system.lua

-- Services
local replicatedStorage = game:GetService("ReplicatedStorage")

local shared = replicatedStorage.shared

local sharedLib = shared.lib
local signalLib = require(sharedLib.signalLib)

local sharedData = shared.data
local entityPool = require(sharedData.entityPool)

local system = {}
system.__index = system

function system.new(componentName)
    local systemPool = entityPool.componentEntityMap[componentName] or {}
    entityPool.componentEntityMap[componentName] = systemPool

    -- Base system intentionally lacks a default update() member
    local self = setmetatable({}, system)
    self.registeredEntityCount = 0
    self.pool = systemPool

    signalLib.subscribeAsync("componentAttached", function(attachedComponentName, entityId)
        if attachedComponentName == componentName then
            self:onEntityRegistered(entityId)
        end
    end)
    signalLib.subscribeAsync("componentDetached", function(detachedComponentName, entityId)
        if detachedComponentName == componentName then
            self:onEntityRegistered(entityId)
        end
    end)
    -- Allows for communication between systems
    signalLib.subscribeAsync("entitySignal", function(component, entityId, ...)
        if component == componentName and self.onEntitySignal then
            self:onEntitySignal(entityId, ...)
        end
    end)

    return self
end

-- Called implicitly by entityPool's addComponent function.
function system:onEntityRegistered(entityId)
    self.registeredEntityCount = self.registeredEntityCount + 1

    -- Calls inhereted class' entity registered callback if it has one.
    if self._onEntityRegistered then
        self:_onEntityRegistered(entityId)
    end

    return entityId
end

-- Called implicitly by entityPool's removeComponent function.
function system:onEntityDeregistered(entityId)
    -- Guard clause not needed, the entity is checked for its existence in entityPool.
    self.registeredEntityCount = self.registeredEntityCount - 1

    -- Calls inhereted class' entity registered callback if it has one.
    if self["_onEntityDeregistered"] then
        self:_onEntityDeregistered(entityId)
    end
end

return system