Hello again! In this post I’m going to talk about building an MVVM framework.
As I said in the previous post, this post should be about the OrderView. This view should allow users to pick items from a list of products, add them to the actual order, choose quantities, view the current total price and submit the order. Also the user should be able to filter, sort and do pagination on the product list. I’m sure you’ve seen enough blog posts from other people talking about this subject, that is getting the selected product, add it to another collection or create an order detail based on it, then update some other data on the UI and finally submit the changes back to the server. The thing is, everyone has its own way for programming and eventually when you end up in a team, you may find that two people coded the same thing in a different way, and one has a bug in situation A and the other has a bug in situation B. Having a good MVVM framework with a well defined methodology is a must to prevent these situations. In this post I want to talk about essential components you must have in an MVVM framework. Later, I’ll describe an MVVM Framework I’ve been working on which was based on WCF RIA Services but doesn’t really depend on it.
Since we’re following best practices, we know that using a good MVVM architecture we can come up with a solution whose fetching, filtering, sorting, paging logic is entirely separated from the view, allowing us to also have different views for the same view model. For example, we can start by using a DataGrid and a DataPager to display our items but later provide a new view that uses comboboxes to select the sort options, an album-like listbox to show the items and custom buttons for paging. Also, we should be able to separate all this logic from its actual data access logic to be able to use mock objects for our model and do unit tests for our viewmodels. That’s not an easy task but that’s what I want to achieve from now on.
Well, to start, .NET / Silverlight already offers us some classes and interfaces that are very handy for MVVM scenarios.
- INotifyPropertyChanged – Used to raise an event when a property changes. WPF / Silverlight Binding framework use this interface to update the view when a property changes.
- INotifyCollectionChanged – Used to raise an event when an insert, remove, clear or replace operation has been done against a collection. WPF / Silverlight controls that have an ItemsSource property usually use this interface to create or delete visual items in a container. For example, ListBoxes display new ListBoxItems, DataGrids display new DataGridRows.
- ICollectionView – Used to provide filter, sort descriptions, group descriptions, and item selection for an IEnumerable collection and have the view display only the filtered items, sorted according to the sort descriptions and highlight the selected item. (Has more features but these are the most relevant for the sake of this post).
- IPagedCollectionView – Used to provide paging options to an IEnumerable collection. This is used by DataPagers mostly, that make calls to the MoveToPage(int pageIndex) method and allows us to register in the PageChanging event and fetch a new page of entities to be displayed.
- There are other important interfaces like IEditableObject, IEditableCollectionView but I’m not going to cover those in this posts. They are used to update property values of an object in an atomic fashion.