ExStub

ExStub provides an easy way to stub a module to facilitate writing clean, isolated unit tests.

Installation

Add ex_stub to your deps in mix.exs

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

Usage

If you have a module in your original application like:

defmodule OriginalModule do
  def process(param), do: :original_process
  def another_method, do: :original_method
end

You can quickly create a stub copy of this module using defstub

use ExStub

defstub MyStub, for: OriginalModule do
  def process(true), do: :stubbed1
  def process(false), do: :stubbed2
  def process(1), do: :stubbed3
end

Now you can pass around MyStub instead of OriginalModule. When you invoke method from the created MyStub, if the method was stubbed it will call the stubbed version. Else the original version will be called.

MyStub.process(true) # returns :stubbed1
MyStub.process(false) # returns :stubbed2
MyStub.process(1) # returns :stubbed3

MyStub.process(20) # returns :original_process

MyStub.another_method # returns :original_method

Notice that Since we did not stub another_method, calling it on MyStub returns the original implementation. Also when calling MyStub.process(20) the original implementation is called since it failed pattern matching with our stub version of the method.

As a safety procedure, if you try to stub a method that is not found in the original module. ExStub will throw a compilation error telling you about the unexpected stubbed method.

defstub MyStub, for: OriginalModule do
def new_method(), do: :stubbed1
end

The following error will be thrown

** (RuntimeError) Cannot provide implementations for methods that are not in the original module
The def `{:new_method, 0}` is not defined in module `OriginalModule`