[Corona SDK] How to support multiple Ads networks - AdMob, iAds, Vungle

InHow to add advertisement in my APP - using AdMob V2, we learn how to add AdMob Ads.
For iAds and Vungle, we can use the similar way to add them.
We just consider how to support multiple networks here.
That it, we want to select AdMob, iAds or Vungle dynamically, no matter it is for Fill Rate, eCPM or any other reasons.

Add plugin
First, we need add plug-in in build.settings:

settings =
{
 plugins =
    {
       ["plugin.google.play.services"] =
        {
            publisherId = "com.coronalabs"
        },
        ["CoronaProvider.ads.iads"] =
        {
            publisherId = "com.coronalabs",
            supportedPlatforms = { iphone=true, ["iphone-sim"]=true },
        },
        ["CoronaProvider.ads.vungle"] =
        {
            publisherId = "com.vungle",
        },      
    },
 android =
    {

        usesPermissions =
        {
            "android.permission.INTERNET",
            "android.permission.ACCESS_NETWORK_STATE",
        },
    },
}

Init the Ads

local adsProvider = {"adMob1","adMob2","vungle","iAds"} 
local ads = require( "ads" )
local adMobAdProvider = "admob"
local iAdsAdProvider = "iads"
local vungleAdProvider = "vungle"
local vungleAppId = "55e4xxxxxx"
local bannerAppID = "ca-app-pub-xxxxxx"
local interstitialAppID = "ca-app-pub-xxxxxx"
local appID = "com.gmail.MyCompany.App"
if ( isAndroid ) then
    bannerAppID = "ca-app-pub-xxxxx"
    interstitialAppID = "ca-app-xxxxxx"
    vungleAppId = "com.gmail.My_Company.App"
end
local lastAdsTypeShown = "none"
local currentAds = 1

local function adMobListener( event )
    local msg = event.response
    -- Quick debug message regarding the response from the library
    print( "Message from the adMob library: ", msg )
 if (event.type == "banner") then
     if ( event.isError ) then
         changeToNextAds()
     else
     end
 elseif(event.type == "interstitial") then
        if ( event.isError ) then
            changeToNextAds()
        elseif ( event.phase == "loaded" ) then
        elseif ( event.phase == "shown" ) then
        end
 end  
end

local function iAdsListener( event )
 local msg = event.response
 print("Message received from the iAds library: ", msg)
 if event.isError then
  changeToNextAds() 
 else
 end  
end

local function vungleListener( event )
   print("Message received from the vungleListener library: ", event.type)

   if ( event.type == "adStart" and event.isError ) then
     changeToNextAds()
   elseif ( event.type == "adEnd" ) then
      -- Ad was successfully shown and ended; hide the overlay so the app can resume.
   else
      print( "Received event", event.type )
   end
   return true
end

function adsInit()
 ads.init( adMobAdProvider, bannerAppID, adMobListener )
 if(isAndroid == false) then
   ads.init( iAdsAdProvider, appID, iAdsListener )    
 end
 ads.init( vungleAdProvider, vungleAppId, vungleListener )
end

We have to call ads.init() for AdMob,iAds and Vungle separately.
The parameters in their callback functions are different.
In above codes, we change to next Ads network when we get error.

Change Ads Network
We set new Ads network by function ads:setCurrentProvider().

local function changeToNextAds()
 currentAds = currentAds + 1
 if(currentAds > 3) then
  currentAds = 1
 end
 if(adsProvider[currentAds] == "iAds") then
  ads:setCurrentProvider(iAdsAdProvider)
 elseif(adsProvider[currentAds] == "vungle") then
  ads:setCurrentProvider(vungleAdProvider)
 else
  ads:setCurrentProvider(adMobAdProvider)
 end
end

Show Ads

function showAds(type,px,py)
 if(adsProvider[currentAds] == "adMob") then
  if(type == "banner") then
   ads.show( type, { x=px, y=py, appId=bannerAppID} )
   return true
  elseif(type == "interstitial") then    
   if(ads.isLoaded(type))then
    ads.show( type, { x=px, y=py, appId=interstitialAppID } )
    return true
   else
    return false
   end 
  end 
 elseif(adsProvider[currentAds] == "vungle") then
  if (type == "interstitial" and ads.isAdAvailable() ) then
      ads.show( "interstitial" )
      return true
  else
   return false
  end   
 elseif(adsProvider[currentAds] == "iAds") then
  ads.show( type, { x=px, y=py} )
  return true
 end
 return false  
end

function loadAds()
 if(adsProvider[currentAds] == "adMob") then
  ads.load( "interstitial", { appId=interstitialAppID} )
 end
end 

Vungle does not have Banner Ads and it will pre-load interstitial Ads itself.
iAds cannot pre-load interstitial Ads，
The codes shown above are very completed. 
You can just copy them directly and make it work.