(Objective-C) 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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#import <CkoHttp.h>
#import <CkoHttpResponse.h>
#import <CkoJsonObject.h>
#import <NSString.h>
#import <CkoJsonArray.h>
#import <CkoHttpRequest.h>

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

CkoHttp *http = [[CkoHttp alloc] init];

// 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

CkoHttpResponse *resp = [http QuickRequest: @"GET" url: @"https://localhost:5000/.well-known/openid-configuration"];
if (http.LastMethodSuccess != YES) {
    NSLog(@"%@",http.LastErrorText);
    return;
}

if ([resp.StatusCode intValue] != 200) {
    NSLog(@"%@%d",@"Received response status code ",[resp.StatusCode intValue]);
    NSLog(@"%@",@"Response body containing error text or JSON:");
    NSLog(@"%@",resp.BodyStr);

    return;
}

CkoJsonObject *json = [[CkoJsonObject alloc] init];
BOOL success = [json Load: resp.BodyStr];

// 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.

NSString *tokenEndpoint = [json StringOf: @"token_endpoint"];

CkoJsonArray *grantTypes = [json ArrayOf: @"grant_types_supported"];
int clientCredentialsIdx = [[grantTypes FindString: @"client_credentials" caseSensitive: YES] intValue];

// If clientCredentialsIdx is less then zero (-1) then the "client_credentials" string was not found.
if (clientCredentialsIdx < 0) {
    NSLog(@"%@",@"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'

CkoHttpRequest *req = [[CkoHttpRequest alloc] init];
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 == NO) {
    NSLog(@"%@",http.LastErrorText);
    return;
}

// Make sure we got a 200 response status code, otherwise it's an error.
if ([resp.StatusCode intValue] != 200) {
    NSLog(@"%@",@"POST to token endpoint failed.");
    NSLog(@"%@%d",@"Received response status code ",[resp.StatusCode intValue]);
    NSLog(@"%@",@"Response body containing error text or JSON:");
    NSLog(@"%@",resp.BodyStr);

    return;
}

success = [json Load: resp.BodyStr];

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

// Get the access token:
NSString *accessToken = [json StringOf: @"access_token"];

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