Fuchsia Interface Definition Language

<<../../../_common/fidl/_fidl_intro.md>>

Creating a FIDL library

FIDL libraries group FIDL source files together. A library acts as a namespace for the protocols it contains, and FIDL source files can implicitly access all other declarations within the same library. FIDL source files must import any declarations from another library.

The Fuchsia SDK provides the fuchsia_fidl_library() build target to compile FIDL source files into a library. The name of the library target must match the library declarations in each source file. See the following BUILD.bazel example for the fuchsia.examples library:

# Import the fidl template.
load("fuchsia_fidl_library")

# Define a FIDL library target.
fuchsia_fidl_library(
    name = "fuchsia.examples",
    srcs = [
        "echo.fidl",
    ],
    library = ""fuchsia.examples"",
    visibility = ["//visibility:public"],
)

At build time, the FIDL Compiler (fidlc) frontend tool validates and compiles the library source files into a JSON Intermediate Representation (IR). This JSON IR format is the basis for the FIDL bindings.

Generating FIDL bindings

Components consume FIDL protocols through generated code called FIDL bindings. Bindings encode and decode requests and responses as FIDL messages and transfer them over the underlying IPC channel. The language-specific binding libraries provide wrappers around these structures to align interactions with familiar programming idioms.

The client interface (sometimes referred to as a proxy) performs translation between higher-level function calls and FIDL messages. On the server side, bindings process incoming request messages and deliver them through an abstract interface for components to implement.

![Diagram showing how FIDL bindings provide generated library code to translate function calls into FIDL messages for transport across process boundaries.] (/get-started/images/fidl/fidl-bindings.png){: width="574"}

Note: For more details on the bindings specification and supported programming languages, see the Bindings Reference.

At build time, the fidlgen backend tools generate bindings for supported programming languages from the JSON IR library produced by fidlc. The Fuchsia SDK provides build templates to generate bindings for each supported language. See the following BUILD.bazel example to generate HLCPP bindings for the fuchsia.examples library:

fuchsia_fidl_hlcpp_library(
    name = "fuchsia.examples.fidl_cc",
    library = ":fuchsia.examples",
    visibility = ["//visibility:public"],
    deps = [
        "@fuchsia_sdk//pkg/fidl_cpp",
    ],
)

Components that consume this library can use the bindings target as a dependency.

Exercise: Echo FIDL Library

In this section, you'll define a new FIDL library with a protocol called Echo containing a single method that returns string values back to the caller.

Start by creating a new directory for the FIDL library target:

mkdir -p fuchsia-codelab/echo-fidl

Add a new FIDL interface file called echo.fidl with the following contents:

echo-fidl/echo.fidl:

{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/routing/fidl/echo.fidl" region_tag="fidl_echo" adjust_indentation="auto" %}

EchoString is a two-way method that accepts an optional (nullable) string value and returns the same value.

Add a BUILD.bazel file with the following contents to declare the library target:

echo-fidl/BUILD.bazel:

{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/routing/fidl/BUILD.bazel" adjust_indentation="auto" %}

Run bazel build and verify that the build completes successfully:

bazel build --config=fuchsia_x64 //fuchsia-codelab/echo-fidl:fidl.examples.routing.echo.fidl_cc

Examine the FIDL bindings

The FIDL bindings target compiles the FIDL interface and generates language-specific bindings in the bazel-bin/ directory:

```none {:.devsite-disable-click-to-copy} bazel-bin/fuchsia-codelab/echo-fidl/_virtual_includes/

Locate and open the `fidl.h` generated header found in the above directory:

```posix-terminal
find bazel-bin/fuchsia-codelab/echo-fidl/_virtual_includes -name fidl.h

Explore the contents of these files. Below is a summary of some of the key generated interfaces:

Interface	Description
EchoPtr	Asynchronous client that transforms protocol methods into FIDL request messages sent over the IPC channel.
EchoSyncPtr	Synchronous client that transforms protocol methods into FIDL request messages sent over the IPC channel.
Echo	Abstract class for a server component to override and handle incoming FIDL requests.
EchoStringCallback	Callback to send a return value for each request as a FIDL response message.

---

最后更新: 2022 年 12 月 31 日（Saturday） 21:07 CST