Developing an ffx plugin

This page describes the basic steps for creating a plugin for ffx.

The plugin system employs a combination of GN build rules and Rust attributes to decouple plugin code from ffx internals.

GN Build Rule

Use the ffx_plugin() build rule template in your project's BUILD.gn file to create a build target for your plugin.

Your BUILD.gn file should look similar to the following example:

import("//src/developer/ffx/build/ffx_plugin.gni")

ffx_plugin("ffx_example") {
  version = "0.1.0"
  edition = "2021"
  with_unit_tests = true
  deps = []
  sources = [
    "src/args.rs",
    "src/lib.rs",
  ]
}

Inside the src/ directory, the project should contain two source files:

src/args.rs: Defines the CLI parameters for your plugin.
src/lib.rs: Contains the main plugin source code implementation.

Arguments

Create the file src/args.rs containing the plugins supported arguments:

use {argh::FromArgs, ffx_core::ffx_command};

#[ffx_command()]
#[derive(FromArgs, Debug, PartialEq)]
#[argh(subcommand, name = "example", description = "an example")]
pub struct ExampleCommand {}

This uses the argh crate and more documentation can be found here. This struct has been decorated by the ffx_command attribute that signifies that your plugin should run when someone types the following command:

fx ffx example

If you want to add more parameters for your plugins, you add them to this struct.

An example parameter would look like this:

use {argh::FromArgs, ffx_core::ffx_command};

#[ffx_command()]
#[derive(FromArgs, Debug, PartialEq)]
#[argh(subcommand, name = "example", description = "an example")]
pub struct ExampleCommand {
    #[argh(positional)]
    /// example optional positional string parameter
    pub example: Option<String>,
}

See more documentation: - Argh

Plugin

Create the file src/lib.rs containing the plugin implementation:

use {
    anyhow::Result,
    ffx_core::ffx_plugin,
    ffx_example_args::ExampleCommand,
};

#[ffx_plugin()]
pub async fn example(_cmd: ExampleCommand) -> Result<()> {
    println!("Hello from the example plugin :)");
    Ok(())
}

Plugin methods need to accept the argh command created in the src/args.rs file as a parameter even if they do not use them.

Integration

Add your plugin library as a dependency to ffx to include it in the build. Edit the plugin_deps array in the ffx build target to add your ffx_plugin() target to the top level:

  plugin_deps = [
    "//path/to/your/plugin/dir:ffx_example",
    ...
  ]

To build and test your plugin, build ffx:

fx build ffx

You should now see the output when you run your example command:

$ fx ffx example
Hello from the example plugin :)

Unit tests

If you want to unit test your plugin, just follow the standard method for testing rust code on a host. The ffx_plugin() GN template generates a <target_name>_lib_test library target for unit tests when the with_unit_tests parameter is set to true.

If your lib.rs contains tests, they can be invoked using fx test:

fx test ffx_example_lib_test

If fx test doesn't find your test, check that the product configuration includes your test. You can include all the ffx tests with this command:

fx set ... --with=//src/developer/ffx:tests

FIDL protocols

FFX plugins can communicate with a target device using FIDL protocols through Overnet. To access FIDL protocols from your plugin, follow the instructions in this section.

Add the FIDL Rust bindings as a dependency to the plugin's BUILD.gn file. The following example adds bindings for the fuchsia.device FIDL library:

import("//src/developer/ffx/build/ffx_plugin.gni")

ffx_plugin("ffx_example") {
  version = "0.1.0"
  edition = "2021"
  with_unit_tests = true
  deps = [
    "//sdk/fidl/fuchsia.device:fuchsia.device_rust",
  ]
  sources = [
    "src/args.rs",
    "src/lib.rs",
  ]
}

Import the necessary bindings int your plugin implementation. The following example imports NameProviderProxy from fuchsia.device:

use {
    anyhow::Result,
    ffx_core::ffx_plugin,
    ffx_example_args::ExampleCommand,
    fidl_fuchsia_device::NameProviderProxy,
};

Include the FIDL proxy can be used in the plugin implementation. Plugins can accept proxies in the parameters list:
```
pub async fn example(
    name_proxy: NameProviderProxy,
    _cmd: ExampleCommand,
) -> Result<()> { }
```
Map the proxy type to a component selector representing the component providing the FIDL protocol in the ffx_plugin() annotation:
```
#[ffx_plugin(
    NameProviderProxy = "core/appmgr:out:fuchsia.device.NameProvider"
)]
```

The example plugin implementation in src/lib.rs should now look like the following:

use {
    anyhow::Result,
    ffx_core::ffx_plugin,
    ffx_example_args::ExampleCommand,
    fidl_fuchsia_device::NameProviderProxy,
};

#[ffx_plugin(
    NameProviderProxy = "core/appmgr:out:fuchsia.device.NameProvider"
)]
pub async fn example(
    name_proxy: NameProviderProxy,
    _cmd: ExampleCommand,
) -> Result<()> {
    if let Ok(name) = name_proxy.get_device_name().await? {
        println!("Hello, {}", name);
    }
    Ok(())
}

Repeat these steps to include additional FIDL proxies to your ffx plugin.

The following FIDL proxies are built into ffx, and do not require additional dependencies or mappings:

You can simply add the above proxies to your plugin's parameter list to access them in your implementation.

Proxy selector maps

ffx and the Remote Control Service (RCS) provide a mechanism for maintaining compatibility with existing selectors used by ffx plugins if the selector representing a given FIDL proxy changes. For example:

The FIDL proxy is provided by a new component
The FIDL protocol name changes
The proxy selector varies across product builds

RCS supports this using selector maps that override the selectors defined in an ffx plugin's source and map it to a different value. To override a given selector, add an entry to //src/developer/remote-control/data/selector-maps.json in the following format:

{
  ...
  "original/moniker:out:fuchsia.MyService": "some/new/moniker:expose:fuchsia.MyOtherService"
}

This example enables RCS to override references to original/moniker:out:fuchsia.MyService in ffx plugins and route them to some/new/moniker:expose:fuchsia.MyOtherService in any build which contains the mapping.