middleclass

When upgrading to version 3.x, note the following breaking changes:

1. No Global Variables: class and Object are no longer set on the global scope. You must use require to access them.
2. Accessing class and Object:
   • Use local class = require 'middleclass' to get the class factory.
   • Access the base object via class.Object.
3. API Refactoring: The standalone functions instanceOf, subclassOf, and includes have been replaced by methods on instances or static methods on the Object class.

Note on Parameter Order: When using the new Object static methods, the parameter order has changed compared to the 2.x standalone functions.

-- 1. Accessing the class factory and Object
local class = require 'middleclass'
local Object = class.Object

-- 2. Using the new method-based API (requires valid objects/classes)
obj:isInstanceOf(MyClass)
aClass:isSubclassOf(Object)
aClass:includes(aMixin)

-- 3. Using the Object static API (safe for any type, but note parameter order)
Object.isInstanceOf(obj, MyClass)
Object.isSubclassOf(aClass, Object)
Object.includes(aClass, aMixin)

Middleclass provides a simple way to implement Object-Oriented Programming in Lua, supporting inheritance, class variables, and superclass initialization.

Key patterns:

• Class Definition: Use class('ClassName') to create a new class. To inherit from another class, use class('SubclassName', ParentClassName).
• Initialization: Define an initialize method to set up instance state.
• Superclass Calls: Use ParentClassName.initialize(self, ...) to invoke the parent's initializer.
• Class Variables/Methods: Use the .static table to define variables or methods that belong to the class itself rather than instances.

local class = require 'middleclass'

local Fruit = class('Fruit') -- 'Fruit' is the class' name

function Fruit:initialize(sweetness)
  self.sweetness = sweetness
end

Fruit.static.sweetness_threshold = 5 -- class variable (also admits methods)

function Fruit:isSweet()
  return self.sweetness > Fruit.sweetness_threshold
end

local Lemon = class('Lemon', Fruit) -- subclassing

function Lemon:initialize()
  Fruit.initialize(self, 1) -- invoking the superclass' initializer
end

local lemon = Lemon:new()

print(lemon:isSweet()) -- false