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A wrapper around MoonSharp that allows easy development of moddable Unity games
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README.md

Unity-Lua

A wrapper around MoonSharp that allows easy development of moddable Unity games

Installation

  • Create "Assets/Plugins/Lua" in your Unity project
  • Download and place the contents of this repository to the newly created folder

Dependancies

MoonSharp & Unity-Logger

Code Breakdown

This codebase allows Unity developers to easily create game specific modding apis for their Unity games! It was created during the development of Off Grid

Executing Lua Code

LuaVM is the main Lua interface, it's the class you use to run Lua scripts or individual Lua command strings.

Executing a string

const string luaCode = @"
  -- Lua Code
  num = 1 + 1
  print(num)
";

LuaVM vm = new LuaVM();
vm.ExecuteString(luaCode); // Prints 2

Executing a script

Most of the time you'll want to load and execute Lua code from a script file written to disk, here's an example of how that's achieved.

Contents of fruits.lua

fruits = {
  "apple",
  "banana",
}

GetRandomFruit = function()
  return fruits[math.random(1, #fruits)]
end

Here's some code that loads the fruits table from the script, as well as getting a reference to the GetRandomFruit and calling it.

LuaVM vm = new LuaVM();
vm.ExecuteScript("/path/to/fruits.lua");

// Get table to iterate
Table fruitTable = vm.GetGlobalTable("fruits");
foreach (DynValue fruit in fruitTable.Values)
{
  Debug.Log(fruit.String); // Prints "apple" then "banana"
}

// Or get a lua function and call it
DynValue fruitFunction = vm.GetGlobal("GetRandomFruit");
Debug.Log(vm.Call(fruitFunction).String); // Prints return of GetRandomFruit

Creating an API

Creating an api with this framework is incredibly simple, let's say we're making a game where the player can interact with supermarkets. We of course want this game to be moddable, so let's write up information about the super market in Lua, things like stock and name for a start.

Here's an example of a very simple Lua api to let the players get random items that can go in their shops stocklist.

[LuaApi(
	luaName = "SuperMarket",
	description = "This is a test lua api")]
public class SuperMarketAPI : LuaAPIBase
{
  private readonly List<string> m_Veggies = new List<string>
  {
    "Aubergine",
    "Broccoli",
    "Cauliflower",
    "Carrot",
    "Kale",
  };

  private readonly List<string> m_Fruits = new List<string>
  {
    "Strawberry",
    "Grape",
    "Lychee",
    "Melon",
    "Apple",
  };

  public SuperMarketAPI()
    : base("SuperMarket")
  {
  }

  protected override void InitialiseAPITable()
  {
    m_ApiTable["GetRandomVeg"] = (System.Func<string>) (Lua_GetRandomVeggies);
    m_ApiTable["GetRandomFruit"] = (System.Func<string>) (Lua_GetRandomFruits);
    m_ApiTable["MaxStock"] = MaxStock;
  }

  [LuaApiEnumValue(description = "The max stock any shop should contain")]
  private const int MaxStock = 10;

  [LuaApiFunction(
    name = "GetRandomVeg",
    description = "Returns a random vegetable that can be stocked by an in-game shop"
    )]
  private string Lua_GetRandomVeggies()
  {
    int randomIndex = Random.Range(0, m_Veggies.Count - 1);
    return m_Veggies[randomIndex];
  }

  [LuaApiFunction(
    name = "GetRandomFruit",
    description = "Returns a random fruit that can be stocked by an in-game shop"
  )]
  private string Lua_GetRandomFruits()
  {
    int randomIndex = Random.Range(0, m_Fruits.Count - 1);
    return m_Fruits[randomIndex];
  }
}

Lua apis become available to LuaVM instances by default, the first time a LuaVM is created reflection is used to cache all types that derive from LuaAPIBase. Here's a Lua script that uses the brand new SuperMarket api:

Shop = {
  Name = "Dumpling's Super Store",
  Stock = {},
}

-- Generate the stock items
for i= 1, SuperMarket.MaxStock do
  table.insert(Shop.Stock, SuperMarket.GetRandomVeg())
  table.insert(Shop.Stock, SuperMarket.GetRandomFruit())
end

Corrisponding C# code

LuaVM vm = new LuaVM();
vm.ExecuteScript("/path/to/DumplingsStore.lua");

// Get the shops name
string shopName = vm.GetGlobal("Shop", "Name").String;
Debug.Log(shopName); // Prints "Dumpling's Super Store"

// Get Items in stock
Table fruitTable = vm.GetGlobalTable("Shop", "Stock");
foreach (DynValue item in fruitTable.Values)
{
  Debug.Log(item.String);
}

So there's a really simple example of how you can add arbitrary apis to your game for use in Lua. LuaApiBase uses the string passed into its constructor as the true name of the Lua api, in this example that's "SuperMarket". It then allows the derived type to fill in m_ApiTable, note how above this is really a MoonSharp wrapper around a Lua table, meaning that it's not just functions. Above we've used a MaxStock int which whilst it's currently a const, could be set by calling into a gameplay system.

Exposing Enums

One thing that can be a pain when dealing with lua is having to use raw ints rather than enums, using this framework fixes this issue by allowing the automatic generation of Lua versions of your enums. Let's say in our game the player can adopt pets, and we want modders to be able to create new pets with different personalities and abilities. Here's how you could go about exposing an enum type to your modders in order to know how to render the correct model/sprite.

[LuaApiEnum(
  name = "PetType",
  description = "Defines what type a pet is")]
public enum PetType
{
  [LuaApiEnumValue(
    description = "Aloof and occasionally affectionate")]
  Cat,

  [LuaApiEnumValue(
    description = "A loyal best friend")]
  Dog,

  [LuaApiEnumValue(
    description = "Slow and a bit snappy")]
  Turtle,

  [LuaApiEnumValue(
    description = "Cute, small and loves grain")]
  Hamster,

  // Not ready for modders yet!
  [LuaApiEnumValue(hidden = true)]
  Dragon,
}

In a similar vein to how Lua apis are automatically detected by LuaVM, any enum with the LuaApiEnum is automatically exposed to all lua scripts run from LuaVM. So modders can now use it like so:

Pet = {
  Name = "Rex",
  Type = PetType.Dog,
  attack = function(target)
    -- Attack Logic
  end
}

** Note: ** It's worth noting that enum values can be hidden from exposure by the LuaApiEnumValue attributes hidden value, just as we've done with the Dragon type above.

Options

The LuaVM constructor optionally takes in an instance of the VMSettings flag, this allows the user to attach only attach apis, enums, both or none at all. If you know you're not going to need any of the attachments, it's more performant to use new LuaVM(VMSettings.None)

Documentation Creation

One of this frameworks most handy features is its automatic documentation creation. Document creation is triggered by using the built in Unity menu items:

Currently the framework supports the creation of the following documentation:

  • MediaWiki pages per api
  • Visual Studio Code snippets for auto complete
  • Atom Snippets for auto complete

These are all created by using the attributes attached to your apis, api function, api variables and enums. All the available attributes are used in the code snippets above, however here's a quick reference:

Attribute Use
LuaApi Above LuaApiBase derived type
LuaApiFunction Above functions that are part of the Lua api table
LuaApiVariable Above variables that are part of the Lua api table
LuaApiEnum Above enums that you want to be attached to LuaVMs
LuaApiEnumValue Above enums values that you'd like to describe or hide

Adding more documentation formats is very easy, why not try adding another doucmentation type yourself in LuaDocGenerator.cs!

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