Elixir - Behaviour01 Nov 2017
- behaviour module
- callback module
- facade callback module
NOTE: the classification below is purely mine.
in all cases specific CM is usually fetched from environment at compile time
- in a single place (see FCM for details)
in an arbitrary application module on demand
# client module defmodule MyApp.Foo do @api Application.get_env(:my_app, :api) def foo, do: @api.fetch() end
behaviour module (BM)
defmodule MyApp.API do @callback fetch() :: any() end
BM combined with CM
BM can be embedded inside CM directly or as a nested module (in the latter case the boundary between the two is more explicit).
this pattern is useful when there are only 2 CMs (real and mock ones) and it’s tiresome to create a separate CM for real implementation each time (or you cannot come up with a proper name for this module).
embedding BM directly:
defmodule MyApp.API do @callback fetch() :: any() @behaviour __MODULE__ def fetch, do: 123 end
nesting BM inside CM:
defmodule MyApp.API do defmodule Behaviour do @callback fetch() :: any() end @behaviour Behaviour def fetch, do: 123 end
callback module (CM)
defmodule MyApp.API.HTTPClient do @behaviour MyApp.API def fetch, do: 123 end
CM combined with BM
same as vice versa.
facade CM (FCM)
unlike pure CM, FCM is not a fully functional CM since it implements callbacks by delegating to specific CM fetched from environment.
FCM is often combined with BM.
=> FCM acts as a facade or proxy forwarding calls to dynamically fetched adapter so that clients are not even aware that they are dealing with switchable adapter under the hood (since they don’t have to fetch it from environment by themselves like in all other cases) - FCM looks just like an ordinary module from outside:
# FCM defmodule MyApp.API do defmodule Behaviour do @callback fetch() :: any() end @behaviour Behaviour @api Application.get_env(:my_app, :api) defdelegate fetch, to: @api end # client module defmodule MyApp.Foo do def foo, do: MyApp.API.fetch() end
also FCM gives an opportunity to do some housekeeping (say, logging) before or after delegating to actual CM.
NOTE: both FCM and actual CM must implement