Padrões de mixins

O que é um Mixin?

local Flyable={}
function Flyable:fly()
  print(self.name.." is flying!")
end
function Flyable:land()
  print(self.name.." landed.")
end

local Bird={} Bird.__index=Bird
-- Mix in Flyable
for k,v in pairs(Flyable) do Bird[k]=v end

function Bird.new(name)
  return setmetatable({name=name},Bird)
end
Bird.new("Eagle"):fly()

Vários Mixins

local Serializable={}
function Serializable:toCSV(fields)
  local row={}
  for _,f in ipairs(fields) do row[#row+1]=tostring(self[f]) end
  return table.concat(row,",")
end

local Loggable={}
function Loggable:log(msg)
  print("["..os.date("%H:%M").."]["..self.name.."] "..msg)
end

local Entity={} Entity.__index=Entity
for k,v in pairs(Serializable) do Entity[k]=v end
for k,v in pairs(Loggable) do Entity[k]=v end

function Entity.new(n,v) return setmetatable({name=n,value=v},Entity) end
local e=Entity.new("item",42)
e:log("created"); print(e:toCSV({"name","value"}))

Conflitos entre Mixins

local A={speak=function(self) print(self.name.." (A)") end}
local B={speak=function(self) print(self.name.." (B)") end}

local C={} C.__index=C
for k,v in pairs(A) do C[k]=v end
for k,v in pairs(B) do C[k]=v end  -- B.speak overwrites A.speak

-- Explicit resolution:
local origA, origB = A.speak, B.speak
function C:speak() origA(self); origB(self) end  -- call both

local c=setmetatable({name="X"},C)
c:speak()

Auxiliar include()

local function include(cls, mixin)
  for k, v in pairs(mixin) do
    if cls[k] == nil then
      cls[k] = v
    end
  end
  return cls
end

local Comparable={
  equals=function(a,b) return a:compareTo(b)==0 end,
  lt=function(a,b) return a:compareTo(b)<0 end,
}

local Temp={} Temp.__index=Temp
include(Temp, Comparable)
function Temp.new(c) return setmetatable({c=c},Temp) end
function Temp:compareTo(other) return self.c-other.c end

local a,b=Temp.new(100),Temp.new(50)
print(b.lt and b:lt(a))  -- true (b < a)

Mixin com Estado

local Observable={}
function Observable._init(self)
  self._listeners={}
end
function Observable:on(event,fn)
  self._listeners[event]=self._listeners[event] or {}
  self._listeners[event][#self._listeners[event]+1]=fn
end
function Observable:emit(event,...)
  for _,fn in ipairs(self._listeners[event] or {}) do fn(...) end
end

local Button={} Button.__index=Button
for k,v in pairs(Observable) do Button[k]=v end

function Button.new(label)
  local self=setmetatable({label=label},Button)
  Observable._init(self)
  return self
end

local btn=Button.new("OK")
btn:on("click",function() print("Clicked!") end)
btn:emit("click")

Módulos de Mixins

-- In a real project:
-- local Loggable = require("mixins.loggable")
-- local Cacheable = require("mixins.cacheable")
-- local MyClass = {}
-- for k,v in pairs(Loggable) do MyClass[k]=v end
-- for k,v in pairs(Cacheable) do MyClass[k]=v end

local TimestampMixin={}
function TimestampMixin:setCreated() self.createdAt=os.time() end
function TimestampMixin:age() return os.time()-self.createdAt end

local Record={} Record.__index=Record
for k,v in pairs(TimestampMixin) do Record[k]=v end
function Record.new(data)
  local self=setmetatable(data or {},Record)
  self:setCreated()
  return self
end
print(Record.new({}).createdAt>0)

Mixins Baseados em Papéis

local Roles={}
Roles.admin={deleteUser=function(self) print(self.name.." deletes user") end}
Roles.editor={editPost=function(self) print(self.name.." edits post") end}
Roles.viewer={viewPost=function(self) print(self.name.." views post") end}

local User={} User.__index=User
function User.new(name,...)
  local self=setmetatable({name=name},User)
  for _,role in ipairs({...}) do
    for k,v in pairs(Roles[role] or {}) do self[k]=v end
  end
  return self
end

local alice=User.new("Alice","admin","editor")
alice:deleteUser(); alice:editPost()

Verificando a Aplicação de Mixins

local function applyMixin(cls, mixin, name)
  cls._mixins = cls._mixins or {}
  if cls._mixins[name] then return end  -- already applied
  for k,v in pairs(mixin) do
    if k~="_init" then cls[k]=cls[k] or v end
  end
  cls._mixins[name] = true
end

local Fly={fly=function(self) print(self.name.." flies") end}
local Bird={} Bird.__index=Bird
applyMixin(Bird,Fly,"Fly")
applyMixin(Bird,Fly,"Fly")  -- no-op, already applied
print(Bird._mixins.Fly)  -- true

Mixins vs. Herança vs. Composição

-- Inheritance: "is-a"
-- local Dog = extends(Animal)

-- Mixin: "can-do"
-- include(Dog, Swimmable)

-- Composition: "has-a"
-- local Dog = {}
-- function Dog.new()
--   return {collar=Collar.new(),chip=Chip.new()}
-- end

print("Pick the right tool for the relationship")

Modelo de Mixin

-- Standard mixin template:
local EventMixin = {}
EventMixin._name = "EventMixin"

function EventMixin._init(self)
  self._events = {}
end

function EventMixin:on(e, fn)
  self._events[e] = self._events[e] or {}
  self._events[e][#self._events[e]+1] = fn
end

function EventMixin:trigger(e, ...)
  for _, fn in ipairs(self._events[e] or {}) do fn(...) end
end

-- return EventMixin
print("EventMixin ready for use")

Pergunta sobre Mixins

Recapitulação: Padrões de Mixin