Question

I am writing a small game engine for XNA. Right now my entity manager has a list of all entities it can update and render. Lets say, however, that I want to split this list up into ‘sub lists’. Maybe some entities do not require rendering, and so we would like to separate the rendering entities from the non-rendering entities, while maintaining another list which shows ALL entities for updating. So thats 3 lists.

Is there a way to maintain consistency between lists easily? To ensure that the exact same objects are being referenced in both lists? (Not a copy of the object?). In C++ we’d just have an array of pointers to entities that render, and another for entities that dont.

The problem isn’t really putting entities into these lists, its disposing them thats throwing me off. To destroy an entity, i simply set a ‘dispose’ bool to true in that entity. The entity manger, when looping through all entities, removes the reference to that entity if ‘dispose’ is set to true, and the garbage cleaner destroys the entity because there is no more references left to it. Would i have to manually go through each and every list that has something to do with Entities and remove that entity from those lists to keep things consistent?

another example of this ‘problem’ is with quad trees. I have a 2d game and i generate a quad tree for all the entities. The entities that lie in between 2 grid locations are in 2 branches of the tree. how would i remove it from the tree? search the entire tree for references to that entity? seems a little pointless to have a quad tree if thats the case..!

Answer 1

Consider instead of 3 lists, have one list and 2 queries of that list.

var list = GetYourMainList(); 
var renderableEntities = list.Where(item => item.IsRenderable); 
var nonrenderableEntities = list.Where(item => !item.IsRenderable); 

// work with the queries
foreach (var item in renderableEntities)
{
    // do whatever
}

Here, you can iterate over each query, the results will stream from the main list, and any update to that list will be reflected in the query results as they are iterated. The caveat is that the list cannot be modified (as in added to or deleted from) while the queries are being iterated.