Adapter

May 01, 2025

The Adapter is a structural design pattern that enables collaboration between objects with incompatible interfaces. Consider we have a game where we save the state in Lua format, but after publishing the game to a kind of platform or gaming service. This service supports only JSON format. So our XML format is not compatible with JSON. Here adapter is the best solution.

Adapter Scheme

Game class

This is a mostly hard-coded demonstration of the class. Implementation of full XML support is out of scope, and not important for this example.

---@class Game
local Game = {}
Game.__index = Game

---@param state table
---@return Game
function Game:new(state)
    local t = {
        state = state,
    }
    return setmetatable(t, self)
end

---@return string
function Game:save()
    local t = {}
    t[#t + 1] = "<savegame>"
    t[#t + 1] = "\t<date>" .. os.date() .. "</date>"
    t[#t + 1] = "\t<level>" .. self.state.level .. "</level>"
    t[#t + 1] = "\t<score>" .. self.state.score .. "</score>"
    t[#t + 1] = "</savegame>"
    return table.concat(t, "\n")
end

return Game

Adapter class

This is the adapter class, which adapts the XML format to JSON. In this case adapter can be a function. I use here class for demonstration and constituency.

---@class SaveGameAdapter
local SaveGameAdapter = {}
SaveGameAdapter.__index = SaveGameAdapter

---@param game Game
---@return SaveGameAdapter
function SaveGameAdapter:new(game)
    local t = {
        game = game,
    }
    return setmetatable(t, self)
end

---@return string
function SaveGameAdapter:save()
    local t = {}
    t[#t + 1] = "{"
    t[#t + 1] = string.format('\t"date": "%s",', os.date())
    t[#t + 1] = string.format('\t"level": %d,', self.game.state.level)
    t[#t + 1] = string.format('\t"score": %d', self.game.state.score)
    t[#t + 1] = "}"
    return table.concat(t, "\n")
end

return SaveGameAdapter

Adapter class usage

local gameState = {
    level = 2,
    score = 32100,
}

local game = Game:new(gameState)
print(game:save())
--[[
<savegame>
        <date>Thu May  1 20:01:24 2025</date>
        <level>2</level>
        <score>32100</score>
</savegame>
--]]

local adapter = SaveGameAdapter:new(game)
print(adapter:save())
--[[
{                                                                               
        "date": "Thu May  1 20:01:24 2025",
        "level": 2,
        "score": 32100
}
--]]

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