Clever Geek Handbook

ViewModel Binding Model (WPF)

I am doing the transition from MVP to MVVM and am a little confused as to how best to bind ViewModel

to Model

. I understand how we can use the WPF data binding framework to route events between View

and ViewModel

using an interface ICommand

and INotifyPropertyChanged

for example View

:

public class MyView
{
    public MyView()
    {
        InitializeComponent();
        DataContext = new MyViewModel();
    }
}

and ViewModel

:

public class MyViewModel : INotifyPropertyChanged
{
    public MyViewModel(){}

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        var handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }

    public ICommand MyCommand ... 
}

This works great!

Now, generally with MVP, I would bind the Presenter

link to Model

via constructor injection and increment events to Model

from Presenter

to update the data in Model

. I've tried the same approach with MVVM, but it requires ViewModel

taking Model

as a dependency in my constructor, which seems to make some things useless with MVVM when using it straight out of the box without any form of IOC (at least with WPF) ...

So my two questions are:

  • Is this introducing Model

    into the ViewModel

    right approach or should I implement the interface INotifyPropertyChanged

    on Model

    and using the WPF binding framework?
  • To take advantage of MVVM, should you almost always implement it with an IOC and a DI container, or better yet, Prism?
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  • This is a "pure" MVVM approach: it View

    should only depend on ViewModel

    . Itself ViewModel

    is a bridge between View

    and Model

    .

    Motivation - definition and responsibilities Model

    , View

    and ViewModel

    as well as their relationship:

    • A model that provides a view-independent view of your business objects. The design of the model is optimized for logical relationships and operations between your business objects, regardless of how the data is presented in the user interface.
    • The View class, which is the user interface . It displays information to the user and fires events in response to user interaction.
    • The ViewModel that is the bridge between the view and the model . Each View class has a corresponding ViewModel class. The ViewModel retrieves data from the Model and processes it in the format required by the View . It notifies the View if the underlying data in the Model has changed, and it updates the data in the Model in response to UI events from the View.

    - Implementation of Model-View-ViewModel Pattern, MSDN .

    Conclusion. Typing Model

    in ViewModel

    seems to be the correct approach. Also, instead of injection, it Model

    may be helpful to type:

    • Model Factory

      to create Model

      - lazy initialization;
    • Service

      ( Service Facade

      ) to get Model

      - lazy loading.

  • As shown in the book "MVVM Unleashed" by Michael Brown , the potential benefits of MVVM can be exploited: maintainability, testability, "mixability", portability. At the very least, dependency injection (in the described case using a dependency injection container) allows the design to follow the principle of dependency inversion:

    Dependency Inversion is at the heart of many of the claimed benefits of object-oriented technology. Its correct application is essential to create reusable frameworks. It is also critical for building code that can be changed. And, because abstractions and details are isolated from each other, the code is much easier to maintain.

    - Dependency Inversion Principle, Robert C. Martin, 1996 .

    As a result, these are the benefits of MVVM, as maintainability and testability seem to be improved by following the Dependency Inversion Principle. The dependency injection container is just a tool to follow the principle.



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