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Edit oauth2-objective-c.md

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 gRPC Basics: Objective-C and the overview, 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. To download the example, clone this repository by running the following commands:

$ git clone -b v1.17.1 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.

Note: OAuth2 libraries need the application to register and obtain an ID from the identity provider (in the case of this example app, Google). The app’s XCode project is configured using that ID, so you shouldn’t copy this project “as is” for your own app: it would result in your app being identified in the consent screen as “gRPC-AuthSample”, and not having access to real Google services. Instead, configure your own XCode project following the instructions here.

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 an RPC object

The other basic tutorials show how to invoke an RPC by calling an asynchronous method in a generated client object. However, to make an authenticated call you need to initialize an object that represents the RPC, and configure it before starting the network request. First let’s look at how to create the RPC object.

Assume you have a proto service definition like this:

option objc_class_prefix = "AUTH";

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

A unaryCallWithRequest:handler: method, with which you’re already familiar, is generated for the AUTHTestService class:

[client unaryCallWithRequest:request handler:^(AUTHResponse *response, NSError *error) {
  ...
}];

In addition, an RPCToUnaryCallWithRequest:handler: method is generated, which returns a not-yet-started RPC object:

#import <ProtoRPC/ProtoRPC.h>

ProtoRPC *call =
    [client RPCToUnaryCallWithRequest:request handler:^(AUTHResponse *response, NSError *error) {
      ...
    }];

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

[call start];

Setting request metadata: Auth header with an access token

Now let’s look at how to configure some settings on the RPC object. The ProtoRPC class has a requestHeaders property (inherited from GRPCCall) defined like this:

@property(atomic, readonly) id<GRPCRequestHeaders> requestHeaders

You can think of the GRPCRequestHeaders protocol as equivalent to the NSMutableDictionary class. Setting elements of this dictionary of metadata keys and values means this metadata will be sent on the wire when the call is started. gRPC metadata are pieces of information about the call sent by the client to the server (and vice versa). They take the form of key-value pairs and are essentially opaque to gRPC itself.

For convenience, the property is initialized with an empty NSMutableDictionary, so that request metadata elements can be set like this:

call.requestHeaders[@"My-Header"] = @"Value for this header";
call.requestHeaders[@"Another-Header"] = @"Its value";

A typical use of metadata is for authentication details, as in our example. If you have an access token, OAuth2 specifies it is to be sent in this format:

call.requestHeaders[@"Authorization"] = [@"Bearer " stringByAppendingString:accessToken];

Getting response metadata: Auth challenge header

The ProtoRPC class also inherits a pair of properties, responseHeaders and responseTrailers, analogous to the request metadata we just looked at but sent back by the server to the client. They are defined like this:

@property(atomic, readonly) NSDictionary *responseHeaders;
@property(atomic, readonly) NSDictionary *responseTrailers;

In OAuth2, if there’s an authentication error the server will send back a challenge header. This is returned in the RPC’s response headers. To access this, as in our example’s error-handling code, you write:

call.responseHeaders[@"www-authenticate"]

Note that, as gRPC metadata elements are mapped to HTTP/2 headers (or trailers), the keys of the response metadata are always ASCII strings in lowercase.

Many uses cases of response metadata involve getting more details about an RPC error. For convenience, when a NSError instance is passed to an RPC handler block, the response headers and trailers dictionaries can also be accessed this way:

error.userInfo[kGRPCHeadersKey] == call.responseHeaders
error.userInfo[kGRPCTrailersKey] == call.responseTrailers