Conduit Design Pattern

I love dependency injection. Specifically I like constructor dependency injection.

Sometimes however you have two objects that need to talk to each other. This means you need create each before the other to use constructor injection.

What I have been doing instead is creating a class I call a conduit.

The Conduit class implements one of the interfaces and delegates to another instance of that interface. It also provies a setter injection method to allow you to initialize the conduit.

To use it therefore you do the following:
1/ Create an instance of IXConduit
2/ create an instance of class Y passing in the IXConduit as an instance of IX
3/ create an instance of class X passing in Y
4/ call the setter injection method on the conduit passing X in.

When Y calls a method on the IX it has (the conduit) it passes on the call the the IX it has (the real instance of X), and X can call method on Y directly.

This means the compilcation of instantiation is not part of class X or Y so they don't get polluted by this design issue.

MockingTestFixture makes NMock tests simpler

I just noticed that not everyone is using a helper class when using NMock2. It seriously makes your code cleaner. Here is the one I use.

using System;
using NMock2;
using NUnit.Framework;


namespace Tests.Utilities
{
   public abstract class MockingTestFixture
   {
       private Mockery mockery;

       protected IDisposable Ordered
       {
           get { return mockery.Ordered; }
       }

       [SetUp]
       public void MockSetUp()
       {
           mockery = new Mockery();
           SetUp();
       }

       protected abstract void SetUp();

       [TearDown]
       public virtual void MockTearDown()
       {
           TearDown();
           VerifyExpectations();
       }

       protected virtual void TearDown()
       {
       }

       public T NewMock()
       {
           return mockery.NewMock();
       }

       public void VerifyExpectations()
       {
           mockery.VerifyAllExpectationsHaveBeenMet();
       }

       protected static void IgnoreReturnValue(object ignored)
       {
       }
   }
}

Derive your test fixture from this class. Override the abstract setup method... there you go. No more need to worry about the Mockery.

Tip#3: Extend your mocking framework

Currently I am using NMock2 in my C# development. I have however made a few extensions to allow me to test some edge cases. Due to the clean design of NMock extending it is really easy.

IAction

A normal use of NMock looks like this:

Expect.Once.On(someObject).Method("methodName").WithAnyArguments().Will(Return.Value("value"));

In this statement the 'Return.Value' method returns an object that implements the IAction interface. The job of this object is to perform some action at the point the expectation matches. In this example the effect is to set the return value of the method to a known string.

However... any class implementing the IAction interface can be passed to the 'Will' method, so if you have some special need to execute code at the point the method is matched, then here is a great place to put it.

Here is an example...

[Test]
public void Move_Sleeps_UntilUpdatedWithTrue
{
    Expect.Once.On(sleeper).Method("Sleep").Will(Execute.Delegate(delegate { mono.Update(false); }));
    Expect.Once.On(sleeper).Method("Sleep").Will(Execute.Delegate(delegate { mono.Update(false); }));
    Expect.Once.On(sleeper).Method("Sleep").Will(Execute.Delegate(delegate { mono.Update(true); }));
    mono.Move(120);
}

Here I have used an IAction class that invokes a delegate at the point your expectation is met. In this case I am using it to test a piece of code that repeatedly sleeps (using a sleeper service) until the object is updated with true. I am intercepting the call to sleep and instead taking the opportunity to update the object.

Here is the code that makes that possible.

public class Execute : IAction
{
  public delegate void DelegateAction();
  private readonly DelegateAction action;

  public static IAction Delegate(DelegateAction action)
  {return new Execute(action);}

  private Execute(DelegateAction action)
  {this.action = action;}

  public void Invoke(Invocation invocation)
  {action();}

  public void DescribeTo(TextWriter writer)
  {}
}

So in way of explanation: the invoke method is want gets called when the expectation is met. It just invokes the delegate the 'Execute' object was constructed with. The rest is just static methods to allow you to say Execute.Delegate(...) rather than new Execute(...)

Next post I'll explain how I test firing events using another extension to NMock.

Installing simple_helpful without edge Rails

DHH has helpfully moved the simply_helpful Ruby on Rails plugin.
If you are not running edge you can still install, you just have to use:

script/plugin install http://dev.rubyonrails.org/svn/rails/plugins/legacy/simply_helpful/

Tip:#2 Smell: Duplicate Tests Indicate a Missing Class

Smell: A class with two public methods on it, both perform the same functionality, or one is a subset of the other. The existence of a private method is a good indication of this.

To fully test these public methods you have to repeat a bunch of tests. There has to be a lazier simpler solution.

Recently I have been getting this smell a lot using MVC in a .Net winforms app.

.Net forms are hard to unit test. It is therefore helpful to keep your Views as thin as possible. Just use them to expose the form fields as a bunch of properties and to catch events and call the corresponding method on the controller. This moves the logic to the Controller where it is far more testable.

This can however lead to methods on the controller like "OnStartButtonClicked" and "OnStartMenuItemSelected". Both are going to perform the same actions. Both are going to need the same set of tests.

The solution is simple. Use the 'Extract Class' refactoring to pull the private method out to another class and use Dependency Injection to pass an instance of this new class back to the original class.

Following this above advice though you end up with another object. In my experience this split makes a lot of sense. I call this new class a Service. I rename the new thinner Controller to Presenter which better resembles it's remaining responsibilities.

(Note: I've read a few things on the differences between MVC and MVP, but I don't really get it. What I have here may be what is meant by the naming. Either way I like this design better.)

I'm really liking the new code. The view is really thin. The presenter translates UI events to service calls, and knows which views to update when the domain changes.

A piece of advice Steve Hayes often gives is "Design your UI layer so you could replace it with a command line and everything would keep working". I have struggled to do this with .Net, even with MVC. Having this new split however I can see keeping the Service and Model layers and replacing the UI would be easy.

So... Look out for duplicates tests... get lazy and write better code!