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 a stub component
Set up a hello world component in
examples/fidl/llcpp/client_sync
. You can name the componentecho-client
, and give the package a name ofecho-llcpp-client-sync
.Note: If necessary, refer back to the previous tutorial.
Once you have created your component, ensure that the following works:
fx set core.x64 --with //examples/fidl/llcpp/client_sync
Build the Fuchsia image:
fx build
In a separate terminal, run:
fx serve
In a separate terminal, run:
fx shell run fuchsia-pkg://fuchsia.com/echo-llcpp-client-sync#meta/echo-client.cmx
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.
Edit component manifest
Include the
Echo
protocol in the client component’s sandbox by editing the component manifest inclient.cmx
.{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/llcpp/client_sync/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.
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 an
unknown event is received, its return value becomes the return value of the
HandleOneEvent
call:
{%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
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
for
the client that provides the server’s protocol, then launches the client in it.
Configure your GN build:
fx set core.x64 --with //examples/fidl/llcpp/server --with
//examples/fidl/client/client_sync --with //examples/fidl/test:echo-launcher
Build the Fuchsia image:
fx build
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-sync#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
).
[189209.659] 859216:859218> Running echo server
[189209.778] 859216:859218> echo_server_llcpp: Incoming connection for fuchsia.examples.Echo
[189209.803] 859554:859556> Got response: hello
[189209.804] 859554:859556> Got event: hi