(Swift 2) OAuth2 Token using IdentityServer4 with Client Credentials

Demonstrates how to get an OAuth2 access token using the client credential flow with IdentityServer4.

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

    let http = CkoHttp()

    // The first step is to fetch your IdentityServer4's discovery document
    // (OpenID Connect defines a discovery mechanism, called OpenID Connect Discovery, where an OpenID server publishes its metadata at a well-known URL, 
    // typically https://server.com/.well-known/openid-configuration

    var resp: CkoHttpResponse? = http.QuickRequest("GET", url: "https://localhost:5000/.well-known/openid-configuration")
    if http.LastMethodSuccess != true {
        print("\(http.LastErrorText)")
        return
    }

    if resp!.StatusCode.intValue != 200 {
        print("Received response status code \(resp!.StatusCode.intValue)")
        print("Response body containing error text or JSON:")
        print("\(resp!.BodyStr)")
        resp = nil
        return
    }

    let json = CkoJsonObject()
    var success: Bool = json.Load(resp!.BodyStr)
    resp = nil

    // We have the discovery document, which contains something like this:

    // You can use this online tool to generate parsing code from sample JSON: 
    // Generate Parsing Code from JSON

    // {
    //   "issuer": "https://localhost:5000",
    //   "jwks_uri": "https://localhost:5000/.well-known/openid-configuration/jwks",
    //   "authorization_endpoint": "https://localhost:5000/connect/authorize",
    //   "token_endpoint": "https://localhost:5000/connect/token",
    //   "userinfo_endpoint": "https://localhost:5000/connect/userinfo",
    //   "end_session_endpoint": "https://localhost:5000/connect/endsession",
    //   "check_session_iframe": "https://localhost:5000/connect/checksession",
    //   "revocation_endpoint": "https://localhost:5000/connect/revocation",
    //   "introspection_endpoint": "https://localhost:5000/connect/introspect",
    //   "frontchannel_logout_supported": true,
    //   "frontchannel_logout_session_supported": true,
    //   "backchannel_logout_supported": true,
    //   "backchannel_logout_session_supported": true,
    //   "scopes_supported": [
    //     "openid",
    //     "profile",
    //     "email",
    //     "MyCompany.profile",
    //     "MyCompany.Identity.WebApi",
    //     "MyCompany.TriHub.WebApi",
    //     "offline_access"
    //   ],
    //   "claims_supported": [
    //     "sub",
    //     "updated_at",
    //     "locale",
    //     "zoneinfo",
    //     "birthdate",
    //     "gender",
    //     "website",
    //     "profile",
    //     "preferred_username",
    //     "nickname",
    //     "middle_name",
    //     "given_name",
    //     "family_name",
    //     "name",
    //     "picture",
    //     "email_verified",
    //     "email",
    //     "userId",
    //     "groups",
    //     "fullname"
    //   ],
    //   "grant_types_supported": [
    //     "authorization_code",
    //     "client_credentials",
    //     "refresh_token",
    //     "implicit",
    //     "password"
    //   ],
    //   "response_types_supported": [
    //     "code",
    //     "token",
    //     "id_token",
    //     "id_token token",
    //     "code id_token",
    //     "code token",
    //     "code id_token token"
    //   ],
    //   "response_modes_supported": [
    //     "form_post",
    //     "query",
    //     "fragment"
    //   ],
    //   "token_endpoint_auth_methods_supported": [
    //     "client_secret_basic",
    //     "client_secret_post"
    //   ],
    //   "subject_types_supported": [
    //     "public"
    //   ],
    //   "id_token_signing_alg_values_supported": [
    //     "RS256"
    //   ],
    //   "code_challenge_methods_supported": [
    //     "plain",
    //     "S256"
    //   ]
    // }
    // 

    // The next steps are to (1) get the token_endpoint,
    // and (2) verify that the client_credentials grant type is supported.

    var tokenEndpoint: String? = json.StringOf("token_endpoint")

    var grantTypes: CkoJsonArray? = json.ArrayOf("grant_types_supported")
    var clientCredentialsIdx: Int = grantTypes!.FindString("client_credentials", caseSensitive: true).intValue
    grantTypes = nil

    // If clientCredentialsIdx is less then zero (-1) then the "client_credentials" string was not found.
    if clientCredentialsIdx < 0 {
        print("The client credentials grant type is not supported.")
        return
    }

    // Request the access token using our Client ID and Client Secret.
    // We're going to duplicate this CURL statement:

    // curl --request POST \
    //   --url '<tokenEndpoint>' \
    //   --header 'content-type: application/x-www-form-urlencoded' \
    //   --data 'grant_type=client_credentials&client_id=CLIENT_ID&client_secret=CLIENT_SECRET'

    let req = CkoHttpRequest()
    req.HttpVerb = "POST"
    req.AddParam("grant_type", value: "client_credentials")
    req.AddParam("client_id", value: "CLIENT_ID")
    req.AddParam("client_secret", value: "CLIENT_SECRET")

    resp = http.PostUrlEncoded(tokenEndpoint, req: req)
    if http.LastMethodSuccess == false {
        print("\(http.LastErrorText)")
        return
    }

    // Make sure we got a 200 response status code, otherwise it's an error.
    if resp!.StatusCode.intValue != 200 {
        print("POST to token endpoint failed.")
        print("Received response status code \(resp!.StatusCode.intValue)")
        print("Response body containing error text or JSON:")
        print("\(resp!.BodyStr)")
        resp = nil
        return
    }

    success = json.Load(resp!.BodyStr)
    resp = nil

    // Our JSON response should contain this:
    // {
    //   "access_token":"eyJz93a...k4laUWw",
    //   "token_type":"Bearer",
    //   "expires_in":86400
    // }

    // Get the access token:
    var accessToken: String? = json.StringOf("access_token")

    // The access token is what gets added to "Authorization: Bearer <access_token>"
    // for the subsequent REST API calls..

}