MVP Pattern Recommendations for C# (.NET)

1. Overview

The main reason for separating view and logic is unit testing.

There are many patterns for separating view and logic (MVC, MVP, MVVM or even MVP-VM). Comparison is not subject of the article and i just note main advantages of MVP:

• MVP is very simple pattern.
• MVP allows to test more than other patterns and tests are more clear.

Classical MVP is too abstract pattern. Real implementation generally has more than 3 participants. This article is practical guide for using and testing of MVP.

2. Class Diagram

3. Sequencing diagram

• Presenter communicates with view directly

• Presenter communicates with view within Model (using binding)

4. MVP binding:

• Inside view

// Constructor
public View()
{
  var presenter = new Presenter(this);
}

// view creation
var view = new View();
view.Show();

• Outside View (for multiple Presenter implementations)

var view = new View();
var presenter = new Presenter(view);
view.Show();

5. Presenter initialization

Presenter(IView view)
{
	this.view = view;
	this.model = new Model();
	view.FormLoadEvent += this.Load;
	view.ButtonClickEvent += this.Click;
}

6. Firing events in View

• Simplifying firing:

form.Load += (s, args) =>{ FormLoadEvent(); };
button.Click += (s, args) =>{ ButtonClickEvent(args); };

• Using Commands: todo

7. Handling events in Presenter:

void view_FieldValidating(CancelEventHandler e)
{
	e.Cancel = /* expression */;
}

8. Presenter interaction with view.

void Presenter.DoSomething()
{
  view.BindModel(model);
  view.AskUser();
  view.MakeBeep();
}

9. Using Services.

• Class diagram

• Binding:

var presenter = new Presenter(view, new Service());

10. Encapsulation.

To hide internal implementation of component inside assembly declare interface as "internal" and implement it explicitly in view (do it if you really need it).

internal interface IView
{
	void f();
	event Action<string> act;
}
public class View : IView
{
	private object objectLock = new Object(); 
	internal event Action<string> act;

	void IView.f() { }

	event Action<string> IView.act
  {
	   add { lock (objectLock) { act += value; } }
	   remove { lock (objectLock) { act -= value; } }
  }
}

11. Unit Testing with Moq

• Raising events:

var viewMock = new Mock<IView>();
var presenter = new Presenter(viewMock.Object);
viewMock.Raise (m =>m.LoadEvent += null, EventArgs.Empty);
Assert.AreEqual( … );

• Check pulling/calling of view/service methods:

/* view methods invocation verification */
viewMock.Verify(v =>v.MakeBeep(), Times.Once()); 
viewMock.Verify(v =>v.AskUser(), Times.Once()); 
/* service methods invocation verification */
viewMock.Verify(s =>s.SaveData(), Times.Once()); 

• Use Friend Assemblies to test “internal” members (attribute InternalsVisibleTo). See details on https://msdn.microsoft.com/en-us/library/0tke9fxk.aspx

12. Complex scenarios

• Extending logic of Presenter (composition/aggregation is preferable from Unit Testing perspective).

var view =new View();
var presenter1 = new Presenter1(view);
var presenter2 = new Presenter2(view, presenter1);
view.Show();

• Interaction of several views.
  • Class diagram:

• Main view interacts with child within public contract of child view.

MainView.SomeMethod()
{
	view1.Refresh();
	view2.Refresh();
}

• Child view interacts with main within itself public events

public MainView()
{
	// event subscription
	view1.DataUpdated += this.DataUpdatedHandler;
}

View1.SomeMethod()
{
	// firing event
	DataUpdated();
}

13. Example of implementation

https://github.com/agre1981/MVP