Flex Mock -- Making Mock Easy

FlexMock is a simple, but flexible, mock object library for Ruby unit testing.

Version

0.9.0.beta.0

Links

Documents

flexmock.rubyforge.org

RubyGems

Install with: gem install flexmock

Download

Download from RubyForge at rubyforge.org/frs/?group_id=3433 (pre 0.6.0 versions may be found at rubyforge.org/frs/?group_id=170)

Issue Tracking

www.pivotaltracker.com/projects/28401

Bug Reports

onestepback.org/cgi-bin/bugs.cgi?project=flexmock

Installation

You can install FlexMock with the following command.

$ gem install flexmock

Simple Example

We have a data acquisition class (TemperatureSampler) that reads a temperature sensor and returns an average of 3 readings. We don't have a real temperature to use for testing, so we mock one up with a mock object that responds to the read_temperature message.

Here's the complete example:

require 'test/unit'
require 'flexmock/test_unit'

class TemperatureSampler
  def initialize(sensor)
    @sensor = sensor
  end

  def average_temp
    total = (0...3).collect { 
      @sensor.read_temperature 
    }.inject { |i, s| i + s }
    total / 3.0
  end
end

class TestTemperatureSampler < Test::Unit::TestCase
  def test_sensor_can_average_three_temperature_readings
    sensor = flexmock("temp")
    sensor.should_receive(:read_temperature).times(3).
      and_return(10, 12, 14)

    sampler = TemperatureSampler.new(sensor)
    assert_equal 12, sampler.average_temp
  end
end

You can find an extended example of FlexMock in Google Example.

Test::Unit Integration

FlexMock integrates nicely with Test::Unit. Just require the 'flexmock/test_unit' file at the top of your test file. The flexmock method will be available for mock creation, and any created mocks will be automatically validated and closed at the end of the individual test.

Your test case will look something like this:

require 'flexmock/test_unit'

class TestDog < Test::Unit::TestCase
  def test_dog_wags
    tail_mock = flexmock(:wag => :happy)
    assert_equal :happy, tail_mock.wag
  end
end

NOTE: If you don't want to automatically extend every TestCase with the flexmock methods and overhead, then require the 'flexmock' file and explicitly include the FlexMock::TestCase module in each test case class where you wish to use mock objects. FlexMock versions prior to 0.6.0 required the explicit include.

RSpec Integration

FlexMock also supports integration with the RSpec behavior specification framework. Starting with version 0.9.0 of RSpec, you will be able to say:

Spec::Runner.configure do |config|
  config.mock_with :flexmock
end

describe "Using FlexMock with RSpec" do
  it "should be able to create a mock" do
    m = flexmock(:foo => :bar)
    m.foo.should === :bar
  end
end

If you wish to try this prior to the release of RSpec 0.9.0, check out the trunk of the RSpec subversion repository.

Quick Reference

Creating Mock Objects

The flexmock method is used to create mocks in various configurations. Here's a quick rundown of the most common options. See FlexMock::MockContainer#flexmock for more details.

NOTE: Versions of FlexMock prior to 0.6.0 used flexstub to create partial mocks. The flexmock method now assumes all the functionality that was spread out between two different methods. flexstub is still available for backward compatibility.

Expectation Declarators

Once a mock is created, you need to define what that mock should expect to see. Expectation declarators are used to specify these expectations placed upon received method calls. A basic expectation, created with the should_receive method, just establishes the fact that a method may (or may not) be called on the mock object. Refinements to that expectation may be additionally declared. FlexMock always starts with the most general expectation and adds constraints to that.

For example, the following code:

mock.should_receive(:average).and_return(12)

Means that the mock will now accept method calls to an average method. The expectation will accept any arguments and may be called any number of times (including zero times). Strictly speaking, the and_return part of the declaration isn't exactly a constraint, but it does specify what value the mock will return when the expectation is matched.

If you want to be more specific, you need to add additional constraints to your expectation. Here are some examples:

mock.should_receive(:average).with(12).once

mock.should_receive(:average).with(Integer).
    at_least.twice.at_most.times(10).
    and_return { rand }

The following methods may be used to create and refine expectations on a mock object. See theFlexMock::Expectation for more details.

Argument Validation

The values passed to the with declarator determine the criteria for matching expectations. The first expectation found that matches the arguments in a mock method call will be used to validate that mock method call.

The following rules are used for argument matching:

Creating Partial Mocks

Sometimes it is useful to mock the behavior of one or two methods in an existing object without changing the behavior of the rest of the object. If you pass a real object to the flexmock method, it will allow you to use that real object in your test and will just mock out the one or two methods that you specify.

For example, suppose that a Dog object uses a Woofer object to bark. The code for Dog looks like this (we will leave the code for Woofer to your imagination):

class Dog
  def initialize
    @woofer = Woofer.new
  end
  def bark
    @woofer.woof
  end
  def wag
    :happy
  end
end

Now we want to test Dog, but using a real Woofer object in the test is a real pain (why? ... well because Woofer plays a sound file of a dog barking, and that's really annoying during testing).

So, how can we create a Dog object with mocked Woofer? All we need to do is allow FlexMock to replace the bark method.

Here's the test code:

class TestDogBarking < Test::Unit::TestCase
  include FlexMock::TestCase

  # Setup the tests by mocking the +new+ method of 
  # Woofer and return a mock woofer.
  def setup
    @dog = Dog.new
    flexmock(@dog, :bark => :grrr)
  end

  def test_dog
    assert_equal :grrr, @dog.bark   # Mocked Method
    assert_equal :happy, @dog.wag    # Normal Method
  end
end

The nice thing about this technique is that after the test is over, the mocked out methods are returned to their normal state. Outside the test everything is back to normal.

NOTE: In previous versions of FlexMock, partial mocking was called "stubs" and the flexstub method was used to create the partial mocks. Although partial mocks were often used as stubs, the terminology was not quite correct. The current version of FlexMock uses the flexmock method to create both regular stubs and partial stubs. A version of the flexstub method is included for backwards compatibility. See Martin Fowler's article Mocks Aren't Stubs (www.martinfowler.com/articles/mocksArentStubs.html) for a better understanding of the difference between mocks and stubs.

This partial mocking technique was inspired by the Stuba library in the Mocha project.

Mocking Class Objects

In the previous example we mocked out the bark method of a Dog object to avoid invoking the Woofer object. Perhaps a better technique would be to mock the Woofer object directly. But Dog uses Woofer explicitly so we cannot just pass in a mock object for Dog to use.

But wait, we can add mock behavior to any existing object, and classes are objects in Ruby. So why don't we just mock out the Woofer class object to return mocks for us.

class TestDogBarking < Test::Unit::TestCase
  include FlexMock::TestCase

  # Setup the tests by mocking the +new+ method of 
  # Woofer and return a mock woofer.
  def setup
    flexmock(Woofer).should_receive(:new).
       and_return(flexmock(:woof => :grrr))
    @dog = Dog.new
  end

  def test_dog
    assert_equal :grrrr, @dog.bark  # Calls woof on mock object
                                    # returned by Woofer.new
  end
end

Mocking Behavior in All Instances Created by a Class Object

Sometimes returning a single mock object is not enough. Occasionally you want to mock every instance object created by a class. FlexMock makes this very easy.

class TestDogBarking < Test::Unit::TestCase
  include FlexMock::TestCase

  # Setup the tests by mocking Woofer to always
  # return partial mocks.
  def setup
    flexmock(Woofer).new_instances.should_receive(:woof => :grrr)
  end

  def test_dog
    assert_equal :grrrr, Dog.new.bark  # All dog objects
    assert_equal :grrrr, Dog.new.bark  # are mocked.
  end
end

Note that FlexMock adds the mock expectations after the original new method has completed. If the original version of new yields the newly created instance to a block, that block will get an non-mocked version of the object.

Note that new_instances will accept a block if you wish to mock several methods at the same time. E.g.

flexmock(Woofer).new_instances do |m|
  m.should_receive(:woof).twice.and_return(:grrr)
  m.should_receive(:wag).at_least.once.and_return(:happy)
end

Default Expectations on Mocks

Sometimes you want to setup a bunch of default expectations that are pretty much for a number of different tests. Then in the individual tests, you would like to override the default behavior on just that one method you are testing at the moment. You can do that by using the by_default modifier.

In your test setup you might have:

def setup
  @mock_dog = flexmock("Fido")
  @mock_dog.should_receive(:tail => :a_tail, :bark => "woof").by_default
end

The behaviors for :tail and :bark are good for most of the tests, but perhaps you wish to verify that :bark is called exactly once in a given test. Since :bark by default has no count expectations, you can override the default in the given test.

def test_something_where_bark_must_be_called_once
  @mock_dog.should_receive(:bark => "woof").once

  # At this point, the default for :bark is ignored, 
  # and the "woof" value will be returned.

  # However, the default for :tail (which returns :a_tail)
  # is still active.
end

By setting defaults, your individual tests don't have to concern themselves with details of all the default setup. But the details of the overrides are right there in the body of the test.

Mocking Law of Demeter Violations

The Law of Demeter says that you should only invoke methods on objects to which you have a direct connection, e.g. parameters, instance variables, and local variables. You can usually detect Law of Demeter violations by the excessive number of periods in an expression. For example:

car.chassis.axle.universal_joint.cog.turn

The Law of Demeter has a very big impact on mocking. If you need to mock the "turn" method on "cog", you first have to mock chassis, axle, and universal_joint.

# Manually mocking a Law of Demeter violation
cog = flexmock("cog")
cog.should_receive(:turn).once.and_return(:ok)
joint = flexmock("gear", :cog => cog)
axle = flexmock("axle", :universal_joint => joint)
chassis = flexmock("chassis", :axle => axle)
car = flexmock("car", :chassis => chassis)

Yuck!

The best course of action is to avoid Law of Demeter violations. Then your mocking exploits will be very simple. However, sometimes you have to deal with code that already has a Demeter chain of method calls. So for those cases where you can't avoid it, FlexMock will allow you to easily mock Demeter method chains.

Here's an example of Demeter chain mocking:

# Demeter chain mocking using the short form.
car = flexmock("car")
car.should_receive( "chassis.axle.universal_joint.cog.turn" => :ok).once

You can also use the long form:

# Demeter chain mocking using the long form.
car = flexmock("car")
car.should_receive("chassis.axle.universal_joint.cog.turn").once.
  and_return(:ok)

That's it. Anywhere FlexMock accepts a method name for mocking, you can use a demeter chain and FlexMock will attempt to do the right thing.

But beware, there are a few limitations.

The all the methods in the chain, except for the last one, will mocked to return a mock object. That mock object, in turn, will be mocked so as to respond to the next method in the chain, returning the following mock. And so on. If you try to manually mock out any of the chained methods, you could easily interfer with the mocking specified by the Demeter chain. FlexMock will attempt to catch problems when it can, but there are certainly scenarios where it cannot detect the problem beforehand.

Examples

Create a simple mock object that returns a value for a set of method calls

require 'flexmock/test_unit'

class TestSimple < Test::Unit::TestCase
  def test_simple_mock
    m = flexmock(:pi => 3.1416, :e => 2.71)
    assert_equal 3.1416, m.pi
    assert_equal 2.71, m.e
  end
end

Create a mock object that returns an undefined object for method calls

require 'flexmock/test_unit'

class TestUndefined < Test::Unit::TestCase
  def test_undefined_values
    m = flexmock("mock")
    m.should_receive(:divide_by).with(0).
      and_return_undefined
    assert_equal FlexMock.undefined, m.divide_by(0)
  end
end

Expect multiple queries and a single update

Multiple calls to the query method will be allows, and calls may have any argument list. Each call to query will return the three element array [1, 2, 3]. The call to update must have a specific argument of 5.

require 'flexmock/test_unit'

class TestDb < Test::Unit::TestCase
  def test_db
    db = flexmock('db')
    db.should_receive(:query).and_return([1,2,3])
    db.should_receive(:update).with(5).and_return(nil).once
    # test code here
  end
end

Expect all queries before any updates

(This and following examples assume that the 'flexmock/test_unit' file has been required.)

All the query message must occur before any of the update messages.

def test_query_and_update
  db = flexmock('db')
  db.should_receive(:query).and_return([1,2,3]).ordered
  db.should_receive(:update).and_return(nil).ordered
  # test code here
end

Expect several queries with different parameters

The queries should happen after startup but before finish. The queries themselves may happen in any order (because they are in the same order group). The first two queries should happen exactly once, but the third query (which matches any query call with a four character parameter) may be called multiple times (but at least once). Startup and finish must also happen exactly once.

Also note that we use the with method to match different argument values to figure out what value to return.

def test_ordered_queries
  db = flexmock('db')
  db.should_receive(:startup).once.ordered
  db.should_receive(:query).with("CPWR").and_return(12.3).
    once.ordered(:queries)
  db.should_receive(:query).with("MSFT").and_return(10.0).
    once.ordered(:queries)
  db.should_receive(:query).with(/^....$/).and_return(3.3).
    at_least.once.ordered(:queries)
  db.should_receive(:finish).once.ordered
  # test code here
end

Same as above, but using the Record Mode interface

The record mode interface offers much the same features as the should_receive interface introduced so far, but it allows the messages to be sent directly to a recording object rather than be specified indirectly using a symbol.

def test_ordered_queries_in_record_mode
  db = flexmock('db')
  db.should_expect do |rec|
    rec.startup.once.ordered
    rec.query("CPWR") { 12.3 }.once.ordered(:queries)
    rec.query("MSFT") { 10.0 }.once.ordered(:queries)
    rec.query(/^....$/) { 3.3 }.at_least.once.ordered(:queries)
    rec.finish.once.ordered
  end
  # test code here using +db+.
end

Using Record Mode to record a known, good algorithm for testing

Record mode is nice when you have a known, good algorithm that can use a recording mock object to record the steps. Then you compare the execution of a new algorithm to behavior of the old using the recorded expectations in the mock. For this you probably want to put the recorder in strict mode so that the recorded expectations use exact matching on argument lists, and strict ordering of the method calls.

Note: This is most useful when there are no queries on the mock objects, because the query responses cannot be programmed into the recorder object.

def test_build_xml
  builder = flexmock('builder')
  builder.should_expect do |rec|
    rec.should_be_strict
    known_good_way_to_build_xml(rec)  # record the messages
  end
  new_way_to_build_xml(builder)       # compare to new way
end

Expect multiple calls, returning a different value each time

Sometimes you need to return different values for each call to a mocked method. This example shifts values out of a list for this effect.

def test_multiple_gets
  file = flexmock('file')
  file.should_receive(:gets).with_no_args.
     and_return("line 1\n", "line 2\n")
  # test code here
end

Ignore uninteresting messages

Generally you need to mock only those methods that return an interesting value or wish to assert were sent in a particular manner. Use the should_ignore_missing method to turn on missing method ignoring.

def test_an_important_message
  m = flexmock('m')
  m.should_receive(:an_important_message).and_return(1).once
  m.should_ignore_missing
  # test code here
end

When should_ignore_missing is enabled, ignored missing methods will return an undefined object. Any operation on the undefined object will return the undefined object.

Mock just one method on an existing object

The Portfolio class calculate the value of a set of stocks by talking to a quote service via a web service. Since we don't want to use a real web service in our unit tests, we will mock the quote service.

def test_portfolio_value
  flexmock(QuoteService).new_instances do |m|
    m.should_receive(:quote).and_return(100)
  end
  port = Portfolio.new
  value = port.value     # Portfolio calls QuoteService.quote
  assert_equal 100, value
end

Other Mock Objects

test-unit-mock

www.deveiate.org/code/Test-Unit-Mock.shtml

mocha/stubba

mocha.rubyforge.org/

Schmock

rubyforge.org/projects/schmock/

License

Copyright 2003, 2004, 2005, 2006, 2007 by Jim Weirich (jim@weirichhouse.org). All rights reserved.

Permission is granted for use, copying, modification, distribution, and distribution of modified versions of this work as long as the above copyright notice is included.

Other stuff

Author

Jim Weirich <jim@weirichhouse.org>

Requires

Ruby 1.8.7 or later (Use Flexmock-0.8.8 for Ruby version 1.8.6)

Warranty

This software is provided "as is" and without any express or implied warranties, including, without limitation, the implied warranties of merchantibility and fitness for a particular purpose.

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