystal Space and adds game specific notions.


The most important concept in nearly every game is an entity. An entity can be anything that has a meaning in the game. This includes objects a player can interact with (like keys, weapons, ...), non player characters (NPC), monsters, boxes that you can open and close, the player itself, the world, a part of the storyline the player needs to solve. And the list goes on. Basically everything that you consider a seperate entity when talking about your game will be an entity in Crystal Entity Layer.

Entities are not always visual entities. i.e. you can't always see an entity on screen. An entity can also be something that is not visual like a story element or some data you want to keep.

An entity is represented by the iCelEntity interface. An entity is created by an iCelPlLayer instance.

On its own an entity in Crystal Entity Layer is pretty useless. It is just an empty stub with a name. That's all. To make an enity useful two important concepts are introduced: property classes and a behaviour.

Property Classes

A property class is a piece of code (typically implemented in a plugin) that you can attach to some entity. By attaching the right property classes to an entity you define the characteristics of that entity. This notion can be explained best with a few examples. Let's say you want to define an entity for a box in a game. Here are the property classes you could use for that entity:

Here is another example of how you could setup your hero:

These are only two examples. There are a lot more possibilities. It is important to note that property classes are dynamic. You can add or remove property classes at runtime so that you can actually change the characteristics of an entity.

A property class is represented by the iCelPropertyClass interface. Property classes are created from property class factories.


The property classes define what an entity can do. The combination of property classes define the characteristics of that entity. The behaviour for an entity defines how the entity actually interacts in and with the game. It is where the game logic is defined. The entity system and property classes represent code that is written in Crystal Entity Layer itself but the behaviours represent code that you as a game creator need to write. There are currently three supported ways to write behaviours:

In the same game you can mix behaviours of different types and in the future more ways will be added.

Property classes send messages to the behaviour when something interesting happens. For example, when an entity has a `pccommandinput' property class then the behaviour will get messages every time a key is pressed. In reaction to those messages the behaviour can then call API calls in `pcactormove' to (for example) move the actor around.

A behaviour is represented by the iCelBehaviour interface. You can create behaviours from iCelBlLayer.

Physical Layer

The physical layer is the core of Crystal Entity Layer. It is responsible for the creation and management of entities and property classes. In addition to the physical layer plugin the plugins that contain the code for the property class factories are also considered part of the physical layer.

A physical layer is represented by the iCelPlLayer interface. You can get an instance of the physical layer from the object registry if you loaded the plugin at initialization time.

Behaviour Layer

The behaviour layer is a simple class which has only one responsability: the creation of behaviours. For XML and `python' there are already two predefined behaviour layers. If you want to use `c++' for the game logic then you must make your own behaviour layer.

The behaviour layer is represented by the iCelBlLayer interface.

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