docs/development/languages/fidl/tutorials/llcpp/basics/client.md

# Implement an LLCPP FIDL client

<!-- TODO(fxbug.dev/58758) <<../../common/client/overview.md>> -->

## Prerequisites

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

## Overview

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

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

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

## Create a stub component

1. Set up a hello world component in `examples/fidl/llcpp/client`.
   You can name the component `echo-client`, and give the package a name of
   `echo-llcpp-client`.

   Note: If necessary, refer back to the [previous tutorial][server-tut].

1. Once you have created your component, ensure that the following works:

   ```
   fx set core.x64 --with //examples/fidl/llcpp/client
   ```

1. Build the Fuchsia image:

   ```
   fx build
   ```

1. In a separate terminal, run:

   ```
   fx serve
   ```

1. In a separate terminal, run:

   ```
   fx shell run fuchsia-pkg://fuchsia.com/echo-llcpp-client#meta/echo-client.cmx
   ```

## Edit GN dependencies

1. Add the following dependencies:

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

1. Then, include them in `main.cc`:

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

These dependencies are explained in the  [server tutorial][server-tut].

## Edit component manifest

1. Include the `Echo` protocol in the client component's sandbox by
   editing the component manifest in `client.cmx`.

   ```cmx
   {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/client.cmx" %}
   ```

## 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.

```cpp
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="2,3,17,27,29,30,42" %}
```

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.

```cpp
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="5,6,7,8,9,10" %}
```

In practice, this 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.
In parallel, the component manager will route the requested service name and channel to the
server component, where the [`connect` function][server-handler] 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](#run) at the end of the tutorial.

The `get_svc_directory()` function is defined as follows:

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

Similar to the `fdio_service_connect_at` call above, this function initializes a channel,
and passes the server end to `fdio_service_connect` to connect to the `/svc` directory, returning
the client end.

Note: This pattern of making a request to connect one 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][pipelining-tut].

### Initialize the client {#proxy}

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

```cpp
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="11,12,13,14,15,16,17,18,19,20,21" %}
```

The event handlers should be a struct with fields corresponding to the events offered by the
protocol (see [LLCPP event handlers][event-handlers]). In this case, a struct is defined with
a single field corresponding to the handler for the `OnString` event. The handler prints the
string and quits the event loop.

### Send requests to the server

The code makes three requests to the server:

* An asynchronous `EchoString` request.
* A synchronous `EchoString` request.
* A `SendString` request (async vs sync is not relevant for this case because it is a fire and
  forget method).

```cpp
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client/main.cc" region_tag="main" highlight="23,24,25,26,27,28,32,33,34,39,40,41" %}
```

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

The asynchronous method call requires the request parameters followed by a response handler
callback, which is called when the response is received.

The client object also allows synchronous calls, which will block until the response is received
and return the response object. These are suffixed with `_Sync` (e.g. `client->EchoString_Sync()`).

In the synchronous case, a [result object][resultof] is returned, since the method call can fail.
In the asynchronous case, the response handler callback takes as arguments the response parameters
directly, since the handler is only called in the case of a successfull method call.

## Run the client {#run}

If you run the client directly, it will not connect to the server correctly because the
client does not automatically get the `Echo` protocol provided in its
sandbox (in `/svc`). To get this to work, a launcher tool is provided
that launches the server, creates a new [`Environment`][environment] for
the client that provides the server's protocol, then launches the client in it.

1. Configure your GN build as follows:

    ```
    fx set core.x64 --with //examples/fidl/llcpp/server --with //examples/fidl/llcpp/client --with //examples/fidl/test:echo-launcher
    ```

2. Build the Fuchsia image:

   ```
   fx build
   ```

3. Run the launcher by passing it the client URL, the server URL, and
   the protocol that the server provides to the client:

    ```
    fx shell run fuchsia-pkg://fuchsia.com/echo-launcher#meta/launcher.cmx fuchsia-pkg://fuchsia.com/echo-llcpp-client#meta/echo-client.cmx fuchsia-pkg://fuchsia.com/echo-llcpp-server#meta/echo-server.cmx fuchsia.examples.Echo
    ```

You should see the print output in the QEMU console (or using `fx log`).

```
[166633.167] 757796:757798> Running echo server
[166633.489] 757796:757798> echo_server_llcpp: Incoming connection for fuchsia.examples.Echo
[166633.528] 758101:758103> Got synchronous response: hello
[166633.531] 758101:758103> Got response: hello
[166633.531] 758101:758103> Got event: hi"
```

<!-- xrefs -->
[bindings-ref]: /docs/reference/fidl/bindings/llcpp-bindings.md
[event-handlers]: /docs/reference/fidl/bindings/llcpp-bindings.md#event-handlers
[resultof]: /docs/reference/fidl/bindings/llcpp-bindings.md#resultof
[client-impl]: /docs/reference/fidl/bindings/llcpp-bindings.md#async-client
[server-handler]: /docs/development/languages/fidl/tutorials/llcpp/basics/server.md#server-handler
[server-tut]: /docs/development/languages/fidl/tutorials/llcpp/basics/server.md
[sync-client]: /docs/development/languages/fidl/tutorials/llcpp/basics/sync-client.md
[overview]: /docs/development/languages/fidl/tutorials/overview.md
[environment]: /docs/concepts/components/v2/environments.md
[pipelining-tut]: /docs/development/languages/fidl/tutorials/llcpp/topics/request-pipelining.md