@title Arcanist User Guide: Customizing Lint, Unit Tests and Workflows
@group userguide

Explains how to build new classes to control how Arcanist behaves.

= Overview =

Arcanist has some basic configuration options available in the ##.arcconfig##
file (see @{article:Arcanist User Guide: Configuring a New Project}), but it
can't handle everything. If you want to customize Arcanist at a deeper level,
you need to build new classes. For instance:

  - if you want to configure linters, or add new linters, you need to create a
    new class which extends @{class@arcanist:ArcanistLintEngine}.
  - if you want to integrate with a unit testing framework, you need to create a
    new class which extends @{class@arcanist:ArcanistBaseUnitTestEngine}.
  - if you you want to change how workflows behave, or add new workflows, you
    need to create a new class which extends
    @{class@arcanist:ArcanistConfiguration}.

Arcanist works through a sort of dependency-injection approach. For example,
Arcanist does not run lint rules by default, but you can set **lint_engine**
in your ##.arcconfig## to the name of a class which extends
@{class@arcanist:ArcanistLintEngine}. When running from inside your project,
Arcanist will load this class and call methods on it in order to run lint. To
make this work, you need to do three things:

  - actually write the class;
  - add the library where the class exists to your ##.arcconfig##;
  - add the class name to your ##.arcconfig## as the **lint_engine**,
    **unit_engine**, or **arcanist_configuration**.

= Write the Class =

Start by creating a new phutil library -- this is a directory which will contain
class files for Arcanist to load. You can either put it inside your project, or
outside (if you want to share lint rules between several projects, for
instance). To create a new library, run ##arc liberate##:

  $ arc liberate path/to/new/library

This will prompt you to name your library and create a new directory on disk.
Create a new ##lint/## directory in this library (or ##unit/##, or whatever you
want). This creates a module. Put ##CustomArcanistLintEngine.php## inside the
##lint/## directory:

  lang=php
  <?php

  class CustomArcanistLintEngine extends ArcanistLintEngine {

  }

Now run ##arc liberate## on the library again. Whenever you add, remove, or
rename modules or classes you should run ##arc liberate## to update the
classmap.

You can either write the class body now (refer to the documentation for the
class you are extending) or continue with integrating it into your project.

= Load the Class =

To make the class loadable, you need to put the path to it in your
##.arcconfig##, under **phutil_libraries**:

  {
    // ...
    "phutil_libraries" : {
      // ...
      "library-a" : "/path/to/my/library", // Absolute path
      "library-b" : "support/arcanist",    // Relative path in this project
      // ...
    }
    // ...
  }

You can either specify an absolute path, or a path relative to the project root.
When you run ##arc list --trace##, you should see a message to the effect that
it has loaded your library.

For debugging or testing, you can also run Arcanist with the
##--load-phutil-library## flag:

  arc --load-phutil-library=/path/to/library <command>

You can specify this flag more than once to load several libraries. Note that
if you use this flag, Arcanist will ignore any libraries listed in
##.arcconfig##.

= Use the Class =

This step is easy: just edit ##.arcconfig## to specify your class name as
the appropriate configuration value.

  {
    // ...
    "lint_engine" : "CustomArcanistLintEngine",
    // ...
  }

Now, when you run Arcanist in your project, it will invoke your class when
appropriate.

For lint and unit tests, you can also use the ##--engine## flag override the
default engine:

  arc lint --engine MyCustomArcanistLintEngine

This is mostly useful for debugging and testing.