Smart Constructor Anti-Pattern
If you look at the definition of a constructor in the context of software development on Wikipedia (a completely factual and reliable source :), you’ll find this simple statement of their responsibility…
“Their responsibility is to initialize the object’s data members and to establish the invariant of the class, failing if the invariant isn’t valid.”
Simple really. Just initialize the (usually) private data members and ensure the arguments won’t put the object in an invalid state. But so often I see constructors doing so much more than that. One example I’ve seen of this recently is in presenters.
The (perhaps contrived) scenario:
When a view loads for the first time, you need to load some lookup data that gets populated in a combo box on the view and set the number of logged in users on the view.
Here is our IView interface for reference:
1: public interface IView
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<pre><span style="color: #606060"> 2:</span> {</pre>
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<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">void</span> BindWith(IEnumerable<User> users);</pre>
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</p>
<pre><span style="color: #606060"> 4:</span> <span style="color: #0000ff">bool</span> HasUserSelected { get; }</pre>
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</p>
<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">void</span> SetNumberOfLoggedInUsersTo(<span style="color: #0000ff">int</span> numberOfUsers);</pre>
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<pre><span style="color: #606060"> 6:</span> }</pre>
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<h3>
“Smart” Constructor Approach
</h3>
<p>
One might be tempted to start writing a spec like the one below.
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> SmartConstructorPresenterSpecs</pre>
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<pre><span style="color: #606060"> 2:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> when_the_presenter_is_constructed : ContextSpecification</pre>
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<pre><span style="color: #606060"> 4:</span> {</pre>
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<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">private</span> IView View;</pre>
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<pre><span style="color: #606060"> 6:</span> <span style="color: #0000ff">private</span> IUserRepository UserRepository;</pre>
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<pre><span style="color: #606060"> 7:</span> <span style="color: #0000ff">private</span> IAuthenticationService AuthenticationService;</pre>
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</p>
<pre><span style="color: #606060"> 8:</span> <span style="color: #0000ff">private</span> SmartConstructorPresenter SUT;</pre>
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<pre><span style="color: #606060"> 9:</span> <span style="color: #0000ff">private</span> IEnumerable<User> Users;</pre>
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<pre><span style="color: #606060"> 10:</span>  </pre>
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<pre><span style="color: #606060"> 11:</span> <span style="color: #0000ff">protected</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> When()</pre>
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<pre><span style="color: #606060"> 12:</span> {</pre>
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<pre><span style="color: #606060"> 13:</span> Users = <span style="color: #0000ff">new</span> List<User> {<span style="color: #0000ff">new</span> User(<span style="color: #0000ff">true</span>), <span style="color: #0000ff">new</span> User(<span style="color: #0000ff">true</span>), <span style="color: #0000ff">new</span> User(<span style="color: #0000ff">false</span>)};</pre>
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<pre><span style="color: #606060"> 14:</span>  </pre>
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<pre><span style="color: #606060"> 15:</span> View = Mock<IView>();</pre>
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</p>
<pre><span style="color: #606060"> 16:</span> UserRepository = Mock<IUserRepository>();</pre>
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</p>
<pre><span style="color: #606060"> 17:</span> AuthenticationService = Mock<IAuthenticationService>();</pre>
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<pre><span style="color: #606060"> 18:</span>  </pre>
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<pre><span style="color: #606060"> 19:</span> UserRepository.Stub(x => x.All()).Return(Users);</pre>
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<pre><span style="color: #606060"> 20:</span>  </pre>
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<pre><span style="color: #606060"> 21:</span> SUT = <span style="color: #0000ff">new</span> SmartConstructorPresenter(View, UserRepository, AuthenticationService);</pre>
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</p>
<pre><span style="color: #606060"> 22:</span> }</pre>
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<pre><span style="color: #606060"> 23:</span>  </pre>
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<pre><span style="color: #606060"> 24:</span> [Test]</pre>
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<pre><span style="color: #606060"> 25:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> it_should_populate_the_view_with_a_list_of_users()</pre>
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<pre><span style="color: #606060"> 26:</span> {</pre>
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<pre><span style="color: #606060"> 27:</span> View.AssertWasCalled(x => x.BindWith(Users));</pre>
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<pre><span style="color: #606060"> 28:</span> }</pre>
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<pre><span style="color: #606060"> 29:</span>  </pre>
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<pre><span style="color: #606060"> 30:</span> [Test]</pre>
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<pre><span style="color: #606060"> 31:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> it_should_set_the_number_of_logged_in_users_on_the_view()</pre>
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<pre><span style="color: #606060"> 32:</span> {</pre>
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<pre><span style="color: #606060"> 33:</span> View.AssertWasCalled(x => x.SetNumberOfLoggedInUsersTo(2));</pre>
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<pre><span style="color: #606060"> 34:</span> }</pre>
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<pre><span style="color: #606060"> 35:</span> }</pre>
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<pre><span style="color: #606060"> 36:</span>  </pre>
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<pre><span style="color: #606060"> 37:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> when_attempting_to_login_and_a_user_is_not_selected : ContextSpecification</pre>
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<pre><span style="color: #606060"> 38:</span> {</pre>
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<pre><span style="color: #606060"> 39:</span> <span style="color: #0000ff">private</span> IView View;</pre>
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<pre><span style="color: #606060"> 40:</span> <span style="color: #0000ff">private</span> IUserRepository UserRepository;</pre>
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<pre><span style="color: #606060"> 41:</span> <span style="color: #0000ff">private</span> IAuthenticationService AuthenticationService;</pre>
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<pre><span style="color: #606060"> 42:</span> <span style="color: #0000ff">private</span> SmartConstructorPresenter SUT;</pre>
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<pre><span style="color: #606060"> 43:</span> <span style="color: #0000ff">private</span> IEnumerable<User> Users;</pre>
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<pre><span style="color: #606060"> 44:</span>  </pre>
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<pre><span style="color: #606060"> 45:</span> <span style="color: #0000ff">protected</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> EstablishContext()</pre>
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<pre><span style="color: #606060"> 46:</span> {</pre>
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<pre><span style="color: #606060"> 47:</span> Users = <span style="color: #0000ff">new</span> List<User> {<span style="color: #0000ff">new</span> User(<span style="color: #0000ff">true</span>), <span style="color: #0000ff">new</span> User(<span style="color: #0000ff">true</span>), <span style="color: #0000ff">new</span> User(<span style="color: #0000ff">false</span>)};</pre>
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<pre><span style="color: #606060"> 48:</span>  </pre>
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<pre><span style="color: #606060"> 49:</span> View = Mock<IView>();</pre>
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</p>
<pre><span style="color: #606060"> 50:</span> UserRepository = Mock<IUserRepository>();</pre>
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</p>
<pre><span style="color: #606060"> 51:</span> AuthenticationService = Mock<IAuthenticationService>();</pre>
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</p>
<pre><span style="color: #606060"> 52:</span>  </pre>
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<pre><span style="color: #606060"> 53:</span> UserRepository.Stub(x => x.All()).Return(Users);</pre>
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</p>
<pre><span style="color: #606060"> 54:</span> View.Stub(x => x.HasUserSelected).Return(<span style="color: #0000ff">false</span>);</pre>
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</p>
<pre><span style="color: #606060"> 55:</span>  </pre>
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<pre><span style="color: #606060"> 56:</span> SUT = <span style="color: #0000ff">new</span> SmartConstructorPresenter(View, UserRepository, AuthenticationService);</pre>
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</p>
<pre><span style="color: #606060"> 57:</span> }</pre>
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<pre><span style="color: #606060"> 58:</span>  </pre>
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<pre><span style="color: #606060"> 59:</span> <span style="color: #0000ff">protected</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> When()</pre>
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<pre><span style="color: #606060"> 60:</span> {</pre>
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<pre><span style="color: #606060"> 61:</span> SUT.Login();</pre>
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<pre><span style="color: #606060"> 62:</span> }</pre>
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<pre><span style="color: #606060"> 63:</span>  </pre>
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<pre><span style="color: #606060"> 64:</span> [Test]</pre>
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<pre><span style="color: #606060"> 65:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> it_should_NOT_perform_a_login()</pre>
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<pre><span style="color: #606060"> 66:</span> {</pre>
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<pre><span style="color: #606060"> 67:</span> AuthenticationService.AssertWasNotCalled(x => x.Login());</pre>
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<pre><span style="color: #606060"> 68:</span> }</pre>
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<pre><span style="color: #606060"> 69:</span> }</pre>
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<pre><span style="color: #606060"> 70:</span> }</pre>
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<p>
And implement the presenter.
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> SmartConstructorPresenter</pre>
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</p>
<pre><span style="color: #606060"> 2:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IView view;</pre>
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</p>
<pre><span style="color: #606060"> 4:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IAuthenticationService authenticationService;</pre>
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</p>
<pre><span style="color: #606060"> 5:</span>  </pre>
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</p>
<pre><span style="color: #606060"> 6:</span> <span style="color: #0000ff">public</span> SmartConstructorPresenter(IView view, IUserRepository userRepository,</pre>
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</p>
<pre><span style="color: #606060"> 7:</span> IAuthenticationService authenticationService)</pre>
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<pre><span style="color: #606060"> 8:</span> {</pre>
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<pre><span style="color: #606060"> 9:</span> <span style="color: #0000ff">this</span>.view = view;</pre>
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<pre><span style="color: #606060"> 10:</span> <span style="color: #0000ff">this</span>.authenticationService = authenticationService;</pre>
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<pre><span style="color: #606060"> 11:</span>  </pre>
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<pre><span style="color: #606060"> 12:</span> var users = userRepository.All();</pre>
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</p>
<pre><span style="color: #606060"> 13:</span>  </pre>
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<pre><span style="color: #606060"> 14:</span> <span style="color: #0000ff">this</span>.view.BindWith(users);</pre>
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</p>
<pre><span style="color: #606060"> 15:</span> <span style="color: #0000ff">this</span>.view.SetNumberOfLoggedInUsersTo(users.Where(x => x.IsLoggedIn).Count());</pre>
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</p>
<pre><span style="color: #606060"> 16:</span> }</pre>
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</p>
<pre><span style="color: #606060"> 17:</span>  </pre>
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</p>
<pre><span style="color: #606060"> 18:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> Login()</pre>
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</p>
<pre><span style="color: #606060"> 19:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 20:</span> <span style="color: #0000ff">if</span> (view.HasUserSelected)</pre>
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</p>
<pre><span style="color: #606060"> 21:</span> authenticationService.Login();</pre>
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<pre><span style="color: #606060"> 22:</span> }</pre>
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<pre><span style="color: #606060"> 23:</span> }</pre>
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<p>
And then the view.
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> View : IView</pre>
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<pre><span style="color: #606060"> 2:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">event</span> VoidHandler loginButtonClick = <span style="color: #0000ff">delegate</span> { };</pre>
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</p>
<pre><span style="color: #606060"> 4:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> SmartConstructorPresenter presenter;</pre>
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</p>
<pre><span style="color: #606060"> 5:</span>  </pre>
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</p>
<pre><span style="color: #606060"> 6:</span> <span style="color: #0000ff">public</span> View()</pre>
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<pre><span style="color: #606060"> 7:</span> {</pre>
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<pre><span style="color: #606060"> 8:</span> presenter = <span style="color: #0000ff">new</span> SmartConstructorPresenter(<span style="color: #0000ff">this</span>, <span style="color: #0000ff">null</span>, <span style="color: #0000ff">null</span>);</pre>
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</p>
<pre><span style="color: #606060"> 9:</span>  </pre>
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</p>
<pre><span style="color: #606060"> 10:</span> <span style="color: #008000">// simulate a possible login button click</span></pre>
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<pre><span style="color: #606060"> 11:</span> loginButtonClick += () => presenter.Login();</pre>
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</p>
<pre><span style="color: #606060"> 12:</span> }</pre>
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</p>
<pre><span style="color: #606060"> 13:</span>  </pre>
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</p>
<pre><span style="color: #606060"> 14:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> BindWith(IEnumerable<User> users)</pre>
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</p>
<pre><span style="color: #606060"> 15:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 16:</span> <span style="color: #008000">// bind this stuff to something</span></pre>
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</p>
<pre><span style="color: #606060"> 17:</span> }</pre>
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</p>
<pre><span style="color: #606060"> 18:</span>  </pre>
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</p>
<pre><span style="color: #606060"> 19:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">bool</span> HasUserSelected</pre>
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</p>
<pre><span style="color: #606060"> 20:</span> {</pre>
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<pre><span style="color: #606060"> 21:</span> get { <span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span>; }</pre>
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</p>
<pre><span style="color: #606060"> 22:</span> }</pre>
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<pre><span style="color: #606060"> 23:</span>  </pre>
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</p>
<pre><span style="color: #606060"> 24:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> SetNumberOfLoggedInUsersTo(<span style="color: #0000ff">int</span> numberOfUsers)</pre>
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</p>
<pre><span style="color: #606060"> 25:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 26:</span> <span style="color: #008000">// set some label or something</span></pre>
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</p>
<pre><span style="color: #606060"> 27:</span> }</pre>
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</p>
<pre><span style="color: #606060"> 28:</span> }</pre>
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</p>
<pre><span style="color: #606060"> 29:</span>  </pre>
<p>
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</p>
<pre><span style="color: #606060"> 30:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">delegate</span> <span style="color: #0000ff">void</span> VoidHandler();</pre>
<p>
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<p>
If you take a look at the code above, you can see that a the view is being populated with a list of users when the presenter is <strong>constructed</strong>.  While this might seem harmless at first, take a look at the second specification : <strong>when_attempting_to_login_and_a_user_is_not_selected.it_should_NOT_perform_a_login</strong>.
</p>
<p>
Notice that the only thing we care about is verifying that when a user is not selected, that we’re NOT attempting to call login on the authentication service.  But since our constructor has some “smarts”, we’re required to stub out the call to the user repository just to keep the test from breaking.  This may not look like a big deal in a simple case like this.  But as your codebase grows, it can make your specs brittle and harder to understand.
</p>
<h3>
“Dumb” Constructor Approach
</h3>
<p>
Another way this could be done is by delegating the initial operations on the view to an explicit “initialize” method.  Let’s see what that might look like.
</p>
<p>
First the specs of course.  🙂
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> DumbConstructorPresenterSpecs</pre>
<p>
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</p>
<pre><span style="color: #606060"> 2:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> when_initializing : ContextSpecification</pre>
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</p>
<pre><span style="color: #606060"> 4:</span> {</pre>
<p>
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</p>
<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">private</span> IView View;</pre>
<p>
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</p>
<pre><span style="color: #606060"> 6:</span> <span style="color: #0000ff">private</span> IUserRepository UserRepository;</pre>
<p>
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</p>
<pre><span style="color: #606060"> 7:</span> <span style="color: #0000ff">private</span> IAuthenticationService AuthenticationService;</pre>
<p>
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</p>
<pre><span style="color: #606060"> 8:</span> <span style="color: #0000ff">private</span> DumbConstructorPresenter SUT;</pre>
<p>
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</p>
<pre><span style="color: #606060"> 9:</span> <span style="color: #0000ff">private</span> IEnumerable<User> Users;</pre>
<p>
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</p>
<pre><span style="color: #606060"> 10:</span>  </pre>
<p>
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</p>
<pre><span style="color: #606060"> 11:</span> <span style="color: #0000ff">protected</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> EstablishContext()</pre>
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</p>
<pre><span style="color: #606060"> 12:</span> {</pre>
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</p>
<pre><span style="color: #606060"> 13:</span> View = Mock<IView>();</pre>
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<!--CRLF-->
</p>
<pre><span style="color: #606060"> 14:</span> UserRepository = Mock<IUserRepository>();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 15:</span> AuthenticationService = Mock<IAuthenticationService>();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 16:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 17:</span> SUT = <span style="color: #0000ff">new</span> DumbConstructorPresenter(View, UserRepository, AuthenticationService);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 18:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 19:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 20:</span> <span style="color: #0000ff">protected</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> When()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 21:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 22:</span> Users = <span style="color: #0000ff">new</span> List<User> {<span style="color: #0000ff">new</span> User(<span style="color: #0000ff">true</span>), <span style="color: #0000ff">new</span> User(<span style="color: #0000ff">true</span>), <span style="color: #0000ff">new</span> User(<span style="color: #0000ff">false</span>)};</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 23:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 24:</span> UserRepository.Stub(x => x.All()).Return(Users);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 25:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 26:</span> SUT.Initialize();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 27:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 28:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 29:</span> [Test]</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 30:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> it_should_populate_the_view_with_a_list_of_users()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 31:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 32:</span> View.AssertWasCalled(x => x.BindWith(Users));</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 33:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 34:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 35:</span> [Test]</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 36:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> it_should_set_the_number_of_logged_in_users_on_the_view()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 37:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 38:</span> View.AssertWasCalled(x => x.SetNumberOfLoggedInUsersTo(2));</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 39:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 40:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 41:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 42:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> when_attempting_to_login_and_a_user_is_not_selected : ContextSpecification</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 43:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 44:</span> <span style="color: #0000ff">private</span> IView View;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 45:</span> <span style="color: #0000ff">private</span> IUserRepository UserRepository;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 46:</span> <span style="color: #0000ff">private</span> IAuthenticationService AuthenticationService;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 47:</span> <span style="color: #0000ff">private</span> DumbConstructorPresenter SUT;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 48:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 49:</span> <span style="color: #0000ff">protected</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> EstablishContext()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 50:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 51:</span> View = Mock<IView>();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 52:</span> UserRepository = Mock<IUserRepository>();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 53:</span> AuthenticationService = Mock<IAuthenticationService>();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 54:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 55:</span> SUT = <span style="color: #0000ff">new</span> DumbConstructorPresenter(View, UserRepository, AuthenticationService);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 56:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 57:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 58:</span> <span style="color: #0000ff">protected</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> When()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 59:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 60:</span> View.Stub(x => x.HasUserSelected).Return(<span style="color: #0000ff">false</span>);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 61:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 62:</span> SUT.Login();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 63:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 64:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 65:</span> [Test]</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 66:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> it_should_NOT_perform_a_login()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 67:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 68:</span> AuthenticationService.AssertWasNotCalled(x => x.Login());</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 69:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 70:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 71:</span> }</pre>
<p>
<!--CRLF--></div> </div>
<p>
Then the implementation of the presenter.
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> DumbConstructorPresenter</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 2:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IView view;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 4:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IUserRepository userRepository;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IAuthenticationService authenticationService;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 6:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 7:</span> <span style="color: #0000ff">public</span> DumbConstructorPresenter(IView view, IUserRepository userRepository,</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 8:</span> IAuthenticationService authenticationService)</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 9:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 10:</span> <span style="color: #0000ff">this</span>.view = view;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 11:</span> <span style="color: #0000ff">this</span>.userRepository = userRepository;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 12:</span> <span style="color: #0000ff">this</span>.authenticationService = authenticationService;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 13:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 14:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 15:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> Initialize()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 16:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 17:</span> var users = userRepository.All();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 18:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 19:</span> view.BindWith(users);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 20:</span> view.SetNumberOfLoggedInUsersTo(users.Where(x => x.IsLoggedIn).Count());</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 21:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 22:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 23:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> Login()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 24:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 25:</span> <span style="color: #0000ff">if</span> (view.HasUserSelected)</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 26:</span> authenticationService.Login();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 27:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 28:</span> }</pre>
<p>
<!--CRLF--></div> </div>
<p>
And a slight change to the view implementation.
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> View : IView</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 2:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">event</span> VoidHandler loginButtonClick = <span style="color: #0000ff">delegate</span> { };</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 4:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">event</span> VoidHandler load = <span style="color: #0000ff">delegate</span> { };</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> DumbConstructorPresenter presenter;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 6:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 7:</span> <span style="color: #0000ff">public</span> View()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 8:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 9:</span> presenter = <span style="color: #0000ff">new</span> DumbConstructorPresenter(<span style="color: #0000ff">this</span>, <span style="color: #0000ff">null</span>, <span style="color: #0000ff">null</span>);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 10:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 11:</span> <span style="color: #008000">// simulate a couple possible events</span></pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 12:</span> load += () => presenter.Initialize();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 13:</span> loginButtonClick += () => presenter.Login();</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 14:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 15:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 16:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> BindWith(IEnumerable<User> users)</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 17:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 18:</span> <span style="color: #008000">// bind this stuff to something</span></pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 19:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 20:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 21:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">bool</span> HasUserSelected</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 22:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 23:</span> get { <span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span>; }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 24:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 25:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 26:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> SetNumberOfLoggedInUsersTo(<span style="color: #0000ff">int</span> numberOfUsers)</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 27:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 28:</span> <span style="color: #008000">// set some label or something</span></pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 29:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 30:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 31:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 32:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">delegate</span> <span style="color: #0000ff">void</span> VoidHandler();</pre>
<p>
<!--CRLF--></div> </div>
<p>
I would prefer this approach since I think it keeps the specs and the presenter implementation itself quite a bit cleaner.  Notice now in the spec: <strong>when_attempting_to_login_and_a_user_is_not_selected.it_should_NOT_perform_a_login.</strong>
</p>
<p>
It cares nothing about retrieving users from the repository.  All it cares about is exactly what the context/spec sentence states.  And if you notice, there is even an opportunity to DRY up these specs, if you prefer, by breaking out a base context class, since they both have identical <strong>EstablishContext()</strong> methods.  This is something that wouldn’t be quite as easy or clean to do in the <strong>SmartConstructorPresenterSpecs</strong> in the previous section.
</p>
<h3>
Quick Tip : Load/Initialize Convention
</h3>
<p>
You might be thinking that you don’t want to have to <em>remember</em> to create or call an Initialize() method all the time.  Well recently, a co-worker of mine pointed out a simple way we could leverage some auto-wiring magic + a base presenter class to make it more of a convention that you can use when needed and to negate the need to manually call <strong>Initialize()</strong> all the time.  So here is one approach to make that easier.
</p>
<p>
NOTE: The presenter specs don’t need to change at all for this refactoring.
</p>
<p>
First we add a Load event handler to our IView interface:
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">interface</span> IView</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 2:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">void</span> BindWith(IEnumerable<User> users);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 4:</span> <span style="color: #0000ff">bool</span> HasUserSelected { get; }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">void</span> SetNumberOfLoggedInUsersTo(<span style="color: #0000ff">int</span> numberOfUsers);</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 6:</span> <span style="color: #0000ff">event</span> EventHandler Load;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 7:</span> }</pre>
<p>
<!--CRLF--></div> </div>
<p>
Then we introduce a base presenter class (which I’m simply calling Presenter ‘cause I think Base* class naming is the new hungarian notation…. :).
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> Presenter<T> <span style="color: #0000ff">where</span> T : IView</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 2:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">public</span> Presenter(T view)</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 4:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 5:</span> view.Load += <span style="color: #0000ff">delegate</span> { Initialize(); };</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 6:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 7:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 8:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">virtual</span> <span style="color: #0000ff">void</span> Initialize()</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 9:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 10:</span> }</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 11:</span> }</pre>
<p>
<!--CRLF--></div> </div>
<p>
Then our <strong>DumbConstructorPresenter </strong>just changes slightly to inherit and override the <strong>Initialize()</strong> method.
</p>
<div>
<div>
<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> DumbConstructorPresenter : Presenter<IView></pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 2:</span> {</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 3:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IView view;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 4:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IUserRepository userRepository;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> IAuthenticationService authenticationService;</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 6:</span>  </pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 7:</span> <span style="color: #0000ff">public</span> DumbConstructorPresenter(IView view, IUserRepository userRepository,</pre>
<p>
<!--CRLF-->
</p>
<pre><span style="color: #606060"> 8:</span> IAuthenticationService authenticationService) : <span style="color: #0000ff">base</span>(view)</pre>
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<pre><span style="color: #606060"> 9:</span> {</pre>
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<pre><span style="color: #606060"> 10:</span> <span style="color: #0000ff">this</span>.view = view;</pre>
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<pre><span style="color: #606060"> 11:</span> <span style="color: #0000ff">this</span>.userRepository = userRepository;</pre>
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<pre><span style="color: #606060"> 12:</span> <span style="color: #0000ff">this</span>.authenticationService = authenticationService;</pre>
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<pre><span style="color: #606060"> 13:</span> }</pre>
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<pre><span style="color: #606060"> 14:</span>  </pre>
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<pre><span style="color: #606060"> 15:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">override</span> <span style="color: #0000ff">void</span> Initialize()</pre>
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<pre><span style="color: #606060"> 16:</span> {</pre>
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<pre><span style="color: #606060"> 17:</span> var users = userRepository.All();</pre>
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<pre><span style="color: #606060"> 18:</span>  </pre>
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<pre><span style="color: #606060"> 19:</span> view.BindWith(users);</pre>
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<pre><span style="color: #606060"> 20:</span> view.SetNumberOfLoggedInUsersTo(users.Where(x => x.IsLoggedIn).Count());</pre>
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<pre><span style="color: #606060"> 21:</span> }</pre>
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<pre><span style="color: #606060"> 22:</span>  </pre>
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<pre><span style="color: #606060"> 23:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> Login()</pre>
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<pre><span style="color: #606060"> 24:</span> {</pre>
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<pre><span style="color: #606060"> 25:</span> <span style="color: #0000ff">if</span> (view.HasUserSelected)</pre>
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<pre><span style="color: #606060"> 26:</span> authenticationService.Login();</pre>
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<pre><span style="color: #606060"> 27:</span> }</pre>
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<pre><span style="color: #606060"> 28:</span> }</pre>
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And finally our view implementation.
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<pre><span style="color: #606060"> 1:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">class</span> View : IView</pre>
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<pre><span style="color: #606060"> 2:</span> {</pre>
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<pre><span style="color: #606060"> 3:</span> <span style="color: #008000">// in the context of a winforms app, </span></pre>
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<pre><span style="color: #606060"> 4:</span> <span style="color: #008000">// this event would be automatically declared and fired</span></pre>
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<pre><span style="color: #606060"> 5:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">event</span> EventHandler Load = <span style="color: #0000ff">delegate</span> { };</pre>
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<pre><span style="color: #606060"> 6:</span>  </pre>
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<pre><span style="color: #606060"> 7:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">event</span> VoidHandler LoginButtonClick = <span style="color: #0000ff">delegate</span> { };</pre>
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<pre><span style="color: #606060"> 8:</span> <span style="color: #0000ff">private</span> <span style="color: #0000ff">readonly</span> DumbConstructorPresenter presenter;</pre>
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<pre><span style="color: #606060"> 9:</span>  </pre>
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<pre><span style="color: #606060"> 10:</span> <span style="color: #0000ff">public</span> View()</pre>
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<pre><span style="color: #606060"> 11:</span> {</pre>
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<pre><span style="color: #606060"> 12:</span> presenter = <span style="color: #0000ff">new</span> DumbConstructorPresenter(<span style="color: #0000ff">this</span>, <span style="color: #0000ff">null</span>, <span style="color: #0000ff">null</span>);</pre>
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<pre><span style="color: #606060"> 13:</span>  </pre>
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<pre><span style="color: #606060"> 14:</span> <span style="color: #008000">// simulate a possible login button click</span></pre>
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<pre><span style="color: #606060"> 15:</span> LoginButtonClick += () => presenter.Login();</pre>
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<pre><span style="color: #606060"> 16:</span> }</pre>
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<pre><span style="color: #606060"> 17:</span>  </pre>
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<pre><span style="color: #606060"> 18:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> BindWith(IEnumerable<User> users)</pre>
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<pre><span style="color: #606060"> 19:</span> {</pre>
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<pre><span style="color: #606060"> 20:</span> <span style="color: #008000">// bind this stuff to something</span></pre>
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<pre><span style="color: #606060"> 21:</span> }</pre>
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<pre><span style="color: #606060"> 22:</span>  </pre>
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<pre><span style="color: #606060"> 23:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">bool</span> HasUserSelected</pre>
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<pre><span style="color: #606060"> 24:</span> {</pre>
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<pre><span style="color: #606060"> 25:</span> get { <span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span>; }</pre>
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<pre><span style="color: #606060"> 26:</span> }</pre>
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<pre><span style="color: #606060"> 27:</span>  </pre>
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<pre><span style="color: #606060"> 28:</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span> SetNumberOfLoggedInUsersTo(<span style="color: #0000ff">int</span> numberOfUsers)</pre>
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<pre><span style="color: #606060"> 29:</span> {</pre>
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<pre><span style="color: #606060"> 30:</span> <span style="color: #008000">// set some label or something</span></pre>
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<pre><span style="color: #606060"> 31:</span> }</pre>
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<pre><span style="color: #606060"> 32:</span> }</pre>
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Notice in the <strong>Presenter<T></strong> class above, we’re simply wiring up the <strong>Load</strong> event from the <strong>IView</strong> to automatically call the <strong>Initialize()</strong> method for us.  Concrete presenters could then simply just override the <strong>Initialize()</strong> method if necessary.
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Then notice in the updated view implementation, we don’t have explicitly call the <strong>Initialize() </strong>method anymore in our views, since that’s done for us automatically in the <strong>Presenter<T></strong> class.
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As I’m writing this I realize that the act of overriding this <strong>Initialize()</strong> method would have to be <em>remembered</em> as well.  But I think what you gain from a convention standpoint and the fact that you don’t have to explicitly call it from the views is enough of a win to warrant this technique.  But as always, I’ll probably change my mind tomorrow when I figure out a better way to do all of this.  🙂
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So, putting aside the possibly poor examples and the cloud of MVP in all of this, my goal was just to simply give an example of why it can be a good idea to keep your constructors simple and lean across the board, in many different types of classes.
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Of course feedback is always welcome.
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