Implement a sync LLCPP FIDL client

Prerequisites

This tutorial builds on the FIDL server tutorial. For the full set of FIDL tutorials, refer to the overview.

Overview

This tutorial implements a client for a FIDL protocol and runs it against the server created in the previous tutorial. The client in this tutorial is synchronous. There is an alternate tutorial for asynchronous clients.

If you want to write the code yourself, delete the following directories:

rm -r examples/fidl/llcpp/client_sync/*

Create the component

Create a new component project at examples/fidl/llcpp/client_sync:

Add a main() function to examples/fidl/llcpp/client_sync/main.cc:

int main(int argc, const char** argv) {
  std::cout << "Hello, world!" << std::endl;
}

Declare a target for the client in examples/fidl/llcpp/client_sync/BUILD.gn:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/BUILD.gn" region_tag="imports" %}

# Declare an executable for the client.
executable("bin") {
  output_name = "fidl_echo_llcpp_client_sync"
  sources = [ "main.cc" ]
}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/BUILD.gn" region_tag="rest" %}

Add a component manifest in examples/fidl/llcpp/client_sync/meta/client.cml:
Note: The binary name in the manifest must match the output name of the executable defined in the previous step.
```
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/meta/client.cml" region_tag="example_snippet" %}
```
Once you have created your component, ensure that you can add it to the build configuration:
```
fx set core.qemu-x64 --with //examples/fidl/llcpp/client_sync:echo-client
```
Build the Fuchsia image:
```
fx build
```

Edit GN dependencies

Add the following dependencies:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/BUILD.gn" region_tag="deps" %}

Then, include them in main.cc:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="includes" %}

These dependencies are explained in the server tutorial. The client requires far fewer dependencies because it does not need to run any asynchronous code.

Connect to the server

The steps in this section explain how to add code to the main() function that connects the client to the server and makes requests to it.

Connect to the server

The client then connects to the service directory /svc, and uses it to connect to the server.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="main" highlight="2,3,4,5,6,8,9,10" %}

The service::OpenServiceRoot function initializes a channel, then passes the server end to fdio_service_connect to connect to the /svc directory, returning the client end wrapped in a zx::status result type. We should check for the is_ok() value on the result to determine if any synchronous error occurred.

Connecting to a protocol relative to the service directory is done by calling fdio_service_connect_at, passing it the service directory, the name of the service to connect to, as well as the channel that should get passed to the server. The service::ConnectAt function wraps the low level fdio call, providing the user with a typed client channel endpoint to the requested protocol.

In parallel, the component manager will route the requested service name and channel to the server component, where the connect function implemented in the server tutorial is called with these arguments, binding the channel to the server implementation.

An important point to note here is that this code assumes that /svc already contains an instance of the Echo protocol. This is not the case by default because of the sandboxing provided by the component framework. A workaround will be when running the example at the end of the tutorial.

Note: This pattern of making a request to connect the server end of the channel to a service, then immediately using the client end to communicate with the service is known as request pipelining. This topic is covered further in a separate tutorial.

Send requests to the server

The code makes two requests to the server:

An EchoString request
A SendString request

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="main" highlight="12,13,16,17,18,19,20,24,25,26,27" %}

The protocol methods on the client object (EchoString and SendString) return a result object, which will contain either an error or the contents of the response (if any). When a response is expected, the client will block until the response is received.

A client object is generated for each protocol, which is described further in the LLCPP bindings reference.

Handle events

The client object allows handling events by specifying an event delegate, where each method corresponds to one of the events of the protocol, plus a Unknown handler for when an unknown event is received.

The code defines a handler, which prints the contents of an OnString event, then calls client.HandleOneEvent() to block until an event is received. If a recognized event was received and successfully decoded, HandleOneEvent returns fidl::Status::Ok(). Otherwise, it returns an appropriate error:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/main.cc" region_tag="main" highlight="29,30,31,32,33,34,35,36,37,38,39,41,42,43,44" %}

Run the client

In order for the client and server to communicate using the Echo protocol, component framework must route the fuchsia.examples.Echo capability from the server to the client. For this tutorial, a realm component is provided to declare the appropriate capabilities and routes.

Note: You can explore the full source for the realm component at //examples/fidl/echo-realm

Configure your build to include the provided package that includes the echo realm, server, and client:
```
fx set core.qemu-x64 --with //examples/fidl/llcpp:echo-llcpp-client-sync
```
Build the Fuchsia image:
```
fx build
```
Run the echo_realm component. This creates the client and server component instances and routes the capabilities:
```
ffx component run fuchsia-pkg://fuchsia.com/echo-llcpp-client-sync#meta/echo_realm.cm
```

Start the echo_client instance:

ffx component start /core/ffx-laboratory:echo_realm/echo_client

The server component starts when the client attempts to connect to the Echo protocol. You should see output similar to the following in the device logs (ffx log):

[echo_server][][I] Running echo server
[echo_server][][I] Incoming connection for fuchsia.examples.Echo
[echo_client][][I] Got response: hello
[echo_client][][I] Got event: hi

Terminate the realm component to stop execution and clean up the component instances:

ffx component destroy /core/ffx-laboratory:echo_realm