(Swift 3,4,5...) Get OAuth 2.0 Access Token using G_SvcOauthAccessToken2

Obtains an OAUTH2 access token from the Google OAuth 2.0 Authorization Server using the G_SvcOauthAccessToken2 method. This is for Server to server applications using Google API's that need an access token. See https://developers.google.com/accounts/docs/OAuth2ServiceAccount

Chilkat Downloads for the Swift Programming Language MAC OS X (Cocoa) Objective-C/Swift Libs iOS Objective-C/Swift Libs

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

    let http = CkoHttp()!

    // When a service account (Client ID) is created at https://code.google.com/apis/console/
    // Google will generate a P12 key.  This is a PKCS12 (PFX) file that you will download
    // and save.  The password to access the contents of this file is "notasecret".
    // NOTE: The Chilkat Pfx API provides the ability to load a PFX/P12 and re-save
    // with a different password.

    // Begin by loading the downloaded .p12 into a Chilkat certificate object:
    let cert = CkoCert()!
    var success: Bool = cert.loadPfxFile("/myDir/API Project-1c43a291e2a1-notasecret.p12", password: "notasecret")
    if success != true {
        print("\(cert.lastErrorText!)")
        return
    }

    // The next (and final) step is to request the access token.  Chilkat internally
    // does all the work of forming the JWT header and JWT claim set, encoding and
    // signing the JWT, and sending the access token request.
    // The application need only provide the inputs: The iss, scope(s), sub, and the
    // desired duration with a max of 3600 seconds (1 hour).
    // 
    // Each of these inputs is defined as follows 
    // (see https://developers.google.com/accounts/docs/OAuth2ServiceAccount
    // iss: The email address of the service account.
    // scope: A space-delimited list of the permissions that the application requests.
    // sub: The email address of the user for which the application is requesting delegated access.
    // numSec: The number of seconds for which the access token will be valid (max 3600).

    let claimParams = CkoHashtable()!

    // The required claim parameters that must be provided by the application are
    // the "iss" and "scope" params:
    success = claimParams.addStr("iss", value: "761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com")
    success = claimParams.addStr("scope", value: "https://mail.google.com/")

    // The "sub" param is required if there is an email address, such as for a
    // Google Apps domain—if you use Google Apps for Work, where the administrator of the Google Apps domain 
    // can authorize an application to access user data on behalf of users in the Google Apps domain.
    // 
    // Comment out this line if there is no such email address.
    // The typical case is that there is no such email address.
    success = claimParams.addStr("sub", value: "user@your-domain.com")

    // The "aud" parameter is optional.  If not set, it defaults to the value shown here:
    success = claimParams.addStr("aud", value: "https://accounts.google.com/o/oauth2/token")

    var numSec: Int = 3600

    var accessToken: String? = http.g_SvcOauthAccessToken2(claimParams, numSec: numSec, cert: cert)
    if http.lastMethodSuccess != true {
        print("\(http.lastErrorText!)")
    }
    else {
        print("access token: \(accessToken!)")
    }


}