(Xojo Plugin) Refresh Expiring OAuth2 Access Token for Azure Registered App

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// We previously obtained an access token and saved the JSON to a file using this example:
// Get OAuth2 Access Token for Azure Registered App

// This example will examine the JSON and expiration date, and if near expiration will
// refresh the access token.

Dim json As New Chilkat.JsonObject
Dim success As Boolean
success = json.LoadFile("qa_data/tokens/_myAzureApp.json")
If (success <> True) Then
    System.DebugLog("Failed to load the access token.")
    Return
End If

// The contents of the JSON look like this:
// {
//   "token_type": "Bearer",
//   "scope": "User.Read Mail.ReadWrite Mail.Send",
//   "expires_in": 3600,
//   "ext_expires_in": 0,
//   "access_token": "EwBAA8l6B...",
//   "refresh_token": "MCRMdbe6Cd...",
//   "id_token": "eyJ0eXAiOiJ...",
//   "expires_on": "1494112119"
// }

// The "expires_on" value is a Unix time.
Dim dtExpire As New Chilkat.CkDateTime
success = dtExpire.SetFromUnixTime(False,json.IntOf("expires_on"))

// If this date/time expires within 10 minutes of the current system time, refresh the token.
If (dtExpire.ExpiresWithin(10,"minutes") <> True) Then
    System.DebugLog("No need to refresh, the access token won't expire within the next 10 minutes.")
    Return
End If

// OK, we need to refresh the access token..
Dim oauth2 As New Chilkat.OAuth2

// Note: The endpoint depends on the Azure App Registration.
// See How to Choose the Correct Endpoints for your Azure App Registration
oauth2.TokenEndpoint = "https://login.microsoftonline.com/common/oauth2/v2.0/token"

// Use your client ID.
oauth2.ClientId = "CLIENT_ID"

// Get the existing refresh token.
oauth2.RefreshToken = json.StringOf("refresh_token")

// Send the HTTP POST to refresh the access token.
success = oauth2.RefreshAccessToken()
If (success = False) Then
    System.DebugLog(oauth2.LastErrorText)
    Return
End If

System.DebugLog("OAuth2 authorization granted!")
System.DebugLog("Access Token = " + oauth2.AccessToken)

// Get the full JSON response:
success = json.Load(oauth2.AccessTokenResponse)
json.EmitCompact = False

// If an "expires_on" member does not exist, then add the JSON member by
// getting the current system date/time and adding the "expires_in" seconds.
// This way we'll know when the token expires.
If (json.HasMember("expires_on") <> True) Then
    success = dtExpire.SetFromCurrentSystemTime()
    success = dtExpire.AddSeconds(json.IntOf("expires_in"))
    success = json.AppendString("expires_on",dtExpire.GetAsUnixTimeStr(False))
End If

System.DebugLog(json.Emit())

// Save the new access token JSON to a file for future requests.
Dim fac As New Chilkat.FileAccess
success = fac.WriteEntireTextFile("qa_data/tokens/_myAzureApp.json",json.Emit(),"utf-8",False)