docs/development/languages/fidl/tutorials/llcpp/basics/client.md - fuchsia - Git at Google

 # 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
	# 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