#OAuth2 on gRPC: Objective-C

This example application demostrates how to use OAuth2 on gRPC to make authenticated API calls on
behalf of a user. By walking through it you'll 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 the [Hello World](examples/objective-c/helloworld)
or [Route Guide](examples/objective-c/route_guide) tutorials,
and are familiar with OAuth2 concepts like _access token_.

- [Example code and setup](#setup)
- [Try it out!](#try)
- [Create an RPC object and start it later](#rpc-object)
- [Set request metadata of a call: Authorization header with an access token](#request-metadata)
- [Get response metadata of a call: Auth challenge header](#response-metadata)

<a name="setup"></a>
## Example code and setup

The example code for our tutorial is in [examples/objective-c/auth_sample](examples/objective-c/auth_sample).
To download the example, clone this repository by running the following command:
```shell
$ git clone https://github.com/grpc/grpc.git
```

Then change your current directory to `examples/objective-c/auth_sample`:
```shell
$ cd examples/objective-c/auth_sample
```

Our example is a simple application with two views. The first one lets a user sign in and out using
the OAuth2 flow of Google's [iOS SignIn library](https://developers.google.com/identity/sign-in/ios/).
(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](https://developers.google.com/identity/sign-in/ios/).

As with the other examples, you also should have [Cocoapods](https://cocoapods.org/#install)
installed, as well as the relevant tools to generate the client library code. You can obtain the
latter by following [these setup instructions](https://github.com/grpc/homebrew-grpc).


<a name="try"></a>
## Try it out!

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

```shell
$ 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.

<a name="rpc-object"></a>
## Create an RPC object and start it later

The other basic tutorials show how to invoke an RPC by calling an asynchronous method in a generated
client object. This shows how to initialize an object that represents the RPC, and configure it
before starting the network request.

Assume you have a proto service definition like this:

```protobuf
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:

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

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

```objective-c
#import <ProtoRPC/ProtoRPC.h>

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

The RPC represented by this object can be started at any later time like this:

```objective-c
[call start];
```

<a name="request-metadata"></a>
## Set request metadata of a call: Authorization header with an access token

The `ProtoRPC` class has a `requestMetadata` property (inherited from `GRPCCall`) defined like this:

```objective-c
- (NSMutableDictionary *)requestMetadata; // nonatomic
- (void)setRequestMetadata:(NSDictionary *)requestMetadata; // nonatomic, copy
```

Setting it to a dictionary of metadata keys and values will have them 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.

```objective-c
call.requestMetadata = @{@"My-Header": @"Value for this header",
                         @"Another-Header": @"Its value"};
```

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

```objective-c
call.requestMetadata[@"My-Header"] = @"Value for this header";
```

If you have an access token, OAuth2 specifies it is to be sent in this format:

```objective-c
call.requestMetadata[@"Authorization"] = [@"Bearer " stringByAppendingString:accessToken];
```

<a name="response-metadata"></a>
## Get response metadata of a call: Auth challenge header

The `ProtoRPC` class also inherits a `responseMetadata` property, analogous to the request metadata
we just looked at. It's defined like this:

```objective-c
@property(atomic, readonly) NSDictionary *responseMetadata;
```

To access OAuth2's authentication challenge header you write:

```objective-c
call.responseMetadata[@"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 are getting more details about an RPC error. For convenience,
when a `NSError` instance is passed to an RPC handler block, the response metadata dictionary can
also be accessed this way:

```objective-c
error.userInfo[kGRPCStatusMetadataKey]
```
