Implement aspects

[Ruin0x11]

Purpose of this PR

• Bug fix
• Enhancement
• New feature

Related issues

Close #15.

Description

Implements an "aspect" system, which is synonymous with the capability system described in previous issues. It allows you to add new functionality to instanced map objects.

local aspect = cargo:get_aspect_or_default(IItemCargo)
aspect.cargo_weight = 25000

cargo:set_aspect(IItemCargo, ItemCargoAspect:new(cargo, { cargo_quality = 5 })

Aspects support temporary values, to allow for using :calc() as before. Using aspects, you'd be able to override the behavior of an interface that encompasses IAspect, even for things like property accesses through temporary values. From the unit tests:

local AspectHolder_ITestAspect = require("test.api.AspectHolder_ITestAspect")
local IAspectModdable = require("api.IAspectModdable")

local AspectHolder_TestAspectModdable = class.class("TestAspectModdable", AspectHolder_ITestAspect)

function AspectHolder_TestAspectModdable:init(target, params)
   self.foo = params.my_foo or 0
end

function AspectHolder_TestAspectModdable:calc(target, prop)
   local v = IAspectModdable.calc(self, target, "foo")

   if prop == "foo" then
      return v * 100
   end

   return v
end

function AspectHolder_TestAspectModdable:mod(target, prop, v, params)
   if prop == "foo" then
      v = v + 42
   end

   return IAspectModdable.mod(self, target, prop, v, params)
end

return AspectHolder_TestAspectModdable

Finally, a new _ext table is now supported on all prototypes for map objects, which lets you specify which aspects should get instantiated on object creation, and their parameters. The parameters will get passed to the constructor along with the object instance.

data:add {
   _type = "base.item",
   _id = "aspected",

   _ext = {
      AspectHolder_ITestAspect
   }
}

data:add {
   _type = "base.item",
   _id = "cargo_art",
   elona_id = 669,
   image = "elona.item_painting_of_landscape",
   value = 3800,
   rarity = 150000,
   coefficient = 100,

   categories = {
      "elona.cargo"
   },

   _ext = {
      [IItemCargo] = {
         cargo_quality = 7,
         cargo_weight = 35000
      }
   }
}

The end goal is to replace the usages of item:has_category() and similar for determining if an item "acts like" equipment, cargo, and so on. Item categories are still useful for things like item generation, so they will stay for the time being.

The parts of the cargo system that were implemented have been updated to use aspects.