1. or How I Learned to Stop Worrying and Love the Binding Bryan Anderson MVVM in the Real World

2. Vocabulary Presentation Model pattern MVVM Architecture Compare MVVM to MVC Religious Fights Common Issues & Solutions What am I going to talk about?

3. //Notifies clients that a property value has changed. publicinterfaceINotifyPropertyChanged { //Occurs when a property value changes. eventPropertyChangedEventHandler PropertyChanged; } INotifyPropertyChanged

4. //Notifies listeners of dynamic changes, such as when items get //added and removedor the whole list is refreshed. publicinterfaceINotifyCollectionChanged { //Occurs when the collection changes. eventNotifyCollectionChangedEventHandler CollectionChanged; } //Describes the action that caused a CollectionChangedevent. publicenumNotifyCollectionChangedAction { Add, Remove, Replace, Move, Reset } INotifyCollectionChanged

5. Keeps data in two properties “bound” together Allows data to flow in either or both directions Uses PropertyChanged & CollectionChanged to keep values updated Data Binding

6. Implements IValueConverter or IMultiValueConverter object Convert(object value, Type targetType, object parameter, CultureInfo culture) object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) Converts inputs from one value to another and back Used extensively in data binding Value Converter

7. Looks like a standard CLR property on the class’s interface Adds additional infrastructure features like Data Binding Change Notification Value Coercion Used extensively by WPF and Silverlight to create fast, reactive user interfaces Dependency Property

8. Groups a related set of controls Reduces repetition – DRYer Has its own code behind – allows new dependency properties User Control

9. An implementation of the Command Pattern In WPF & Silverlight the ICommand interface is used to define a class that represents a command bool CanExecute(object parameter) void Execute(object parameter) event EventHandler CanExecuteChanged Frequently implemented by creating a DelegateCommand class Commands

10. INotifyPropertyChanged INotifyCollectionChanged Data Binding Value Converter Dependency Property User Control Commands Questions?

11. Guide the separation of concerns Encapsulate what changes UI Web services Databases Anything else that interfaces with something outside the application The Purpose of Architecture Patterns

12. Separate presentation from functionality Testability Consistency Maintainability Allow a combined Blend/Visual Studio workflow Goals of the Presentation Model

13. Abstract Pattern that defines a family of platform/framework specific patterns MVVM – WPF, Silverlight MVC – Web MVP – Winforms Designed to pull the state and behavior out of the View into a more stable and testable layer Presentation Model View Presentation Model Model

14. Abstract Pattern that defines a family of platform/framework specific patterns MVVM – WPF, Silverlight MVC – Web MVP – Winforms Designed to pull the state and behavior out of the View into a more stable and testable layer Presentation Model View Presentation Model Model

15. MVVM Architecture – Model Responsible for business logic Interacts with persistent storage, e.g. database, web services, files, etc. Capable of being shared among related applications View Unit Tests View Model Model File System Web Services Database

16. MVVM Architecture – View Model Should be thought of as an Abstract View Exposes properties and commands for use in a View Gets data from the Model and updates it according to property changes and command calls View Unit Tests View Model Model File System Web Services Database

17. MVVM Architecture - View Presents data to the user Handles input Interacts with the View Model by data binding to properties and commands Should contain no logic that isn’t entirely a view concern, e.g. which screen to show View Unit Tests View Model Model File System Web Services Database

18. MVVM Architecture - Tests Acts as another View Interacts with the Model and View Model to ensure correct behavior Provide an early warning that the architecture isn’t correct If testing is difficult, coupling is too high or concerns aren’t separated View Unit Tests View Model Model File System Web Services Database

19. The Model holds data and handles business logic The Controller receives input and makes changes to the View and Model accordingly The View renders the Model into a form suitable for interaction MVC Controller Model View

20. Religious Fights

21. Multiple, mutually exclusive options People form strong opinions and will argue the benefits of their choice for all eternity Each side has its own benefits and drawbacks A decision needs to be made before the “correct choice” can be determined In the end it comes down to a combination of reason and instinct Religious Fights

22. View Model Driven Allows more complete testing of logic to open new Views/View Models Tends to be DRYer as applications grow More difficult to use in Silverlight projects Religious Fights – Round 1 View Driven Easy to track which View Model is used by a View Much easier to use when sharing code between WPF and Silverlight Difficult or confusing for a single View to be used with multiple View Models

23. How much code should I actually have in the View’s code behind? Value converters are fine as long as they’re only converting values User Control code behind should only contain Dependency Property definitions and rare view-specific event handlers DataTemplateSelectors as long as their code is limited to choosing between DataTemplates Otherwise there should be no code-behind Religious Fights – Round 2

24. Should the View be allowed to use/display a Model class directly or should all interactions be through the View Model? The View should never edit a Model instance My rule of thumb – Only allow the view to display immutable Model objects, even then try to avoid it It should be considered technical debt if done, but it’s a relatively harmless area to take on debt Religious Fights – Round 3

25. Where should INotifyPropertyChanged and INotifyCollectionChanged be used? Definitely in the View Model Occasionally in the View - usually in Custom Controls and more rarely User Controls It’s becoming more accepted to them in the Model unless added to push updates to a View Religious Fights – Round 4

26. Notifying users from a View Model, i.e. How do I show a message box? How do I handle non-command events without code behind? Something should or should not be shown based on state. What goes in the View vs. View Model? Should I use an additional property or the command parameter to pass contextual data to a View Model’s command? View Models seem to copy all of my Model’s properties Common Issues & Solutions

27. Message boxes are annoying Notifying users from a View Model

28. Message boxes are annoying Messages to the user are useful Use a mediator – Allows you to aggregate messages from all of your VMs, log them, and display them. Also easy to test. If you only need it in one or two places an Error/Notification property or, more rarely, event works well. Implement the IDataErrorInfo interface for notifications related to property validation Notifying users from a View Model

29. Usually done using an attached behavior, search for “Attached Command Behavior” Can it be handled by binding? If absolutely necessary and there’s no logic involved it can be put into code behind. Handling events without code behind

30. Code Behind & Gotos http://xkcd.com/292 Beware of Raptor Attacks

31. Is it a View-only concern? E.g. Master/Detail view Bind the detail view to the selected master item Is it a View Model concern? E.g. IsEditMode Use a DataTemplateSelector if available to switch Views Use Triggers, Visual State Manager, or another method to switch views based on the View Model properties Is it a combination? E.g. IsBusy Use the Visual State Manager if available Visibility based on state, what goes in the View vs. View Model?

32. Would the property be referenced anywhere outside of the command(s)? Does the property make sense in the context of the View Model’s interface? Is the command used in multiple places that would need to set the property to different objects? Property or command parameter?

33. View Models are not just Façades or Adapters for Model classes View Models are not the code behind for a View View Models are an Abstract View or the Model for a View View Models will present multiple Models and multiple View Models might use the same Model View Models just copy my Models

34. I love feedback! Bryan Anderson bryan.anderson@ilmservice.com @SolaAesir Questions?