Mimic

A sane way of using mocks in Elixir. It borrows a lot from both Meck & Mox! Thanks @eproxus & @josevalim

Installation

Just add mimic to your list of dependencies in mix.exs:

def deps do
  [
    {:mimic, "~> 0.2", only: :test}
  ]
end

If :applications key is defined inside your mix.exs or you run mix test --no-start, you probably want to add Application.ensure_all_started(:mimic) in your test_helper.exs

Using

Modules need to be prepared so that they can be used.

You must first call copy in your test_helper.exs for each module that may have the behaviour changed.

Mimic.copy(Calculator)

ExUnit.start()

Calling copy will not change the behaviour of the module.

The user must call stub/1, stub/3, expect/4 or reject/1 so that the functions can behave differently.

Then for the actual tests one could use it like this:

use ExUnit.Case, async: true

import Mimic

# Make sure mocks are verified when the test exits
setup :verify_on_exit!
test "invokes add once and mult twice" do
  Calculator
  |> stub(:add, fn x, y -> :stub end)
  |> expect(:add, fn x, y -> x + y end)
  |> expect(:mult, 2, fn x, y -> x * y end)

  assert Calculator.add(2, 3) == 5
  assert Calculator.mult(2, 3) == 6

  assert Calculator.add(2, 3) == :stub
end

Stub, Expect and Reject

Stub

stub/1 will change every module function to throw an exception if called.

stub(Calculator)

** (Mimic.UnexpectedCallError) Stub! Unexpected call to Calculator.add(3, 7) from #PID<0.187.0>
     code: assert Calculator.add(3, 7) == 10

stub/3 changes a specific function to behave differently. If the function is not called no verification error will happen.

Expect

expect/4 changes a specific function and it works like a queue of operations. It has precedence over stubs and if not called a verification error will be thrown.

If the same function is called with expect/4 the order will be respected:

Calculator
|> stub(:add, fn _x, _y -> :stub end)
|> expect(:add, fn _, _ -> :expected_1 end)
|> expect(:add, fn _, _ -> :expected_2 end)

assert Calculator.add(1, 1) == :expected_1
assert Calculator.add(1, 1) == :expected_2
assert Calculator.add(1, 1) == :stub

expect/4 has an optional parameter which is the amount of calls expected:

Calculator
|> expect(:add, 2, fn x, y -> {:add, x, y} end)

assert Calculator.add(1, 3) == {:add, 1, 3}
assert Calculator.add(4, 5) == {:add, 4, 5}

Reject

One may want to reject calls to a specific function. reject/1 can be used to achieved this behaviour.

reject(&Calculator.add/2)
assert_raise Mimic.UnexpectedCallError, fn -> Calculator.add(4, 2) end

Private and Global mode

The default mode is private which means that only the process and explicitly allowed process will see the different behaviour.

Calling allow/2 will permit a different pid to call the stubs and expects from the original process.

Global mode can be used with set_mimic_global like this:

setup :set_mimic_global

test "invokes add and mult" do
  Calculator
  |> expect(:add, fn x, y -> x + y end)
  |> expect(:mult, fn x, y -> x * y end)

  Task.async(fn ->
    assert Calculator.add(2, 3) == 5
    assert Calculator.mult(2, 3) == 6
  end)
  |> Task.await
end

This means that all processes will get the same behaviour defined with expect & stub. This option is simpler but tests running concurrently will have undefined behaviour. It is important to run with async: false. One could use :set_mimic_from_context instead of using :set_mimic_global or :set_mimic_private. It will be private if async: true, global otherwise.

Implementation Details & Performance

After calling Mimic.copy(MyModule), calls to functions belonging to this module will first go through an ETS table to check which pid sees what (stubs, expects or call original).

It is really fast but it won't be as fast as calling a no-op function. Here's a very simple benchmark:

defmodule Enumerator do
 def to_list(x, y), do: Enum.to_list(x..y)
end

Benchmarking Enumerator.to_list(1, 100) :

Name               ips        average  deviation         median         99th %
mimic         116.00 K        8.62 μs   ±729.13%           5 μs          29 μs
original       19.55 K       51.15 μs   ±302.46%          34 μs         264 μs

Comparison:
mimic         116.00 K
original       19.55 K - 5.93x slower

Benchmarking Enumerator.to_list(1, 250) :

Name               ips        average  deviation         median         99th %
original      131.49 K        7.61 μs   ±167.90%           7 μs          16 μs
mimic         105.47 K        9.48 μs   ±145.21%           9 μs          27 μs

Comparison:
original      131.49 K
mimic         105.47 K - 1.25x slower

There's a small fixed price to pay when mimic is used but it is unnoticeable for tests purposes.

Acknowledgements

Thanks to @jamesotron and @alissonsales for all the help! 🎉

tteerawat/mimic