Lua实现继承

Lua实现继承

我们知道在Lua脚本语言中是没有对象这个概念的。但是Lua为我们提供了一堆的元方法，可以实现类的继承

需求背景

• 实现类似c++中的类
• 实现类对类的继承
• 实现类的多继承

什么是继承

继承是面向对象最显著的一个特性，继承是从已有的类中派生出新的类，新的类能吸收已有类的数据属性和行为，并能扩展新的能力。

实现类似C++中的类

• 我们先不考虑继承的问题，首先来实现一个类的定义。

  • 实现

    local class = function (className)
    
        if type(className) ~= "string" then
            error("class name must string type")
        end
        local cls = {__className = className}
        cls.ctor = function ( ... ) 
            -- default constructor
        end
        cls.new = function ( ... )
            local instance = {}
            setmetatable(instance, {__index = cls})
            instance.__class = cls
            instance.ctor(...)
            return instance
        end
        cls.create = function ( ... )
            return cls.new(...) 
        end
        return cls
    end
    

  • 测试

        local ClassA = class("classA")
    
        function ClassA:ctor( ... )
        
            print("ClassA:ctor params is ", ...)
            self.name = ...
        end
        
        function ClassA:sayHello()
        
            print(string.format("my name is %s", self.name))
        end
        
        local classA_1 = ClassA:create("ClassA_1")
        classA_1:sayHello()
        print("=============================")
        local classA_2 = ClassA:create("ClassA_2")
        classA_2:sayHello()
        print("=============================")
        classA_1.name = "XXX"
        classA_1:sayHello()
        classA_2:sayHello()
    

  • 测试结果

        ClassA:ctor params is   ClassA_1
        my name is ClassA_1
        =============================
        ClassA:ctor params is   ClassA_2
        my name is ClassA_2
        =============================
        my name is XXX
        my name is ClassA_2
        [Finished in 0.0s]
    

• 错误演示

  • 可能会有同学好奇，在测试代码中为什有如下代码

        classA_1.name = "XXX"
        classA_1:sayHello()
        classA_2:sayHello()
    

  这里是为了检测是否做到了由一个类，创建出来的两个对象是否做到了数据的隔离

  • 将class函数中的new函数做出如下修改，就会发现对象与对象之间数据混乱

        cls.new = function ( ... )
            cls.ctor(...)
            return cls
        end
    

  • 使用上面测试代码测试结果为

        ClassA:ctor params is   ClassA_1
        my name is ClassA_1
        =============================
        ClassA:ctor params is   ClassA_2
        my name is ClassA_2
        =============================
        my name is XXX
        my name is XXX
        [Finished in 0.0s]
    

  注意：一定要做好数据隔离，不然创建出来的对象数据就会混乱。这里只为类创建添加了构造函数，没有添加析构函数，可以仿造上面添加一个dtor析构函数

实现类对类的继承

在上面的取得的小成果上，继续扩展和完成class函数，从而实现类对类的继承。其实在我们只需要借鉴类的定义，将super作为cls的__index元表即可。

• 给cls添加一个super属性，并将super设置为cls的__index元表，对上面class函数做出如下扩展

      local cls = {__className = className, __isClass = true}
      if type(super) == "table" then
          if not cls.__super then
              cls.__super = super
          end
      end
  
      cls.__index = cls
      if cls.__super then
          setmetatable(cls, {__index = cls.__super})
      end
  

• 测试代码

      local BaseClass = class("BaseClass")
  
      function BaseClass:ctor( ... )
      
          print("BaseClass ctor")
      end
      
      function BaseClass:sayName(name)
      
          print(string.format("My name is %s", name))
      end
      
      
      local ChildClass = class("ChildClass", BaseClass)
      
      function ChildClass:ctor( ... )
      
          self.__super:create()
          print("ChildClass ctor")
          self:init()
      end
      
      function ChildClass:init( ... )
      
          self.name = "Hello"
      end
      
      function ChildClass:askName( ... )
      
          self:sayName(self.name)
          print("What's your name")
      end
      
      local childClass_1 = ChildClass:create()
      childClass_1:askName()
      print("childClass_1's name is ", childClass_1.name)
      print("=========================================")
      
      local childClass_2 = ChildClass:create()
      childClass_2:askName()
      childClass_1.name = "World"
      print("childClass_2's name is ", childClass_2.name)
      print("childClass_1's name is ", childClass_1.name)
      print("=========================================")
      
      local GrandsonClass = class("GrandsonClass", ChildClass)
      
      function GrandsonClass:ctor( ... )
      
          self.__super:create()
          print("GrandsonClass ctor")
      end
      
      local grandsonClass = GrandsonClass:create()
      grandsonClass.name = "grandsonClass"
      grandsonClass:askName()
  

• 测试结果

  BaseClass ctor
  ChildClass ctor
  My name is Hello
  What's your name
  childClass_1's name is  Hello
  =========================================
  BaseClass ctor
  ChildClass ctor
  My name is Hello
  What's your name
  childClass_2's name is  Hello
  childClass_1's name is  World
  =========================================
  BaseClass ctor
  ChildClass ctor
  GrandsonClass ctor
  My name is grandsonClass
  What's your name
  [Finished in 0.0s]
  

注意：此处实现的是显示调用父类的构造函数，因此在子类的ctor函数中需要先初始化父类self.__super:create()

实现类的多继承

有了上面的基础，实现多继承也是轻而易举的事情了。我们只需要将__index设置成一个函数，然后遍历父类去用key去检索。当然这样多继承，也会存在一个问题，就是不同父类中有同名方法，或者同名属性，这个时候就和继承顺序与关系。在这里我们按照class("ClassName", SuperA, SuperB, ...)中书写先后顺序作为继承顺序，第一个设置为类的__super

• 多继承实现

  class = function (className, ...)
  
      if type(className) ~= "string" then
          error("class name must string type")
      end
      local cls = {__className = className, __isClass = true}
      local supers = {...}
      for _, super in ipairs(supers) do
          local superType = type(super)
          assert(superType == nil or superType == "table" or superType == "function", string.format("create class %s with invalid super class type %s", className, type(super)))
          if superType == "function" then
              assert(cls.__create ~= nil, string.format("create class %s with more than one create", className))
              cls.__create = super
          elseif type(super) == "table" then
              cls.__supers = cls.__supers or {}
              cls.__supers[#cls.__supers + 1] = super
              if not cls.__super then
                  cls.__super = super
              end
          else
              error(string.format("create class %s with invalid super class type %s", className, type(super)))
          end
      end
      
  
      cls.__index = cls
      if not cls.__supers or #cls.__supers == 1 then
          setmetatable(cls, {__index = cls.__super})      
      else
          setmetatable(cls, {__index = function (_, key)
              local supers = cls.__supers
              for _, super in ipairs(supers) do
                  if super[key] then
                      return super[key]
                  end
              end
          end})
      end
      if not cls.ctor then -- default constructor
          cls.ctor = function ( ... )
  
          end
      end
      cls.new = function ( ... )
          local instance = {}
          setmetatable(instance, {__index = cls})
          instance.__class = cls
          instance.ctor(...)
          return instance
      end
      cls.create = function ( ... )
          return cls.new(...) 
      end
      return cls
  end
  

• 测试

      local BaseClass = class("BaseClass")
      
      function BaseClass:ctor( ... )
      
          print("BaseClass ctor")
      end
      
      function BaseClass:sayName(name)
      
          print(string.format("My name is %s", name))
      end
      
      
      local SuperClass = class("SuperClass")
      
      function SuperClass:ctor( ... )
      
          print("SuperClass ctor")
      end
      
      function SuperClass:askName( ... )
      
          print(string.format("What's your name"))
      end
      
      
      local MakeFrinedClass = class("MakeFrinedClass", BaseClass, SuperClass)
      
      function MakeFrinedClass:ctor( ... )
      
          self.__super:create()
          print("MakeFrinedClass ctor")
      end
      
      function MakeFrinedClass:makeFriend( ... )
          
          self:sayName("MakeFrinedClass")
          self:askName()
      end
      
      local makeFrinedClass = MakeFrinedClass:create()
      makeFrinedClass:makeFriend()
  

• 测试结果

      BaseClass ctor
      MakeFrinedClass ctor
      My name is MakeFrinedClass
      What's your name
      [Finished in 0.0s]
  

注意：这里的__super和创建类的时候父类的书写书序有关系，父类如果有同名函数或属性，具体条用那个属性或者函数也和父类书写书序有关系

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