Multi-process collaboration in Mox

Mox enforces process-based verification, meaning only the process that defines expectations is allowed to call the mock by default. This can prove problematic when you are executing code that runs in a different process for example inside a Task:

def process_users(user_ids) do
    Task.async(fn ->
      result = MyApp.Accounts.process_users_details(user_ids)
    end)
end

Here process_users/1 executes on a different process. When defining tests:

      expect(MockAccounts, :process_users_details, fn _user_ids ->
        {:ok, user_details}
      end)

       assert {:ok, user_details} == MyApp.Accounts.process_users(user_ids)

This will not work because we are using expectations defined in a different process than the one calling the actual function. To fix this we need to tell Mox to use global mode so that mocks can be consumed by any process (not necessarily the one that authored them). To do this however the ExUnit case must be set to async: false.

use MyApp.DataCase, async: false

then we can immediately set the mode depending on the context ; when async: true, Mox will use :set_mox_private but we want to set global mode so:

use MyApp.DataCase, async: false
setup :set_mox_from_context