Creating a FIDL library

Prerequisites

This tutorial expects that you have completed the Getting Started guide and are able to build and run Fuchsia (using FEMU or a physical device). You should be familiar with running components on Fuchsia, which is covered in Run an example component. This tutorial is the first of the sequence of FIDL tutorials listed in the overview.

Overview

In this tutorial, we will define a FIDL library fuchsia.examples. Defining the FIDL library lets us compile the .fidl files using fx build and check for any errors. It also creates targets that can be included to depend on the bindings for the new library. The bindings tutorials, as well as the prerequisites for this tutorial are listed in the FIDL tutorials overview.

Define the FIDL library

The example code for this tutorial is in //examples/fidl/fuchsia.examples.

We've chosen the directory name to match the library name, which is the convention taken by the libraries defined in the Fuchsia IDK in //sdk/fidl such as //sdk/fidl/fuchsia.url or //sdk/fidl/fuchsia.ui.text.

FIDL file names use the .fidl extension. Analogous to C header files, FIDL files define data types and declare functional interfaces. These declarations are used in conjunction with FIDL-specific data types to communicate between FIDL endpoints.

The following are some examples of various FIDL language features as defined in //examples/fidl/fuchsia.examples/types.test.fidl:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="lib" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="consts" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="bits" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="enums" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="structs" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="unions" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="tables" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="protocols-preface" %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="protocols" %}

and also add a protocol to examples/fidl/fuchsia.examples/echo.test.fidl:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/echo.test.fidl" %}

The test.fidl extension is used instead of just .fidl, since exemptions to certain requirements like linting or API review are applied to test.fidl files. You should use .fidl when writing your own FIDL files.

Create a GN target for the FIDL library

Now we can define a target for our FIDL library that other code can depend on:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/BUILD.gn" %}

The fuzzers = [...] line is only needed to enable fuzzing of protocols from the library (see FIDL fuzzing), and can otherwise be omitted.

The bindings tutorials can use the bindings for our FIDL library by depending on targets generated by the GN fidl template.

Compile the FIDL library

You can build the .fidl files and check for syntax errors by including the target in your build config:

fx set core.x64 --with //examples/fidl/fuchsia.examples

Then run fx build examples/fidl/fuchsia.examples.

Note: If you are rewriting the FIDL files yourself, don't forget to add the Fuchsia copyright notice at the top of each file.