Thursday, August 23, 2012

Caliburn.Micro: Using a local resource as a proxy to your viewmodel

For a project we are doing we are using the ArcGis WPF components. As we want to use a MVVM architecture we try to use databind wherever possible(by using Caliburn.Micro). The problem is that one of the WPF components (the GpsLayer) inherits from DependencyObject, but the DependencyObject is not linked to a DataContext. Microsoft states that the DO should inherit the DataContext from the containing FrameworkElement, but that in fact doesn't happen.
The only samples we found that worked were using the StaticResource approach:
<Window.Resources>
     <local:MainViewModel x:Key="MainViewModel" />
</Window.Resources>

<Grid DataContext="{StaticResource MainViewModel}">

    <esri:Map Extent="-14268281.1311858,2195120.17402859,-7232639.54776086,7467160.93387503">
    <esri:ArcGISTiledMapServiceLayer Url="http://services.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer" />
      <esri:GraphicsLayer ID="MyGraphicsLayer" 
                          GraphicsSource="{Binding SearchResults, Source={StaticResource MainViewModel}}"/>
    </esri:Map>
</Grid>

But in this case, the MainViewModel was instantiated when the View is constructed, resulting in a View-First approach. We wanted a ViewModel first approach and let Caliburn.Micro do the hard work for us.

After trying lots of possible solutions, the ArcGis forums finally brought some help. There they suggested to create a DataContextProxy object to proxy access to the ViewModel.

public class DataContextProxy : Freezable
    {
        #region Overrides of Freezable

        protected override Freezable CreateInstanceCore()
        {
            return new DataContextProxy();
        }

        #endregion

        public object Data
        {
            get { return (object)GetValue(DataProperty); }
            set { SetValue(DataProperty, value); }
        }

        public static readonly DependencyProperty DataProperty =
            DependencyProperty.Register("Data", typeof(object), typeof(DataContextProxy), new UIPropertyMetadata(null));
    }

And in the XAML you can use it like this:

<esri:Map>
    <esri:Map.Resources>
            <local:DataContextProxy x:Key="proxy" Data="{Binding}" />
    </esri:Map.Resources>
    <esri:ArcGISTiledMapServiceLayer ID="PhysicalTiledLayer"
              Url="http://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer"/>

    <esri:GraphicsLayer ID="MyGraphicsLayer"
            GraphicsSource="{Binding Path=Data.SearchResults, Source={StaticResource proxy}}" Renderer="{StaticResource MySimpleRenderer}" />
</esri:Map>

1 comment:

Павел Крымец said...

Thank you SO MUCH. I had problems wit he same ArcGis layer that you did and struggled two days on it. You are my saviour. Thanks again.