Basics tutorial
A basic tutorial introduction to gRPC in Objective-C.
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
. 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
and the
Objective-C generated code
guide
.
Why use gRPC?
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.
With gRPC we can define our service once in a
.proto
file and generate clients
and servers in any of gRPC’s supported languages, which in turn can be run in
environments ranging from servers inside a large data center to your own tablet ?
all the complexity of communication between different languages and environments is
handled for you by gRPC. We also get all the advantages of working with protocol
buffers, including efficient serialization, a simple IDL, and easy interface
updating.
Example code and setup
The example code for our tutorial is in
grpc/grpc/examples/objective-c/route_guide
.
To download the example, clone the
grpc
repository by running the following
commands:
$ git clone -b v1.64.0 --depth
1
--shallow-submodules https://github.com/grpc/grpc
$
cd
grpc
$ git submodule update --init
Then change your current directory to
examples/objective-c/route_guide
:
$
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
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
.
Try it out!
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:
$
pushd
../../cpp/route_guide
$ make
$ ./route_guide_server &
$
popd
Now have Cocoapods generate and install the client library for our .proto files:
(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
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
. You can
see the complete .proto file for our example in
examples/protos/route_guide.proto
.
To define a service, you specify a named
service
in your .proto file:
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.
// 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.
// 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.
// 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.
// 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:
// 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:
option
objc_class_prefix
=
"RTG"
;
Generating client code
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
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
):
which, before installing the generated library in the XCode project of this sample, runs:
$ 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
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
.
Note
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).
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:
#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
. 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.
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:
- (
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.
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.
- (
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];
}