grpc.io/content/en/docs/languages/objective-c/basics.md

---
title: Basics tutorial
description: A basic tutorial introduction to gRPC in Objective-C.
weight: 50
---

This tutorial provides a basic Objective-C programmer's
introduction to working with gRPC.

By walking through this example you'll learn how to:

- Define a service in a .proto file.
- Generate client code using the protocol buffer compiler.
- Use the Objective-C gRPC API to write a simple client for your service.

It assumes a passing familiarity with [protocol
buffers](https://developers.google.com/protocol-buffers/docs/overview). Note
that the example in this tutorial uses the proto3 version of the protocol
buffers language: you can find out more in
the [proto3 language
guide](https://developers.google.com/protocol-buffers/docs/proto3) and the
[Objective-C generated code
guide](https://developers.google.com/protocol-buffers/docs/reference/objective-c-generated).

### Why use gRPC?

{{< why-grpc >}}

### Example code and setup {#setup}

The example code for our tutorial is in
[grpc/grpc/examples/objective-c/route_guide](https://github.com/grpc/grpc/tree/{{< param grpc_vers.core >}}/examples/objective-c/route_guide).
To download the example, clone the `grpc` repository by running the following
commands:

```sh
$ git clone -b {{< param grpc_vers.core >}} https://github.com/grpc/grpc
$ cd grpc
$ git submodule update --init
```

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

```sh
$ cd examples/objective-c/route_guide
```

Our example is a simple route mapping application that lets clients get
information about features on their route, create a summary of their route, and
exchange route information such as traffic updates with the server and other
clients.

You also should have [Cocoapods](https://cocoapods.org/#install) installed, as
well as the relevant tools to generate the client library code (and a server in
another language, for testing). You can obtain the latter by following [these
setup instructions](https://github.com/grpc/homebrew-grpc).

## Try it out! {#try}

To try the sample app, we need a gRPC server running locally. Let's compile and
run, for example, the C++ server in this repository:

```sh
$ pushd ../../cpp/route_guide
$ make
$ ./route_guide_server &
$ popd
```

Now have Cocoapods generate and install the client library for our .proto files:

```sh
$ 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. You can
check the calling code in `ViewControllers.m` and see the results in XCode's log
console.

The next sections guide you step-by-step through how this proto service is
defined, how to generate a client library from it, and how to create an app that
uses that library.

### Defining the service {#proto}

First let's look at how the service we're using is defined. A gRPC *service* and
its method *request* and *response* types using [protocol
buffers](https://developers.google.com/protocol-buffers/docs/overview). You can
see the complete .proto file for our example in
[`examples/protos/route_guide.proto`](https://github.com/grpc/grpc/blob/{{< param grpc_vers.core >}}/examples/protos/route_guide.proto).

To define a service, you specify a named `service` in your .proto file:

```protobuf
service RouteGuide {
   ...
}
```

Then you define `rpc` methods inside your service definition, specifying their
request and response types. Protocol buffers let you define four kinds of
service method, all of which are used in the `RouteGuide` service:

- A *simple RPC* where the client sends a request to the server and receives a
  response later, just like a normal remote procedure call.

  ```protobuf
  // Obtains the feature at a given position.
  rpc GetFeature(Point) returns (Feature) {}
  ```

- A *response-streaming RPC* where the client sends a request to the server and
  gets back a stream of response messages. You specify a response-streaming
  method by placing the `stream` keyword before the *response* type.

  ```protobuf
  // Obtains the Features available within the given Rectangle.  Results are
  // streamed rather than returned at once (e.g. in a response message with a
  // repeated field), as the rectangle may cover a large area and contain a
  // huge number of features.
  rpc ListFeatures(Rectangle) returns (stream Feature) {}
  ```

- A *request-streaming RPC* where the client sends a sequence of messages to the
  server. Once the client has finished writing the messages, it waits for the
  server to read them all and return its response. You specify a
  request-streaming method by placing the `stream` keyword before the *request*
  type.

  ```protobuf
  // Accepts a stream of Points on a route being traversed, returning a
  // RouteSummary when traversal is completed.
  rpc RecordRoute(stream Point) returns (RouteSummary) {}
  ```

- A *bidirectional streaming RPC* where both sides send a sequence of messages
  to the other. The two streams operate independently, so clients and servers
  can read and write in whatever order they like: for example, the server could
  wait to receive all the client messages before writing its responses, or it
  could alternately read a message then write a message, or some other
  combination of reads and writes. The order of messages in each stream is
  preserved. You specify this type of method by placing the `stream` keyword
  before both the request and the response.

  ```protobuf
  // Accepts a stream of RouteNotes sent while a route is being traversed,
  // while receiving other RouteNotes (e.g. from other users).
  rpc RouteChat(stream RouteNote) returns (stream RouteNote) {}
  ```

Our `.proto` file also contains protocol buffer message type definitions for all
the request and response types used in our service methods - for example, here's
the `Point` message type:

```protobuf
// Points are represented as latitude-longitude pairs in the E7 representation
// (degrees multiplied by 10**7 and rounded to the nearest integer).
// Latitudes should be in the range +/- 90 degrees and longitude should be in
// the range +/- 180 degrees (inclusive).
message Point {
  int32 latitude = 1;
  int32 longitude = 2;
}
```

You can specify a prefix to be used for your generated classes by adding the
`objc_class_prefix` option at the top of the file. For example:

```protobuf
option objc_class_prefix = "RTG";
```

### Generating client code {#protoc}

Next we need to generate the gRPC client interfaces from our .proto service
definition. We do this using the protocol buffer compiler (`protoc`) with a
special gRPC Objective-C plugin.

For simplicity, we've provided a [Podspec
file](https://github.com/grpc/grpc/blob/{{< param grpc_vers.core >}}/examples/objective-c/route_guide/RouteGuide.podspec)
that runs `protoc` for you with the appropriate plugin, input, and output, and
describes how to compile the generated files. You just need to run in this
directory (`examples/objective-c/route_guide`):

```sh
$ pod install
```

which, before installing the generated library in the XCode project of this sample, runs:

```sh
$ protoc -I ../../protos --objc_out=Pods/RouteGuide --objcgrpc_out=Pods/RouteGuide ../../protos/route_guide.proto
```

Running this command generates the following files under `Pods/RouteGuide/`:

- `RouteGuide.pbobjc.h`, the header which declares your generated message
  classes.
- `RouteGuide.pbobjc.m`, which contains the implementation of your message
  classes.
- `RouteGuide.pbrpc.h`, the header which declares your generated service
  classes.
- `RouteGuide.pbrpc.m`, which contains the implementation of your service
  classes.

These contain:

- All the protocol buffer code to populate, serialize, and retrieve our request
  and response message types.
- A class called `RTGRouteGuide` that lets clients call the methods defined in
  the `RouteGuide` service.

You can also use the provided Podspec file to generate client code from any
other proto service definition; just replace the name (matching the file name),
version, and other metadata.

### Creating the client application {#client}

In this section, we'll look at creating an Objective-C client for our
`RouteGuide` service. You can see our complete example client code in
[examples/objective-c/route_guide/ViewControllers.m](https://github.com/grpc/grpc/blob/{{< param grpc_vers.core >}}/examples/objective-c/route_guide/ViewControllers.m).

{{< alert title="Note" color="info" >}}
In your apps, for maintainability and readability reasons, you shouldn't
put all of your view controllers in a single file; it's done here only to
simplify the learning process).
{{< /alert >}}

#### Constructing a service object

To call service methods, we first need to create a service object, an instance
of the generated `RTGRouteGuide` class. The designated initializer of the class
expects a `NSString *` with the server address and port we want to connect to:

```objective-c
#import <GRPCClient/GRPCCall+Tests.h>
#import <RouteGuide/RouteGuide.pbrpc.h>
#import <GRPCClient/GRPCTransport.h>

static NSString * const kHostAddress = @"localhost:50051";
...
GRPCMutableCallOptions *options = [[GRPCMutableCallOptions alloc] init];
options.transport = GRPCDefaultTransportImplList.core_insecure;

RTGRouteGuide *service = [[RTGRouteGuide alloc] initWithHost:kHostAddress callOptions:options];
```

Notice that we our service is constructed with an insecure transport. This is
because the server we will be using to test our client doesn't use
[TLS](https://en.wikipedia.org/wiki/Transport_Layer_Security). This is fine
because it will be running locally on our development machine. The most common
case, though, is connecting with a gRPC server on the internet, running gRPC
over TLS. For that case, the setting the option `options.transport` isn't
needed because gRPC will use a secure TLS transport by default.


#### Calling service methods

Now let's look at how we call our service methods. As you will see, all these
methods are asynchronous, so you can call them from the main thread of your app
without worrying about freezing your UI or the OS killing your app.

##### Simple RPC

Calling the simple RPC `GetFeature` is as straightforward as calling any other
asynchronous method on Cocoa.

```objective-c

RTGPoint *point = [RTGPoint message];
point.latitude = 40E7;
point.longitude = -74E7;

GRPCUnaryResponseHandler *handler =
    [[GRPCUnaryResponseHandler alloc] initWithResponseHandler:
        ^(RTGFeature *response, NSError *error) {
          if (response) {
            // Successful response received
          } else {
            // RPC error
          }
        }
                                        responseDispatchQueue:nil];

[[service getFeatureWithMessage:point responseHandler:handler callOptions:nil] start];
```

As you can see, we create and populate a request protocol buffer object (in our
case `RTGPoint`). Then, we call the method on the service object, passing it the
request, and a block to handle the response (or any RPC error). If the RPC
finishes successfully, the handler block is called with a `nil` error argument,
and we can read the response information from the server from the response
argument. If, instead, some RPC error happens, the handler block is called with
a `nil` response argument, and we can read the details of the problem from the
error argument.

##### Streaming RPCs

Now let's look at our streaming methods. Here's where we call the
response-streaming method `ListFeatures`, which results in our client app
receiving a stream of geographical `RTGFeature`s:

```objective-c

- (void)didReceiveProtoMessage(GPBMessage *)message {
  if (message) {
    NSLog(@"Found feature at %@ called %@.", response.location, response.name);
  }
}

- (void)didCloseWithTrailingMetadata:(NSDictionary *)trailingMetadata error:(NSError *)error {
  if (error) {
    NSLog(@"RPC error: %@", error);
  }
}

- (void)execRequest {
  ...
  [[service listFeaturesWithMessage:rectangle responseHandler:self callOptions:nil] start];
}
```

Notice that instead of providing a response handler object, the view controller
object itself handles the responses. The method `didReceiveProtoMessage:` is
called when there's a message received; it can be called any number of times.
The method `didCloseWithTrailingMetadata:` is called when the call is complete
and the gRPC status is received from the server (or when there's any error
happens during the call).

The request-streaming method `RecordRoute` expects a stream of `RTGPoint`s from
the cient. This stream can be written to the gRPC call object after the call
starts.

```objective-c
RTGPoint *point1 = [RTGPoint message];
point.latitude = 40E7;
point.longitude = -74E7;

RTGPoint *point2 = [RTGPoint message];
point.latitude = 40E7;
point.longitude = -74E7;

GRPCUnaryResponseHandler *handler =
    [[GRPCUnaryResponseHandler alloc] initWithResponseHandler:
        ^(RTGRouteSummary *response, NSError *error) {
            if (response) {
              NSLog(@"Finished trip with %i points", response.pointCount);
              NSLog(@"Passed %i features", response.featureCount);
              NSLog(@"Travelled %i meters", response.distance);
              NSLog(@"It took %i seconds", response.elapsedTime);
            } else {
              NSLog(@"RPC error: %@", error);
            }
        }
                                        responseDispatchQueue:nil];
GRPCStreamingProtoCall *call =
    [service recordRouteWithResponseHandler:handler callOptions:nil];
[call start];
[call writeMessage:point1];
[call writeMessage:point2];
[call finish];
```

Note that since the gRPC call object does not know the end of the request
stream, users must invoke `finish:` method when the request stream is complete.

Finally, let's look at our bidirectional streaming RPC `RouteChat()`. The way to
call a bidirectional streaming RPC is just a combination of how to call
request-streaming RPCs and response-streaming RPCs.

```objective-c

- (void)didReceiveProtoMessage(GPBMessage *)message {
  RTGRouteNote *note = (RTGRouteNote *)message;
  if (note) {
    NSLog(@"Got message %@ at %@", note.message, note.location);
  }
}

- (void)didCloseWithTrailingMetadata:(NSDictionary *)trailingMetadata error:(NSError *)error {
  if (error) {
    NSLog(@"RPC error: %@", error);
  } else {
    NSLog(@"Chat ended.");
  }
}

- (void)execRequest {
  ...
  GRPCStreamingProtoCall *call =
      [service routeChatWithResponseHandler:self callOptions:nil];
  [call start];
  [call writeMessage:note1];
  ...
  [call writeMessage:noteN];
  [call finish];
}
```