(Swift 3,4,5...) Get Google Cloud Storage Access Token using Service Account JSON Private Key

Demonstrates how to get a Google Cloud Storage access token using a JSON service account private key. Obtaining an access token for a service account requires no user interaction (it does not require a browser, nor callback URI's, etc.).

When an access token expires, an application can be (and should be) designed to automatically fetch a new access token.

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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/ChilkatCloud-13a07a2e8b3f.json", charset: "utf-8")
    if fac.lastMethodSuccess != true {
        print("\(fac.lastErrorText!)")
        return
    }

    // A JSON private key should look something 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://www.googleapis.com/auth/cloud-platform"

    // 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!)")

    // We're going to save this access token to a file to be used
    // by other examples.
    let sbToken = CkoStringBuilder()!
    sbToken.append(gAuth.accessToken)
    sbToken.writeFile("qa_data/tokens/googleCloudStorageAccessToken.txt", charset: "utf-8", emitBom: false)

}