[Corona SDK] How to add Apple Game Center and Android Google Play Game Service

For Apple Game Center or Android Google Play Game Service, 
we just need to utilize the function of gameNetwork.
It is not difficulty to implement them.
To consider Apple and Google at the same time, 
let's check the codes first and see how them work:

local gameNetwork = require( "gameNetwork" )
local playerName
local playerId

local leaderboardsListId=
{
 ["HighestScore"]=
 {
  ["Description"] = "Highest Score",
  ["Identifier"] = "com.gmail.mycom.demo.Leaderboards.HighestScore"
 }
}

local achievementsListId = 
{
 ["FinishLevel1"]=
 {
  ["Description"] = "Finish Level 1",
  ["Identifier"] = "com.gmail.mycom.demo.Achievements.FinishLevel1"
 },
 ["FinishLevel2"]=
 {
  ["Description"] = "Finish Level 2",
  ["Identifier"] = "com.gmail.mycom.demo.Achievements.FinishLevel2"
 }
}

if(isAndroid) then
 leaderboardsListId=
 {
  ["HighestScore"]=
  {
   ["Description"] = "Highest Score",
   ["Identifier"] = "Cgkxxxxxxxx"
  }
 }

 achievementsListId = 
 {
  ["FinishLevel1"]=
  {
   ["Description"] = "Finish Level 1",
   ["Identifier"] = "Cgkxxxxxxx"
  },
  ["FinishLevel2"]=
  {
   ["Description"] = "Finish Level 2",
   ["Identifier"] = "Cgkxxxxxxxx"
  }
 }
end



function showLeaderboards( event )
 print("showLeaderboards()")
 if (playerId ~= nil) then
    if ( isAndroid ) then
       gameNetwork.show( "leaderboards" )
    else
       gameNetwork.show( "leaderboards", { leaderboard = {timeScope="AllTime"} } )
    end
 else
  gameNetworkSetup()
 end
    return true
end

local function postScoreSubmit( event )
   --whatever code you need following a score submission...
   return true
end

local function updateLeaderBoards(type, myScore)
 if (playerId == nil) then
  return
 end

 gameNetwork.request( "setHighScore",
 {
    localPlayerScore = { category=leaderboardsListId[type]["Identifier"], value=tonumber(myScore) },
    listener = postScoreSubmit
 } )
 print("gameNetwork.request( setHighScore) finished")
end

function showAchievements( event )
 print("showAchievements()")
 if (playerId ~= nil) then
    gameNetwork.show( "achievements" )
 else
  gameNetworkSetup()
 end
    return
end

local function achievementRequestCallback( event )
   
   return true
end

local function updateAchievement(type)
 if (playerId == nil) then
  return
 end
 gameNetwork.request( "unlockAchievement",
 {
    achievement = { identifier=achievementsListId[type]["Identifier"], percentComplete=100, showsCompletionBanner=true },
    listener = achievementRequestCallback
 } )
end

local function loadScoresCallback( event ) 
 if(event.data == nil) then
  print("event.data is nil")
  return
 end
 for i=1,25 do
  if(event.data[i] == nil) then
   break
  end
  print("event.data[",i,"].playerID:",event.data[i].playerID)
  print("event.data[",i,"].category:",event.data[i].category)
  if(event.data[i].playerID == playerId) then
   print("matched playID...........")
   break
  end
 end
end

local function loadLocalPlayerCallback( event ) 
 if(event.data == nil) then
  print("event.data is nil")
  return
 end

 playerId = event.data.playerID
 playerName = event.data.alias
 print("loadLocalPlayerCallback(),playerName:",playerName,",playerId:",playerId)
 if(playerId == nil) then
  return
 end
 myGameSettings.gameCerterHasEverLoggedIn = true
 saveData()   

 gameNetwork.request( "loadScores",
            {
                leaderboard =
                {
                    category = leaderboardsListId["HighestScore"]["Identifier"], 
                    playerScope = "Global",   -- Global, FriendsOnly
                    timeScope = "AllTime",    -- AllTime, Week, Today
                    --range = {1,5},
                    playerCentered = true,
                },
                listener = loadScoresCallback
            }) 
end

local function loadAchievementCallback( event )    
 if(event.data == nil) then
  print("event.data is nil")
  return
 end

 for i=1,#event.data do
  if(event.data[i] ~= nil) then
   print("event.data[",i,"].identifier:",event.data[i].identifier)
   print("event.data[",i,"].title:",event.data[i].title)
   print("event.data[",i,"].isCompleted:",event.data[i].isCompleted)
  end
 end 
end

local function gameNetworkLoginCallback( event )
 if(isAndroid==false) then
  if(event.data) then
   print("User has logged into iOS Game Center")
  else
   print("User has NOT logged into iOS Game Center")
   return 
  end
 end
 gameNetwork.request( "loadLocalPlayer", { listener=loadLocalPlayerCallback } )
 gameNetwork.request( "loadAchievements", { listener=loadAchievementCallback } )
 return true
end

local function gpgsInitCallback( event )
 if(event.data) then
  print("User has logged into Google App Play Center")
     gameNetwork.request( "login", { userInitiated=true, listener=gameNetworkLoginCallback } )
 else
  print("User has NOT logged into Google App Play Center")
 end
end

function gameNetworkSetup()
 if ( isAndroid ) then
  myGameSettings.gameCerterHasEverLoggedIn = false
  gameNetwork.init( "google", gpgsInitCallback )
 else
  gameNetwork.init( "gamecenter", gameNetworkLoginCallback )
 end
end
}

Init
In general, we can call gameNetworkSetup() in main.lua，
For Android, we need to check if we have logged in successfully in gpgsInitCallback().
For iOS, we need to check if we have logged in successfully in gameNetworkLoginCallback().
The code gameNetwork.request( "loadLocalPlayer", { listener=loadLocalPlayerCallback } ) is to confirm the user who logs in.
You can go without it.
The code gameNetwork.request( "loadAchievements", { listener=loadAchievementCallback } ) is to recover the users's records.
For example, users may have ever installed this APP, remove this APP, and re-install it again.
If you think that you don't need to recover the records for them, you can go with it.

Show leaderboard
To show the leaderboard, we just need to call function showLeaderboards().
In general, we do it when user press some button

Show achievement
To show the leaderboard, we just need to call function showAchievements().
In general, we do it when user press some button.

Update leaderboard
To update leaderboard, we just need to call function updateLeaderBoards().
In general, wwe do it when user finish the game.

Update achievements 
To update leaderboard, we just need to call function updateAchievement().
In general, wwe do it when user finish some round.


Below are what differ between Apple Game Center and Android Google Play Game Service:

Apple Game Center

The data in leaderboardsListId{} and achievementsListId{}:
e.g. "com.gmail.mycom.demo.Leaderboards.HighestScore".
They are user defined, as long as they are  not duplicated.

Google Play Game Service

The data in leaderboardsListId{} and achievementsListId{}:
e.g. "Cgkxxxxxxxx".
They are defined by play store publish when you finished the setting in game center.
For Google Play Game Service， we also need to add the googlePlayGamesAppId in build.settings:

android =
    {
        usesPermissions =
        {
            "android.permission.INTERNET",
            "android.permission.ACCESS_NETWORK_STATE",
            "com.android.vending.BILLING",
        },
        googlePlayGamesAppId = "123456789012" 
    },

usesPermissions are not concerned for game service，
googlePlayGamesAppId is what you get when you link the APP in play store publish game center.