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The ffx subtool interface has the concept of a Writer that manages IO between the tool and the user. This interface distinguishes between human and machine users so that we can use our tools in other tooling more effectively.

We currently implement two kinds of writers:

  • SimpleWriter: Errors if the program is run with --machine json and can only output strings.
  • MachineWriter: adapts it output to match whether the --machine json argument is passed, outputting only JSON on stdout if it is and outputting user-readable strings if not.

How to specify your Writer type

In the legacy plugin interface you could add a writer argument to your plugin definition using an #[ffx(writer = OutputType)] attribute on the writer argument, and that would cause some generation of schema information from the type you specified.

In this subtool interface, you specify this as an associated type on the FfxMain trait for your tool, and the machine type is a generic argument to the MachineWriter type. If your tool doesn't implement machine output, it should use SimpleWriter instead of something like MachineWriter<String> to avoid having people depend on your unstructured output.

Using the Writer

For the most part, you can use the writer as you would any implementation of std::io::Write, and it's fine to just use writeln!() or similar built-in macros against it for any unstructured string output.

All writers also implement some convenience functions for basic writing needs like print and line. These functions will all only produce output if the command wasn't run in machine mode. Otherwise they will be ignored.

All writers also implement item, which will either print the object given in machine mode, or use its Display implementation to output it in non-machine mode as text.

If you're using a machine writer, you also get a few more methods to help with outputting structured output:

  • machine: Only print the given object in machine mode.
  • machine_many: Print the given objects in machine mode.
  • machine_or: If in machine mode, print the object. Otherwise print the textual information in the other argument (implementing Display).
  • machine_or_else: If in machine mode, print the object. Otherwise print the textual information resulting from the function in the other argument.