无法正确实施WPF MVVM

Question

Using ArcGIS Runtime .Net SDK 10.2.7, with MVVM pattern I am getting 'System.NullReferenceException' . in ViewModel constructor at:

this.mapView.Map.Layers.Add(localTiledLayer);

What am I doing wrong?

What I have is:

1- ViewModel.cs

public class ViewModel : INotifyPropertyChanged
{ 
    private Model myModel = null;
    private MapView mapView = null;
    public event PropertyChangedEventHandler PropertyChanged;
    public ViewModel()
    {
        var URI = "D:/GIS/Runtime/Tutorial/VanSchools.tpk";
        ArcGISLocalTiledLayer localTiledLayer = new ArcGISLocalTiledLayer(URI);
        localTiledLayer.ID = "SF Basemap";
        localTiledLayer.InitializeAsync();

        this.mapView.Map.Layers.Add(localTiledLayer);

    }
    public string TilePackage
    {
        get { return this.myModel.TilePackage; }
        set
        {
            this.myModel.TilePackage = value;

        }
    }

2- Model.cs

public class Model
{
    private string tilePackage = "";

    public Model() { }

    public string TilePackage
    {
        get { return this.tilePackage; }
        set
        {
            if (value != this.tilePackage)
            {
                this.tilePackage = value;
            }
        }
    }

3- MainWindow.xaml

<Window x:Class="SimpleMVVM.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:esri="http://schemas.esri.com/arcgis/runtime/2013"
        xmlns:local="clr-namespace:SimpleMVVM"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">

        <Window.Resources>
            <local:ViewModel x:Key="VM"/>
        </Window.Resources>

        <Grid DataContext="{StaticResource VM}">
            <Grid.RowDefinitions>
                <RowDefinition Height="400" />
                <RowDefinition Height="200" />
            </Grid.RowDefinitions>

            <esri:MapView x:Name="MyMapView" Grid.Row="0"
                      LayerLoaded="MyMapView_LayerLoaded" >
                <esri:Map>  </esri:Map>
            </esri:MapView>

        </Grid>
</Window>

4- MainWindow.xaml.cs

  public partial class MainWindow : Window
        {
            public MainWindow()
            {
                InitializeComponent();
            }

            private void MyMapView_LayerLoaded(object sender, LayerLoadedEventArgs e)
            {
                if (e.LoadError == null)
                    return;

                Debug.WriteLine(string.Format("Error while loading layer : {0} - {1}", e.Layer.ID, e.LoadError.Message));
            }

        }

Answer 1

It looks like the problem is that mapView has not been set to an instance of an object. Do you need to say something like:

this.mapView = new Mapview();
this.mapView.Map.Layers.Add(localTiledLayer);

Answer 2

It seems that you aren't hooking up the binding but in general you shouldn't have reference to the MapView from your ViewModel and should bind MapView.Map instead. Map is considered being a model that defines how your MapView looks.

In your view make sure that you have the binding setup

<Window.Resources>
    <vm:MainViewModel x:Key="vm"></vm:MainViewModel>
</Window.Resources>
<Grid DataContext="{StaticResource vm}">
    <esri:MapView x:Name="MyMapView"
                  Map="{Binding Map}"
                  LayerLoaded="MyMapView_LayerLoaded">
    </esri:MapView>
</Grid>

Then in your ViewModel you can do something like this

public class MainViewModel : INotifyPropertyChanged
{
    public MainViewModel()
    {
        // Create map that is concidered to being a model that defines the content
        // of the map
        var map = new Map();
        // Add your layers to the map
        var testServicePath = "https://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer";
        map.Layers.Add(new ArcGISTiledMapServiceLayer(new Uri(testServicePath)));
        // map.Layers.Add(new ArcGISLocalTiledLayer("add your path here"));
        Map = map;
    }

    private Map _map;
    public Map Map
    {
        get
        {
            return _map;
        }
        set
        {
            _map = value;
            NotifyPropertyChanged(nameof(Map));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}