bsy / syringe

Dependency injection for Elixir

Syringe

Syringe is a injection framework that also opens the opportunity for clearer mocking and to run mocked test asynchronously.

To use the injector, it behaves similar to alias, except you use the word inject.

Example

defmodule MyThing do
  def do_mine_things do
    1 + 2
  end
end

defmodule MyModule do
  use Injector

  inject MyThing, as: Mine

  def do_things do
    Mine.do_mine_things
  end
end

Now that we are injecting our module we can mock it in test.

defmodule MyModuleTest do
  use ExUnit.Case, async: true
  import Mocker # need this to use easy mocking functions

  test "Mine must be called" do
    mock(MyThing)
    MyModule.do_things
    assert was_called(MyThing, :do_mine_things, nil) == once # success
  end
end

You can even take control and handle how the mocked functions can fit your test data

defmodule MyModuleTest do
  use ExUnit.Case, async: true
  import Mocker

  test "Mine must be called" do
    mock(MyThing)
    assert MyModule.do_things == nil
    intercept(MyThing, :do_mine_things, nil, with: fn() -> "my mocked return" end)
    assert MyModule.do_things == "my mocked return"
    assert was_called(MyThing, :do_mine_things, nil) == twice # success
  end
end

You can also just call the original function if you want.

defmodule MyModuleTest do
  use ExUnit.Case, async: true
  import Mocker 

  test "Mine must be called" do
    mock(MyThing)
    assert MyModule.do_things == nil
    intercept(MyThing, :do_mine_things, nil, with: :original_function)
    assert MyModule.do_things == 3
    assert was_called(MyThing, :do_mine_things, nil) == twice # success
  end
end

It gets better, you can control the order in which the functions return data.

defmodule MyModuleTest do
  use ExUnit.Case, async: true
  import Mocker 

  test "Mine must be called" do
    mock(MyThing)
    assert MyModule.do_things == nil
    intercept(MyThing, :do_mine_things, nil, with: fn -> "do the things" end)
    intercept(MyThing, :do_mine_things, nil, with: fn -> "do some other things" end)
    intercept(MyThing, :do_mine_things, nil, with: :original_function)
    intercept(MyThing, :do_mine_things, nil, with: fn -> "one more thing" end)
    assert MyModule.do_things == "do the things"
    assert MyModule.do_things == "do some other things"
    assert MyModule.do_things == 3
    assert MyModule.do_things == "one more thing"
    
    # The last specified intercept will persist
    assert MyModule.do_things == "one more thing"
    assert MyModule.do_things == "one more thing"
    assert was_called(MyThing, :do_mine_things, nil) == times(6) # success
  end
end

Finally you can match against function arguments.

defmodule MyThing do
  def do_mine_things(arg1, arg2, arg3) do
    {arg1, arg2, arg3}
  end
end

defmodule MyModuleTest do
  use ExUnit.Case, async: true
  import Mocker 

  test "Mine must be called with correct arguments" do
    mock(MyThing)
    assert MyModule.do_things(:a, :b, :c) == nil
    intercept(MyThing, :do_mine_things, [:b, :c, :d], with: fn(_, _, _) -> :ok)
    intercept(MyThing, :do_mine_things, [:a, :b, :c], with: :original_function)
    
    assert MyModule.do_things(:b, :c, :d) == :ok
    assert MyModule.do_things(:a, :b, :c) == {:a, :b, :c}
    
    MyModule.do_things(:x, :y, :z)

    assert was_called(MyThing, :do_mine_things, [:b, :c, :d) == once # success
    assert was_called(MyThing, :do_mine_things, [:a, :b, :c) == once # success
    assert was_called(MyThing, :do_mine_things, [:x, :y, :z) == never # success
  end
end

Sometimes you want match on any arguments

defmodule SampleModule do
  def do_some_work(how_much, call_me_when_i_am_done) do
    call_me_when_i_am_done.()
  end
end

defmodule ModuleImTesting do
  use Injector
  inject SampleModule

  def do_work(how_much) do
    SampleModule.do_some_work(how_much, &on_complete/0)
  end

  def on_complete do
    IO.inspect "I'm done!"
  end

end

defmodule ModuleImTestingTest do
  use ExUnit.Case, async: true
  import Mocker

  setup do
    mock(SampleModule)
    :ok
  end

  test "should notify SampleModule to do some work" do
    intercept(SampleModule, :do_some_work, [10, any], fn(_, on_complete) -> on_complete.() end) # the arguments are passed in and you can do what you want here
    ModuleImTesting.do_work(10)
    assert was_called(SampleModule, :do_some_work, [10, any]) == once # truthy
  end
end

Based on the nature of how tests execute, sometimes you need to be able to mock modules that are running in different processes. Generally used when interacting with GenServers referred by name, but can be used whenever things are being run in a different process than your test.

defmodule MyWork do
  def handle_work(state) do
    # I'm out of fake implementations. Does it matter
    # what this is at this point?
  end
end

defmodule MyServer do
  use GenServer
  use Injector

  inject MyWork

  def start_link do
    GenServer.start_link(__MODULE__, 0, name: __MODULE__)
  end

  def increment do
    GenServer.call(__MODULE__, :increment)
  end

  def handle_call(:increment, _from, state) do
    output = MyWork.handle_work(state)
    {:reply, output, state}
  end
end

defmodule MyServerTest do
  use ExUnit.Case, async: true
  use Mocker

  test "should outsource work to MyWork module in the GenServer process" do
    {:ok, pid} = MyServer.start_link

    # now that we're operating on a different pid we need to notify the
    # mocker to work within that pid
    mock(MyWork, pid)
    
    # now you can intercept the functions as before
    intercept(MyWork, :handle_work, [0], fn(_) -> 100 end)

    assert MyServer.increment() == 100
    assert was_called(MyWork, :handle_work, [0]) == once #truthy
  end
end

Installation

1. Add syringe to your list of dependencies in mix.exs:

```elixir
def deps do
  [{:syringe, "~> 1.0.0"}]
end
```

2. Configure syringe for your environments in test/config.exs (if you want to use mocking)

```elixir
config :syringe, injector_strategy: MockInjectingStrategy
```
in your other configs
```elixir
config :syringe, injector_strategy: AliasInjectingStrategy
```

3. Turn on mocking for your tests. In your test/test_helper.exs

```elixir
Mocker.start_link
```

4. You're ready to start injecting implementations