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Implement an 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 asynchronous. There is an alternate tutorial for synchronous clients.

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

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

Create the component

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

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

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

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

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

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

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

  3. Add a component manifest in examples/fidl/llcpp/client/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/meta/client.cml" region_tag="example_snippet" %}

  4. 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:echo-client

  5. Build the Fuchsia image:

    fx build

Edit GN dependencies

  1. Add the following dependencies:

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

  2. Then, include them in main.cc:

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

These dependencies are explained in the server tutorial.

Connect to the server {#main}

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.

Initialize the event loop

As in the server, the code first sets up an async loop so that the client can listen for incoming responses from the server without blocking.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="2,3,20,25,34,36,52,53,67" %}

The dispatcher is used to run two pieces of async code. It is first used to run the EchoString method, and quits when the response is received. It is then run after calling the SendString in order to listen for events, and quits when an OnString event is received. The call to ResetQuit() in between these two instances allows the client to reuse the loop.

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/main.cc" region_tag="main" highlight="5,6,7,8,9,11,12,13,14,15" %}

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.

Initialize the client {#proxy}

In order to make Echo requests to the server, initialize a client using the client end of the channel from the previous step, the loop dispatcher, as well as an event handler delegate:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="17,18,22,28,36,38,39" %}

The event handler delegate should be an object that implements the fidl::AsyncEventHandler<Echo> virtual interface, which has methods corresponding to the events offered by the protocol (see LLCPP event handlers). In this case, a local class is defined with a method corresponding to the OnString event. The handler prints the string and quits the event loop. The class also overrides the on_fidl_error method, which is called when the client encounters an error and is going to teardown.

Send requests to the server

The code makes four requests to the server:

  • An asynchronous EchoString call taking a result callback.
  • An asynchronous EchoString call taking a response callback.
  • A synchronous EchoString call.
  • A one way SendString request (async vs sync is not relevant for this case because it is a fire and forget method).

The client object works by overriding the dereference operator to return a protocol specific client implementation, allowing calls such as client->EchoString().

Asynchronous call with result callback

The asynchronous method call requires the request parameters followed by a result callback, which is called either when the method succeeds or an error happens.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="41,42,43,44,45,46,47,48,49,50,51" %}

Synchronous call

The client object also allows synchronous calls, which will block until the response is received and return the response object. These may be selected using the .sync() accessor. (e.g. client.sync()->EchoString()).

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="55,56,57" %}

In the synchronous case, a result object is returned, since the method call can fail. In the asynchronous or fire-and-forget case, a lightweight status object is returned, which communicates any synchronous errors.

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

  1. 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

  2. Build the Fuchsia image:

    fx build

  3. 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#meta/echo_realm.cm

  4. 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 (result callback): hello
[echo_client][][I] Got response (response callback): hello
[echo_client][][I] Got synchronous 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