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I am designing my game with Entity/Component concepts of GameplayKit in iOS 9, for ShootComponent, should define bullet/missile as Entity?

Reason for Yes:

separate logic from its parent, e.g. playerTank or enemyTank;

if not, TankEntity need distinguish whether its bullet collide with other Entities or itself.

Reason for No:

it is not actual entity in logic world, which is fired by my tank or enemy turret;

bullet always be shot and disappeared, so game need add/remove it now and then;

For your comments pls.

Robin
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3 Answers3

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Finally decided to define bullet/missile as entity, so it acts as entity in contact test, rendering and other components.

Robin
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I would have add it as a component for the entity using it. So you will be able to make any entity fire bullet or missile. Keep in mind that your entity should only act as a simple reference with no logic in it.

Elivard
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First lets read Adam Martins original description of his terms. It appears Apple got the idea of entities and components from Martin:

Entity: The entity is a general-purpose object. Usually, it only consists of a unique id.

Component: the raw data for one aspect of the object, and how it interacts with the world.

System: "Each System runs continuously (as though each System had its own private thread) and performs global actions on every Entity that possesses a Component or Components that match that Systems query."

Martin is just defining terms for doing compositional design, which is an alternative to inheritance that is more recombinable and flexible. So entities are what you might recognize as instances of a class, but classes have been stripped of all their data and methods, which has been moved out into components - and the entities just delegate to the components.

So your missile... it would be an instance of a class in normal OO terms - an object, right? And a missile can behave in a variety of ways... it can seek out an enemy, it can fly straight ahead, it can speed up, etc. It also has properties that indicate if it's hit an enemy, properties for its total damage, health, and so on.

So the missile is an entity while these various methods / data would be components of the missile entity.

Martins approach is interesting, and there hasn't been as much focus on compositional design as there has been OO (for what reason I don't really know), so I can see why Apple would adopt it for a game framework like this.

But his ideas don't seem very well fleshed out. For example, usually in compositional design there is a delegation hierarchy, where objects will keep delegating up a chain until some data or method is found. At the top there's one meta-object that everything delegates to. In this way objects are both components and entities - they act as both the delegating and the delegated to. But Martins terms don't support this... his model is flat - there are only entities, and then components that can be added to them, but no delegation between entities and no meta-object.

Maybe he felt this flat design was appropriate for game development. I have my doubts... you seem to want some kind of hierarchical structure of objects. I would look for a way to mix in inheritance, or setup some kind of custom delegation hierarchy where objects could act as both entities and components. You might look to see if this is possible within that framework, or if it isn't just write your own.

ibrust
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