Developing for FFX

FFX is divided into some areas of responsibility, with core functionality related to finding and running fuchsia-interacting programs being in the core program (the ffx cli) and the actual programs being built as 'subtools' that may either be compiled in with the ffx cli, or run as external tools (using the FHO subtool interface).

In order to add functionality to be accessible through ffx, you can write your own subtools and integrate them into the build.

Get started with subtools

To get started building a simple subtool for ffx, see developing an ffx subtool.

We also keep a reference implementation of a subtool that does some basic things called ffx-echo that should always act as a canonical source of how to write a subtool.

New subtool checklist

If this is the start of a larger project, there are some things you should try to do to make sure your new subtool will work over time:

  • Mark your new subtool as experimental to establish an expectation that it will be changing before stabilization.
  • Use a Writer for all output related to your subtool. If inclusion in the SDK is expected at some point, make sure that you use a MachineWriter with a concrete serialize-able type and output works correctly when --machine json is passed to ffx.
  • Properly delineate between user-actionable errors and bugs at the subtool boundary using the fho::Error type.
  • If the subtool lives in the ffx tree, it should have a clear OWNERS file describing both the team that owns it and individuals who can check CLs against that tool.
  • If it lives outside the ffx tree, but inside the fuchsia.git repository, it must have the ffx owners file referenced (ie. file:/src/developer/ffx/OWNERS in its OWNERS file) applying to any files at the api boundary between ffx and the subtool so that we can make pro-active changes to that api boundary over time.