Programming in Lua(Thrid Edition)笔记

16 Object-oriented Programming

• 对象和方法

  Account = {
  	balance = 0,
  	withdraw = function (self, v)
  		self.balance = self.balance - v
  	end
  }
  function (v)
  	self.balance = self.balance + v
  end
  Account.deposit(Account, 200.00)
  Account:withdraw(100.00)

在用冒号:而不是.创建或使用一个table的函数时，会自动生成或调用一个self指代table自身

• 类，用__index来类的实例化

  function Account:new(o)
  	o = o or {} 
  	setmetatable(o, self)
  	self.__index = self
  	return o
  end
  a = Account:new{balance = 0}
  print(a.balance) --> 0
  a:deposit(100.00)
  print(a.balance) --> 100

初次调用a.balance = a.balance + v，右侧的balance是类的默认值，右侧是对象的新域，之后再调用，右侧的balance为对象的域

• 继承，子类可以在父类的基础上修改方法，修改后的方法是在子类自身的域

  SpecialAccount = Account:new()
  s = SpecialAccount:new{limit = 1000.00}
  function SpecialAccount:withdraw(v)
  	if v - self.balance >= self:getLimit() then
  		error"insufficient funds"
  	end
  	self.balance = self.balance - v
  end
  function SpecialAccount:getLimit()
  	return self.limit or 0
  end
  print(s.balance) --> 0
  s:deposit(100.00)
  print(s.balance) --> 100
  s:withdraw(200.00)
  print(s.balance) --> 200

• 多重继承

  -- look up for 'k' in list of tables 'plist'
  local function search(k, plist)
  	for i = 1, #plist do
  		local v = plist[i][k] -- try 'i'-th superclass
  		if v then return v end
  	end
  end
  function createClass(...)
  	local c = {} -- new class
  	local parents = {...}
  	-- class will search for each method in the list of its parents
  	setmetatable(c, {__index = function (t,k)
  		return search(k, parents)
  	end})
  	-- prepare 'c' to be the metatable of its instances
  	c.__index = c
  	-- define a new constructor for this new class
  	function c:new(o)
  		o = o or {}
  		setmetatable(o, c)
  		return o
  	end
  	return c -- return new class
  end
  Named = {}
  function Named:getname()
  	return self.name
  end
  function Named:setname(n)
  	self.name = n
  end
  NamedAccount = createClass(Account, Named)
  account = NamedAccount:new{name = "Paul"}
  print(account:getname()) --> Paul
  print(account)

用createClass()创造一个继承自两个父类的子类。为了减少search()在父类中搜索的复杂度，可以采用记忆化搜索，将搜索的结果保存在原table中

setmetatable(c, {__index = function (t, k)
	local v = search(k, parents)
	t[k] = v -- save for next access
	return v
end})

这样做的缺点：It is difficult to change method definitions after the system is running, because these changes do not propagate down the hierarhy chain.（说实话，我没看懂这句话……）

• privacy，用局部变量和闭包构造私有成员，使得私有成员从外部无法直接获得，只能在内部获得

  function newAccount(initialBalance)
  	local self = {
  		balance = initialBalance,
  		LIM = 10000.00,
  	}
  	local extra = function ()
  		if self.balance > self.LIM then
  			return self.balance * 0.10
  		else
  			return 0
  		end
  	end
  	local withdraw = function (v)
  		self.balance = self.balance - v
  	end
  	local deposit = function (v)
  		self.balance = self.balance + v
  	end
  	local getBalance = function ()
  		return self.balance + extra()
  	end
  	return {
  		withdraw = withdraw,
  		deposit = deposit,
  		getBalance = getBalance
  	}
  end
  acc1 = newAccount(100.00)
  acc1.withdraw(110.00)
  print(acc1.getBalance()) --> -10
  acc1.deposit(50.00)
  print(acc1.getBalance()) --> 40

• single-method object，用闭包创建对象，无需创建一个接口table，根据参数调用不同的方法，没有继承，但是有私有性

  function newObject(value)
  	return function (action, v)
  		if action == "get" then return value
  		elseif action == "set" then value = v
  		else error("invalid action")
  		end
  	end
  end
  
  d = newObject(0)
  print(d("get")) --> 0
  d("set", 10)
  print(d("get")) --> 10