TDD Beyond Basics : How to Fake User Input


I used highline gem to test drive my pair ranking gem, rasam. I knew that StringIO can be used in tests as VirtualFile to simulate File. But how do you simulate user input? Reading the source code of the highline library, I came across HighLine::Simulate class that simulates user input that can be used in tests. It uses meta-programming. It is based on the gist by Jason Morrison. I have simplified the original gist. Here is the source:

require 'test/unit'

class VirtualInput
  def initialize(strings)
    @strings = strings

  def gets
    next_string = @strings.shift
    # Uncomment the following line if you'd like to see the faked $stdin#gets
    puts "(DEBUG) Faking #gets with: #{next_string}"

  def self.with_fake_input(strings)
    $stdin =
    $stdin = STDIN

class Waiter
  attr_accessor :orders

  def take_orders
    self.orders = {}

    puts "Hello, diner.  What is your name?"
    name = gets

    puts "Please to meet you. #{name}, what would you like for dinner?"
    order = gets

    orders[name] = order

class WaiterTest < Test::Unit::TestCase
  def test_ordering
    VirtualInput.with_fake_input(["Harry", "The tarte tomate"]) do
      waiter =

      assert waiter.orders.has_key?("Harry")
      assert_equal "The tarte tomate", waiter.orders["Harry"]

You can see how the with_fake_input hijacks the $stdin value to the VirtualInput and reverts it back once it completes.

We can use Highline::Simulate class to automate the script that tests the rasam gem. This will be useful to write our acceptance test.


In this article, you learned how to simulate user input so that we can write tests without having to manually enter the data to the script.

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