(Swift 3,4,5...) OAuth2 for a GMail using a JSON Service Account Key

Demonstrates how get an OAuth2 access token for GMail (Google Service Account) using a JSON private key.

After the access token is obtained, it may be used to send email. When the access token expires, a new access token needs to be obtained (using this same procedure).

Note: This is for OAuth2 using Google Service account keys.

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func chilkatTest() {
    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // First load the JSON key into a string.
    let fac = CkoFileAccess()!
    var jsonKey: String? = fac.readEntireTextFile("qa_data/googleApi/ChilkatTest-ab2ecd52ef98.json", charset: "utf-8")
    if fac.lastMethodSuccess != true {
        print("\(fac.lastErrorText!)")
        return
    }

    // A JSON private key should look like this:

    // 	{
    // 	  "type": "service_account",
    // 	  "project_id": "chilkattest-1350",
    // 	  "private_key_id": "fa2e36ee26986eab628b59868af8bec1d1c64c38",
    // 	  "private_key": "-----BEGIN PRIVATE KEY-----\nMIIEvgIjFa...28N64N2n1E4FYzBZjSdy\n-----END PRIVATE KEY-----\n",
    // 	  "client_email": "598922945226-00rb0ppfg0sndajo6bhvd4v17jtj2d3a@developer.gserviceaccount.com",
    // 	  "client_id": "598922945226-00rb0ppfg0snd9jo7bhvd4v17jtj2d3a.apps.googleusercontent.com",
    // 	  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
    // 	  "token_uri": "https://accounts.google.com/o/oauth2/token",
    // 	  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
    // 	  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/598922945226-00rb0ppfg0sndajo6bhvd4v17jtj2d3a%40developer.gserviceaccount.com"
    // 	}

    let gAuth = CkoAuthGoogle()!
    gAuth.jsonKey = jsonKey

    // Choose a scope.
    gAuth.scope = "https://mail.google.com/"

    // Request an access token that is valid for this many seconds.
    gAuth.expireNumSeconds = 3600

    // If the application is requesting delegated access:
    // The email address of the user for which the application is requesting delegated access,
    // then set the email address here. (Otherwise leave it empty.)
    gAuth.subEmailAddress = ""

    // Connect to www.googleapis.com using TLS (TLS 1.2 is the default.)
    // The Chilkat socket object is used so that the connection can be established
    // through proxies or an SSH tunnel if desired.
    let tlsSock = CkoSocket()!
    var success: Bool = tlsSock.connect("www.googleapis.com", port: 443, ssl: true, maxWaitMs: 5000)
    if success != true {
        print("\(tlsSock.lastErrorText!)")
        return
    }

    // Send the request to obtain the access token.
    success = gAuth.obtainAccessToken(tlsSock)
    if success != true {
        print("\(gAuth.lastErrorText!)")
        return
    }

    // Examine the access token:
    print("Access Token: \(gAuth.accessToken!)")

    // Now that we have the access token, it may be used to send email
    // while it remains valid.  Once the access token expires, a new access token 
    // should be obtained using the same code as above.

    // -----------------------------------------------------------------------
    let mailman = CkoMailMan()!

    // Set the properties for the GMail SMTP server:
    mailman.smtpHost = "smtp.gmail.com"
    mailman.smtpPort = 587
    mailman.startTLS = true

    mailman.smtpUsername = "YOUR_GOOGLE_APPS_USER_EMAIL_ADDRESS"
    mailman.oAuth2AccessToken = gAuth.accessToken

    // Create a new email object
    let email = CkoEmail()!

    email.subject = "This is a test"
    email.body = "This is a test"
    email.from = "YOUR NAME <YOUR_GOOGLE_APPS_USER_EMAIL_ADDRESS>"
    success = email.add(to: "Chilkat Admin", emailAddress: "admin@chilkatsoft.com")
    // To add more recipients, call AddTo, AddCC, or AddBcc once per recipient.

    // Call SendEmail to connect to the SMTP server and send.
    // The connection (i.e. session) to the SMTP server remains
    // open so that subsequent SendEmail calls may use the
    // same connection.  
    success = mailman.send(email)
    if success != true {
        print("\(mailman.lastErrorText!)")
        return
    }

    // You may close the connection here.  If the connection is kept open, 
    // the next call to mailman.SendEmail will continue using the already-established connection
    // (and automatically re-connect if needed).
    success = mailman.closeSmtpConnection()
    if success != true {
        print("Connection to SMTP server not closed cleanly.")
    }

    print("Email Sent via GMail using an OAuth2 Service Account Key.")

}