# Implement a FIDL client

## 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:

```posix-terminal
rm -r examples/fidl/hlcpp/client/*
```

## Create the component

Create a new component project at `examples/fidl/hlcpp/client`:

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

   ```cpp
   int main(int argc, const char** argv) {
     printf("Hello, world!\n");
     return 0;
   }
   ```

1. Declare a target for the client in `examples/fidl/hlcpp/client/BUILD.gn`:

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

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

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

1. Add a component manifest in `examples/fidl/hlcpp/client/meta/client.cml`:

   Note: The binary name in the manifest must match the output name of the
   `executable` defined in the previous step.

   ```json5
   {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/meta/client.cml" region_tag="example_snippet" %}
   ```

1. Once you have created your component, ensure that you can add it to the
   build configuration:

   ```posix-terminal
   fx set core.x64 --with //examples/fidl/hlcpp/client:echo-client
   ```

1. Build the Fuchsia image:

   ```posix-terminal
   fx build
   ```

## Edit GN dependencies

1. Add the following dependencies:

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

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

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

   The reason for including these dependencies is explained in the
   [server tutorial][server-tut-deps].

## 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/hlcpp/client/main.cc" region_tag="main" highlight="2,28" %}
```

### Initialize a proxy class {#proxy}

In the context of FIDL, proxy designates the code
generated by the FIDL bindings that enables users to make
remote procedure calls to the server. In HLCPP, the proxy takes the form
of a class with methods corresponding to each FIDL protocol method.

The code then creates a proxy class for the `Echo` protocol, and connects it
to the server.

```cpp
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/main.cc" region_tag="main" highlight="4,5,6" %}
```

* [`fuchsia::examples::EchoPtr`][proxy] is an alias for
  `fidl::InterfaceRequest<fuchsia::examples::Echo>` generated by the bindings.
* Analogous to the `fidl::Binding<fuchsia::examples::Echo>` used in the server,
  `fidl::InterfaceRequest<fuchsia::examples::Echo>` is parameterized by a FIDL
  protocol and a channel it will proxy requests over the channel, and listen for
  incoming responses and events.
* The code calls `EchoPtr::NewRequest()`, which creates a channel,
  binds the class to one end of the channel, and returns the other end of the
  channel.
* The returned end of the channel is passed to `sys::ServiceDirectory::Connect()`.
  * Analogous to the call to `context->out()->AddPublicService()` on the server
    side, `Connect` has an implicit second parameter here, which is the protocol
    name (`"fuchsia.examples.Echo"`). This is where the input to the handler
    defined in the [previous tutorial][server-tut-handler] comes from: the
    client passes it in to `Connect`, which then passes it to the handler.

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.

### Set an error handler

Finally, the code sets an error handler for the proxy:

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

### Send requests to the server

The code makes two requests to the server:

* An `EchoString` request
* A `SendString` request

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

For `EchoString` the code passes in a callback to handle the response.
`SendString` does not require such a callback because the method does not
have any response.

### Set an event handler

The code then sets a handler for any incoming `OnString` events:

```cpp
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/main.cc" region_tag="main" highlight="21,22,23,24,25,26" %}
```

### Terminate the event loop

The code waits to receive both a response to the `EchoString` method as well as an
`OnString` event (which in the [current implementation][server-tut-impl] is sent after receiving a
`SendString` request) before quitting from the loop. The code returns a successful exit
code only if it receives both a response and an event:

```cpp
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/main.cc" region_tag="main" highlight="13,17,18,23,24,29" %}
```

## 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][glossary.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`](/examples/fidl/echo-realm)

1. Configure your build to include the provided package that includes the
   echo realm, server, and client:

    ```posix-terminal
    fx set core.x64 --with //examples/fidl/hlcpp:echo-hlcpp-client
    ```

1. Build the Fuchsia image:

   ```posix-terminal
   fx build
   ```

1. Run the `echo_realm` component. This creates the client and server component
   instances and routes the capabilities:

    ```posix-terminal
    ffx component run /core/ffx-laboratory:echo_realm fuchsia-pkg://fuchsia.com/echo-hlcpp-client#meta/echo_realm.cm
    ```

1. Start the `echo_client` instance:

    ```posix-terminal
    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`):

```none {:.devsite-disable-click-to-copy}
[echo_server][][I] Running echo server
[echo_client][][I] Got event hi
[echo_client][][I] Got response hello
```

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

```posix-terminal
ffx component destroy /core/ffx-laboratory:echo_realm
```

<!-- xrefs -->
[glossary.realm]: /docs/glossary/README.md#realm
[server-tut]: /docs/development/languages/fidl/tutorials/hlcpp/basics/server.md
[server-tut-component]: /docs/development/languages/fidl/tutorials/hlcpp/basics/server.md#component
[server-tut-impl]: /docs/development/languages/fidl/tutorials/hlcpp/basics/server.md#impl
[server-tut-deps]: /docs/development/languages/fidl/tutorials/hlcpp/basics/server.md#dependencies
[server-tut-handler]: /docs/development/languages/fidl/tutorials/hlcpp/basics/server.md#handler
[sync-client]: /docs/development/languages/fidl/tutorials/hlcpp/basics/sync_client.md
[proxy]: /docs/reference/fidl/bindings/hlcpp-bindings.md#protocols-client
[overview]: /docs/development/languages/fidl/tutorials/overview.md
[environment]: /docs/concepts/components/v2/environments.md