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Defining Entities using XML/Lua

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#1 SeiryuEnder   Members   


Posted 02 June 2012 - 05:24 PM

I've been developing a data-driven Entity/Component system in C++ using Lua. There's been a lot of good documentation out there, but I've hit a bit of a snag with Lua.

-edit 1:

My problem is likely to do with the need to bind classes to Lua rather than using a factory to create components... Still doing research on that

-edit 2:

After some thought and playing with Luna, I'm going to try registering components with my factory and feeding the base pointer address back to Lua.

From there I should be able to call the expected mapped function. I don't know what kind of error handling this will have (if any), but at least it's moving in a direction.

Summarized Question
How do I create objects with a C++ factory using Lua and define necessary parameters while maintaining type-safety?
My goal is to be able to determine at runtime if there is a parameter mismatch when defining a component.

More Information
The system works like this:

Open OnLoad.lua
-OnLoad.lua loads _Level_.lua
--_Level_.lua loads and posititions Entities in the world
---Game parses XML file for (each) Entity definition

[One of two ways to handle this]

----Entity tag references _Entity_.lua
-----_Entity_.lua loads components


----Entity tag body contains a list of components along with component property definitions

Problem 1:
Not a huge problem, but a mild design question. Method A is more expandable (Lua can perform other actions when loading different types of entities, may be useful), but Method B is simpler and centralized. Right now I'm leaning towards defining entities in Lua to err on the side of caution, as I may need the extra functionality later. Which do you think is the preferable method?

Problem 2:
I'm stuck on how to actually construct the components. I'm sure there's a simple solution, but for some reason it eludes me. Different components may need different parameters. For instance, I may need no parameters at all for a Timer component but will need a name/ID for a Mesh component. I'd like to figure out a way to register the component with my object factory but that would mean having to have a default constructor and defining a different Init(...) function for each type of component. When I create the object with the factory I will get a base class pointer back, so I have no (type-safe) way of knowing what parameters the init function will need. I'm just not making the connection on how to define properties of a component in XML/Lua then pass that information to its appropriate place in C++. I've considered more elaborate methods of defining property tables in my components, but that seems inefficient and overcomplicated.

As always, thanks in advance for any help!

Edited by SeiryuEnder, 02 June 2012 - 07:36 PM.

#2 JTippetts   Moderators   


Posted 02 June 2012 - 07:40 PM

My main question is why are you using both XML and Lua? If you've already embedded Lua, you have at your fingertips an excellent data description language, making any reliance upon XML completely redundant, and needlessly complicating things.

As for the rest of it, I'm not 100% sure what it is you're asking. Guess my reading comprehension fails today. If the problem is with actually binding classes to Lua, they have libraries for that including tolua++ and luabind.

I did a writeup on how I do things for my own projects here, if you are curious.

#3 SeiryuEnder   Members   


Posted 02 June 2012 - 08:35 PM

Yeah it's a bit of a complex problem to describe without coding examples.

The only real reason for using XML right now is that at some point I intend to support other scripting languages such as Python.
XML acts as a simple layer of abstraction between the game and the scripting language.

The problem was twofold - my lack of understanding how to properly use Lua and how to initialize my components.

For instance, until now I've done something like this (just to get things running):
Entity* ent = new Entity;
Cmp_Transform* cmpTransform = new Cmp_Transform( ent );
Cmp_Mesh* cmpMesh = new Cmp_Mesh( ent, "SomeMesh.gmx" );
ent->AttachComponent( cmpMesh );
ent->AttachComponent( cmpTransform );

I moved the constructor args to an Init function so that I could use the default constructor.
Now, I can register components to an object factory and create them like this:

Factory<IComponent*> factory;
IComponent* cmp = factory.Create("Transform");

Entity entity;
entity.AttachComponent( cmp );

With a simple glue function I can create any registered component by knowing its name, which gives me a lot of flexibility from the scripting end.
The problem was, I couldn't call an Init(...) function from the C++ end because it is a member function of an unknown derived class.

I think I've finally figured this one out. I've set up Luna to bind component member functions to Lua, and I'm going to push the component pointer onto the stack.
From there I should be able to call the Initialize function. I haven't gotten that far in code yet, but theoretically it should pipe to the proper function where I can at least check the number of arguments and hopefully later enforce better type safety.

Edited by SeiryuEnder, 02 June 2012 - 08:55 PM.

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