DynamicDispatch

Are you injecting dependencies on test time? Are you using Mox? Do you miss dialyzer when doing something like dependency().do_something()?

This libs solves this problem by automatically implementing a module that fetches the implementation in runtime but also implements all the methods of a behaviour without you doing anything.

defmodule MyApp.Repo do
  use DynamicDispatch,
    for: Ecto.Repo,
    to: Application.get_env(:my_app, :repo, MyApp.Repo.DefaultImpl)
end

defmodule MyApp.Repo.DefaultImpl do
  use Ecto.Repo, otp_app: :repo
end

If you don't set the config :my_app, :repo to anything, all Ecto.Repo methods will be delegated to MyApp.Repo.DefaultImpl, so it will work normally.

But if you want, in some test, to mock the Repo, you can do so by using Mox and this:

Mox.defmock(MyApp.Repo.MockedImpl, for: Ecto.Repo)
Application.put_env(:my_app, :repo, MyApp.Repo.MockedImpl)

Now, all calls defined by Ecto.Repo behaviour sent to MyApp.Repo will be dispatched to MyApp.Repo.MockedImpl and you can do your Mox.expect/Mox.stub as usual.

Installation

The package can be installed by adding dynamic_dispatch to your list of dependencies in mix.exs:

def deps do
  [
    {:dynamic_dispatch, "~> 0.1.0"}
  ]
end

Documentation can be found at https://hexdocs.pm/dynamic_dispatch.

Credits

This lib was inspired by this talk from Aaron Renner.

We were already doing some kind of dynamic dispatching but we were having some problems with dialyzer, this talk however, gave us some ideas on how to do Dynamic Dispatching while keeping dialyzer happy, but it required a lot of boilerplate so we came up with this idea.

So the main credits here are:

• Aaron Renner for the talk
• Paulo Valente for the macro idea