Visitor
The Visitor pattern is a behavioral design pattern that enables you to separate an algorithm from the object structure it operates on. In other words, the visitor extends the functionality of the object by changing the initial implementation.
Monster class
Every monster instance can be visited by the visitor and print information about the monster. Also, the monster can accept as many visitors as needed. This is the simplest example of a usage visitor. Visitor is a good pattern if you, for some reason, do not want, or you cannot to add the printMonster method in the Monster class.
---@class Monster
---@field public name string
---@field public hp number
---@field public damange number
local Monster = {}
Monster.__index = Monster
---@return Monster
function Monster:new(name, hp, damage)
local t = {
name = name,
hp = hp,
damange = damage,
}
return setmetatable(t, self)
end
---@param visitor Visitor
function Monster:accept(visitor)
visitor:visit(self)
end
return Monster
PrintVisitor class
The PrintVisitor class only prints the information about the monster. The class has a method called visit, but this method can also be used as a single function.OOP principles are used only for consistency reasons.
---@class PrintVisitor
local PrintVisitor = {}
PrintVisitor.__index = PrintVisitor
---@return PrintVisitor
function PrintVisitor:visit(instance)
for k, v in pairs(instance) do
print(k, v)
end
end
return PrintVisitor
Usage
local Monster = require("Monster")
local PrintVisitor = require("PrintVisitor")
local goblin = Monster:new("Goblin", 100, 5)
local skelly = Monster:new("Skeleton", 80, 7)
local demon = Monster:new("Demon", 200, 15)
goblin:accept(PrintVisitor)
skelly:accept(PrintVisitor)
demon:accept(PrintVisitor)
--[[
hp 100
damange 5
name Goblin
hp 80
damange 7
name Skeleton
hp 200
damange 15
name Demon
--]]
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