ViewModel First – ViewCaching

With View model first approach, view are usually created via DataTemplate, and each time a view model is injected in the content control, the corresponding view is recreated.

If you have complex view where it take bit time to create view, you may see the performance hits. To avoid the performance hit you may want to cache the view and use the cached view when available. This can be done via creating CacheContentControl and you choice of view factory.

The basic idea is to wrap the view inside CacheContentControl and delegate the view creation logic to your view factory. The attached sample uses weakreference view factory,say if GC has not collected the view, you may use the cached view instead of creating view each time. This will work with controls like tab control docking controls etc.

The CacheContentControl  code

public class CacheContentControl : ContentControl
    {
        public CacheContentControl()
        {
            Unloaded += ViewCache_Unloaded;
            ViewFactory = WeakReferenceViewFactory.Instance;
        }

        void ViewCache_Unloaded(object sender, RoutedEventArgs e)
        {
            Content = null;
        }

        private Type _contentType;
        public Type ContentType
        {
            get { return _contentType; }
            set
            {
                _contentType = value;
               //  use you favorite factory
                Content = ViewFactory.GetView(value);
            }
        }

        public IViewFactory ViewFactory { get; private set; }
    }

In the DataTemplate, use the ViewCache, pass the type of the real view you want to use:

<DataTemplate DataType=”{x:Type moduleB:Panel3Vm}”>

<Border Background=”Green”>

<local:CacheContentControl ContentType=”{x:Type moduleB:Pane3 }” Margin=”5″/>

</Border>

</DataTemplate>

The example uses standard data template (red option) to demonstrate each time you navigate to panes the corresponding views are recreated with new hash code.

If you select CacheContentControl (green option) and navigate the view is only created once. If you click on button to collected the GC then view are created on navigation if they are claimed by GC.

Why ViewModel first ?

I prefer to use view model first approach. For many reasons:

• Vms are your application containing most of logic apart from glue code in form of behaviors or triggers.
• If you creates views then you are responsible for its life and cleanup code. You have to deal with threading and other issues which are difficult to test. On the other hand of you create vms and leave the view creation logic with WPF via data template.. you don’t have to worry about threading issue. And there will be better separation of concerns.
• With vm first approach zero code behind.
• With a project level isolation for view and vms you can restrict developers using view specific things like dispatcher in the view model leaving more cleaner and testable code base. I.e view project sprojec to vm. And vm project should not refer to any presentation lib.
• If there is clear boundary between view and vm. Both can evolve and will be less fragile.
• Allows more complete testing of logic to open new Views and ViewModels
• Tends to be DRYer (don’t repeat yourself ) as applications get larger
• View and ViewModel are more independent and can be worked on separately more easily.