rewire
===
[](https://travis-ci.org/stephanos/rewire)
[](https://hex.pm/packages/rewire)
Keep your application code free from dependency injection and mocking concerns by using `rewire` in your unit tests to inject module dependencies.
If you use `mox`, which is strongly recommended, this will **reduce your entire mocking setup to 2 lines of code**: 1 to define the mock and 1 to rewire.
## Usage
```elixir
# this module has a hard-wired dependency on the `English` module
defmodule Conversation do
def start(), do: English.greet()
end
```
If you define the following `mox` mock:
```elixir
# defining the mock in test_helper.exs
Mox.defmock(EnglishMock, for: English)
```
You can rewire the dependency in your unit test:
```elixir
defmodule MyTest do
use ExUnit.Case
use Rewire
import Mox
# rewire dependency on `English` to `EnglishMock`
rewire Conversation, English: EnglishMock
test "start/0" do
stub(EnglishMock, :greet, fn -> "g'day" end)
assert Conversation.start() == "g'day" # using the mock!
end
end
```
You can also give the alias a different name using `as`:
```elixir
rewire Conversation, English: EnglishMock, as: SmallTalk
```
Alternatively, you can also rewire a module on a test-by-test basis:
```elixir
defmodule MyTest do
use ExUnit.Case
use Rewire
import Mox
test "start/0" do
rewire Conversation, English: EnglishMock do
# within the block `Conversation` is rewired
stub(EnglishMock, :greet, fn -> "g'day" end)
assert Conversation.start() == "g'day" # using the mock!
end
end
end
```
## FAQ
**Why?**
I haven't been happy with the existing tradeoffs of injecting dependencies into Elixir modules that allows me to alter their behavior in my unit tests.
For example, if you don't use `mox`, the best approach known to me is to pass-in dependencies via a function's parameters:
```elixir
defmodule Conversation do
def start(mod \\ English), do: mod.greet()
end
```
The downsides to that approach are:
1) Your application code is now littered with testing concerns.
2) Navigation in your code editor doesn't work as well.
3) Searches for usages of the module are more difficult.
4) The compiler is not able to warn you in case `greet/0` doesn't exist on the `English` module.
If you use `mox` for your mocking, there's a slightly better approach:
```elixir
defmodule Conversation do
def start(), do: english().greet()
defp english(), do: Application.get(:myapp, :english, English)
end
```
In this approach we use the app's config to replace a module with a `mox` mock during testing. This is a little better in my opinion, but still comes with most of the disadvantages described above.
**Witchcraft! How does this work??**
Simply put, `rewire` will create a copy of the module to rewire under a new name, replacing all hard-coded module references that should be changed in the process. Plus, it rewrites the test code in the `rewire` block to use the generated module instead.
**Will that slow down my tests?**
Possibly just a little? Conclusive data isn't in yet.
**Does it work with `mox`?**
It works great with [mox](https://github.com/dashbitco/mox) since `rewire` focuses on the _injection_ and doesn't care about where the _mock_ module comes from. `rewire` and `mox` are a great pair!
