OAuth2

This example demonstrates how to use OAuth2 on gRPC to make authenticated API calls on behalf of a user.

By walking through it you’ll also learn how to use the Objective-C gRPC API to:

• Initialize and configure a remote call object before the RPC is started.
• Set request metadata elements on a call, which are semantically equivalent to HTTP request headers.
• Read response metadata from a call, which is equivalent to HTTP response headers and trailers.

It assumes you know the basics on how to make gRPC API calls using the Objective-C client library, as shown in Basics tutorial and the Introduction to gRPC, and are familiar with OAuth2 concepts like access token.

Example code and setup

The example code for our tutorial is in [gprc/examples/objective-c/auth_sample](https://github.com/grpc/grpc/tree/ v1.35.0/examples/objective-c/auth_sample). To download the example, clone this repository by running the following commands:

$ git clone -b v1.35.0 https://github.com/grpc/grpc
$ cd grpc
$ git submodule update --init

Then change your current directory to examples/objective-c/auth_sample:

$ cd examples/objective-c/auth_sample

Our example is a simple application with two views. The first view lets a user sign in and out using the OAuth2 flow of Google’s iOS SignIn library. (Google’s library is used in this example because the test gRPC service we are going to call expects Google account credentials, but neither gRPC nor the Objective-C client library is tied to any specific OAuth2 provider). The second view makes a gRPC request to the test server, using the access token obtained by the first view.

As with the other Objective-C examples, you also should have CocoaPods installed, as well as the relevant tools to generate the client library code. You can obtain the latter by following these setup instructions.

Try it out!

To try the sample app, first have CocoaPods generate and install the client library for our .proto files:

$ pod install

(This might have to compile OpenSSL, which takes around 15 minutes if CocoaPods doesn’t have it yet on your computer’s cache).

Finally, open the XCode workspace created by CocoaPods, and run the app.

The first view, SelectUserViewController.h/m, asks you to sign in with your Google account, and to give the “gRPC-AuthSample” app the following permissions:

• View your email address.
• View your basic profile info.
• “Test scope for access to the Zoo service”.

This last permission, corresponding to the scope https://www.googleapis.com/auth/xapi.zoo doesn’t grant any real capability: it’s only used for testing. You can log out at any time.

The second view, MakeRPCViewController.h/m, makes a gRPC request to a test server at https://grpc-test.sandbox.google.com, sending the access token along with the request. The test service simply validates the token and writes in its response which user it belongs to, and which scopes it gives access to. (The client application already knows those two values; it’s a way to verify that everything went as expected).

The next sections guide you step-by-step through how the gRPC call in MakeRPCViewController is performed. You can see the complete code in MakeRPCViewController.m.

Create a call with access token

To make an authenticated call, first you need to initialize a GRPCCallOptions object and configure it with the access token.

GRPCMutableCallOptions *options = [[GRPCMutableCallOptions alloc] init];
options.oauth2AccessToken = myAccessToken;

Then you need to create and start your call with this call options object. Assume you have a proto service definition like this:

option objc_class_prefix = "AUTH";

service TestService {
  rpc UnaryCall(Request) returns (Response);
}

A unaryCallWithMessage:responseHandler:callOptions: method, with which you’re already familiar, is generated for the AUTHTestService class:

- (GRPCUnaryProtoRPC *)unaryCallWithMessage:(AUTHRequest *)message
                            responseHandler:(id<GRPCProtoResponseHandler>)responseHandler
                                callOptions:(GRPCCallOptions *)callOptions;

Use this method to generated the RPC object with your request options object:

GRPCUnaryProtoRPC *rpc = [client unaryCallWithMessage:myRequestMessage
                                      responseHandler:myResponseHandler
                                          callOptions:options];

You can then start the RPC represented by this object at any later time like this:

[rpc start];

An alternative way to provide access token

Rather than setting oauth2AccessToken option in GRPCCallOptions before the RPC object is created, an alternative approach allows users providing access token at call start time.

To use this approach, first create a class in your project that conforms to GRPCAuthorizationProtocol protocol.

@interface TokenProvider : NSObject<GRPCAuthorizationProtocol>
...
@end

@implementation TokenProvider

- (void)getTokenWithHandler:(void (^)(NSString* token))handler {
  ...
}

@end

When creating an RPC object, pass an instance of this class to call option authTokenProvider:

GRPCMutableCallOptions *options = [[GRPCMutableCallOptions alloc] init];
options.authTokenProvider = [[TokenProvider alloc] init];
GRPCUnaryProtoCall *rpc = [client unaryCallWithMessage:myRequestMessage
                                       responseHandler:myResponseHandler
                                           callOptions:options] start];
[rpc start];

When the call starts, it will call the TokenProvider instance’s getTokenWithHandler: method with a callback handler and waits for the callback. The TokenProvider instance may call the handler at any time to provide the token for this call and resume the call process.