Writes an .app file.

An .app file is a file containing Erlang terms that defines your application. Mix automatically generates this file based on your mix.exs configuration.

In order to generate the .app file, Mix expects your project to have both :app and :version keys. Furthermore, you can configure the generated application by defining an application/0 function in your mix.exs with the following options.

The most commonly used options are:

• :extra_applications - a list of Erlang/Elixir applications that you want started before your application. For example, Elixir’s :logger or Erlang’s :crypto. Mix guarantees that any application given here and all of your runtime dependencies are started before your application starts.

• :registered - the name of all registered processes in the application. If your application defines a local GenServer with name MyServer, it is recommended to add MyServer to this list. It is most useful in detecting conflicts between applications that register the same names.

• :env - default values for the application environment. The application environment is one of the most common ways to configure applications. See the Application module for mechanisms to read and write to the application environment.

For example:

def application do
  [extra_applications: [:logger, :crypto],
   env: [key: :value],
   registered: [MyServer]]
end

Other options include:

• :applications - all applications your application depends on at runtime. By default, this list is automatically inferred from your dependencies. Mix and other tools use the application list in order to start your dependencies before starting the application itself.

• :mod - specifies a module to invoke when the application is started. It must be in the format {Mod, args} where args is often an empty list. The module specified must implement the callbacks defined by the Application module.

• :start_phases - specifies a list of phases and their arguments to be called after the application is started. See the “Phases” section below.

• :included_applications - specifies a list of applications that will be included in the application. It is the responsibility of the primary application to start the supervision tree of all included applications, as only the primary application will be started. A process in an included application considers itself belonging to the primary application.

Besides the options above, .app files also expect other options like :modules and :vsn, but these are automatically added by Mix.

Command line options

• --force - forces compilation regardless of modification times

Phases

Applications provide a start phases mechanism which will be called, in order, for the application and all included applications. If a phase is not defined for an included application, that application is skipped.

Let’s see an example MyApp.application/0 function:

def application do
  [start_phases: [init: [], go: [], finish: []],
   included_applications: [:my_included_app]]
end

And an example :my_included_app defines on its mix.exs the function:

def application do
  [mod: {MyIncludedApp, []},
   start_phases: [go: []]]
end

In this example, the order that the application callbacks are called in is:

Application.start(MyApp)
MyApp.start(:normal, [])
MyApp.start_phase(:init, :normal, [])
MyApp.start_phase(:go, :normal, [])
MyIncludedApp.start_phase(:go, :normal, [])
MyApp.start_phase(:finish, :normal, [])