Writing MiniTest extensions

account_test.rb

require 'minitest/autorun'

module MiniTest::Assertions
  def assert_changes(obj, method, exp_diff)
    before = obj.send method
    yield
    after  = obj.send method
    diff = after - before

    assert_equal exp_diff, diff, "Expected #{obj.class.name}##{method} to change by #{exp_diff}, changed by #{diff}"
  end
end

module MiniTest::Expectations
  infect_an_assertion :assert_changes, :must_change
end

class Account
  attr_reader :balance

  def initialize(balance = 0)
    @balance = balance
  end

  def credit(amount)
    check_amount amount

    @balance += amount
  end

  def debit(amount)
    check_amount amount

    raise RuntimeError, "Unsufficient funds" if amount > balance

    @balance -= amount
  end

  private

  def check_amount(amount)
    raise ArgumentError, "Amount must be > 0" unless amount > 0
  end
end

describe Account do
  before { @account = Account.new 100 }

  describe '#credit' do
    it "fails when amount <= 0" do
      lambda { @account.credit 0 }.must_raise ArgumentError
    end

    it "increases balance" do
      lambda { @account.credit 50 }.must_change @account, :balance, +50.0
    end
  end

  describe '#debit' do
    it "fails when amount <= 0" do
      lambda { @account.debit 0 }.must_raise ArgumentError
    end

    it "fails when amount is greater than balance" do
      lambda { @account.debit 500 }.must_raise RuntimeError
    end

    it "decreases balance" do
      lambda { @account.debit 50 }.must_change @account, :balance, -50.0
    end
  end
end