Feeds:
Posts
Comments

Posts Tagged ‘SharpMap’

On the way of introducing a compass to my GIS prototype, I came across an implementation idea that I would like to share.

Well structured and designed applications often separate user interface and domain business layers. Data binding is one of the great means introduced in WPF that aims and helps developers efficiently decouple their user interfaces from the rest of the application.

The following XAML code creates a window containing a slider and an image control. The idea is to rotate the image by a rotation angle given by the slider control.
<Window x:Class="Exercises.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="278" Width="356">
<StackPanel Name="stackPanel">
<Label Content="{Binding ElementName=slider, Path=Value}" Height="28" Name="label1" />
<Slider Name="slider" Height="23" Margin="12,58,0,0" Width="307" Maximum="360" ValueChanged="slider_ValueChanged" />
</StackPanel>
</Window>

If you are coming from WinForms world, you would capture the ValueChangedEvent of the slider and do the following:

private void slider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e) {
   double rotationAngle = slider.Value;
   stackPanel.Children.Remove(compassDrawing);
   compassDrawing = DrawImage(rotationAngle);
   stackPanel.Children.Add(compassDrawing);
}
UIElement DrawImage(double rotationAngle) {
   // Create a 100 by 100 image with an upper-left point of (75,75).
   ImageDrawing compassImage = new ImageDrawing();
   compassImage.Rect = new Rect(0, 0, 200, 200);
   compassImage.ImageSource = new BitmapImage(
    new Uri(@"D:\Documents\Visual Studio 2010\Projects\Exercises\Exercises\Images\compassImage.png", UriKind.Relative));
    // Use a DrawingImage and an Image control to display the drawing.
    DrawingImage drawingImageSource = new DrawingImage(compassImage);
    // Freeze the DrawingImage for performance benefits.
    drawingImageSource.Freeze();

    Image imageControl = new Image();
    imageControl.Stretch = Stretch.None;
    imageControl.Source = drawingImageSource;
    imageControl.RenderTransform = new RotateTransform(rotationAngle, compassImage.Rect.Width/2, compassImage.Rect.Height/2);
    // Create a border to contain the Image control.
    Border imageBorder = new Border();
    imageBorder.Child = imageControl;
    return imageBorder;
}

The slider_ValueChanged event handler will replace the previous image in the main stack panel by a newly generated image. The DrawImage() method will draw a new rotated image given an angle as a parameter.

Figure 1 : Non rotated image

Figure 1 shows a non rotated image that is shown either at application start up or when the slider’s value is set to 0 or 360 degrees.

Figure 2 in the other hand shows the result when the slider is set to a given value:

Figure2: Rotated image

If we look back to the code now, there is clearly a smell of a binding possibility out there!

The compass image can have a property, say Rotation which, whenever changed, the image rotates automatically without involving event handlers.

Looking to the UI controls used above, none of them supports this functionality. A great opportunity for us to do some custom data binding.

Advertisements

Read Full Post »

Hi,

Now that I could see a visual representation of my project, next is sharpening the architecture and design of the application.

One design pattern that comes  very often when reading about WPF is MVVM (http://en.wikipedia.org/wiki/Model_View_ViewModel). I used this pattern and separate my GUI business from my domain objects, I struggled a bit at the beginning but finally managed to get something out.

Before moving to the steps taken to get an MVVM architecture out of my application, let’s have a quick look at the current implementation. In my previous post I directly used the map object from SharpMap and displayed it in the GUI as an image:

1- Load a shape file using SharpMap API

2- Render the shape file into an image

3- Use the resulting image and display it directly in the main window

Here is the pieces of code used for this purpose

// Event handler for button click
public void OpenShapeFileClicked(object sender, EventArgs e){
      // ... ask the user to provide a shape file using a file open dialog control
      LoadShapeFile(path_provided_by_user);
}
public void LoadShapeFile(string fileName){
      SharpMap.Layers.VectorLayer myLayer = new SharpMap.Layers.VectorLayer("ShapeFiles");
      myLayer.DataSource = new SharpMap.Data.Providers.ShapeFile("path_to_file", true);
      myLayer.Style.Fill = System.Drawing.Brushes.GreenYellow;
      myLayer.Style.Outline = new System.Drawing.Pen(System.Drawing.Brushes.Gray);
      myLayer.Style.Line.Width = 2;
      myLayer.Style.Line = new System.Drawing.Pen(System.Drawing.Brushes.Black);
      myMap.Layers.Add(myLayer);
      myMap.ZoomToExtents();
      DrawImage();
}
private void DrawImage(){
      // Convert System.Drawing.Image to ImageSource to wich an image is bound
      using (System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(myMap.GetMap())){
             IntPtr hBitmap = bitmap.GetHbitmap();
             // Use System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap
             BitmapSource bitmapSource = CreateBitmapSourceFromHBitmap(
                hBitmap,
                IntPtr.Zero,
                Int32Rect.Empty,
                System.Windows.Media.Imaging.BitmapSizeOptions.FromEmptyOptions());
             image1.Source = bitmapSource;
      }
}

This is the usual way most programmers will do and especially those writing WinForms applications. Obviously not  the way to go as the domain code is linked to the GUI which makes it very hard to read and maintain.

What we want here is certainly not coupling the GUI with the business domain layer. In order to achieve separation of concerns, we will make use of the MVVM pattern and data binding.

Before looking on how to bind the map to the main GUI, let’s first have a look at the different artifacts of the application and where do they fit in MVVM:

Model : the map data structure, contains GIS data organized in layers as per the GIS simple feature specifications

View : a very thin layer communicating directly to the WPF main window

ViewModel: A new class that is introduced to separate the view from the model

Let’s see this pattern in action in a real life example: loading shape files! Let’s look at how shape files are loaded after moving the application’s architecture to MVVM.

The main control worth mentioning here is the Canevas that will hold the map. This control is updated upon model change notifications.

In order to be aware of all domain objects data changes, the main window will subscribe to change notifications coming from the map model (domain objects):

// Listen to model changes
mapView.PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(MapViewModelPropertyChanged);

This bit of code is placed on the main window constructor. This is a simple way of subscribing to all model data changes, albeit not a perfect one ( WPF encourages binding to the model directly on the XAML)

The ‘Load shape file’ button on the main window is bound to a command on the view class that will take care of loading all needed data

public ICommand LoadShapeFileCommand {
 get {
   if (loadShapeFileCommand == null) {
        // Use Lambda calculus to run the proper method when this command is applied
        loadShapeFileCommand = new GenericCommand(param => LoadShapeFile(), param => (mapViewModel != null));
   }
   return loadShapeFileCommand;
 }
}

The command patter used relies on Josh Smith’s relayCommand implementation (http://msdn.microsoft.com/en-us/magazine/dd419663.aspx)

The shape file is loaded as previously done, using SharpMap API. The map rendering is however now done in a different way using WPF rendering services instead of GDI+.
public void LoadShapeFile(string shapeFileName) {
      ...
      // The model has changed, notify the GUI of the change
      PropertyChangedEventArgs args = new PropertyChangedEventArgs("MapSnapshot");
      PropertyChanged(this, args);
}

The PropertyChanged event will end up in the main window.

        private void MapViewModelPropertyChanged(object sender, PropertyChangedEventArgs args) {
            string propertyName = args.PropertyName;
            if (propertyName == "MapSnapshot") {
                // Map snapshot has changed, reload it
                canvas1.Children.Clear();
                canvas1.ClipToBounds = true;
                canvas1.Children.Add(mapView.MapDrawing);
            }
        }
When doing this, watch out to properly setup the property name that has changed in the PropertyChangedEventArg. This is very important as it helps identify which part of the model did change (needs redrawing)
As you can see from the above code snapshot, the Canvas is updated by adding “mapView.Drawing” visual elements into its children.
public class MapView : FrameworkElement, INotifyPropertyChanged, IDisposable {
        private System.Windows.Media.VisualCollection children;
        ....
        public UIElement MapDrawing {
            get {
                return GetMapDrawing(mapViewModel.map);
            }
        }
        private UIElement GetMapDrawing(Map map) {
            children.Clear();
            VectorRenderer.map = map;
            foreach (SharpMap.Layers.ILayer layer in map.Layers) {
                // Render layer into layer drawing
                System.Windows.Media.DrawingVisual layerDrawing = VectorRenderer.Render(layer, map.Envelope);
                // Add rendered layer into drawing group
                AddVisualLayer(layerDrawing);
            }
            return this;
        }
        private void AddVisualLayer(System.Windows.Media.DrawingVisual visualLayer) {
            children.Add(visualLayer);
        }

As the rendering engine I’m working on these days is still “work in progress” only vector data can be rendered.

In this post, we have been through the main steps taken to put the MVVM pattern in action.

I also made some progress on writing my own WPF rendering engine which seems to do the work as expected. (see screenshot below)

Next, I’ll try to improve the rendering engine to support drawing other layer types, stay tuned…

Read Full Post »

WPF is making a lot of noise these days, I was wondering what this new technology is all about and decided to give it a try.

Coming from WinForm world, describing GUIs in text (XAML) seemed awkward to me, I struggled a bit  getting my first window up and running but managed to do so at the end.

In order to understand what is laying underneath WPF, I thought I should write my proper user control that can display something like a map.

Here is a screenshot of my first attempt to write such a control.

This control is based on SharpMap (http://sharpmap.codeplex.com/), reads and displays ESRI shape files.

For now, I am using SharpMap as it is. All what I did is to use the library, load some shape files and display them in an image control.

// Event handler for button click
public void OpenShapeFileClicked(object sender, EventArgs e){
      // ... ask the user to provide a shape file using a file open dialog control
      LoadShapeFile(path_provided_by_user);
}
public void LoadShapeFile(string fileName){
      SharpMap.Layers.VectorLayer myLayer = new SharpMap.Layers.VectorLayer("ShapeFiles");
      myLayer.DataSource = new SharpMap.Data.Providers.ShapeFile("path_to_file", true);
      myLayer.Style.Fill = System.Drawing.Brushes.GreenYellow;
      myLayer.Style.Outline = new System.Drawing.Pen(System.Drawing.Brushes.Gray);
      myLayer.Style.Line.Width = 2;
      myLayer.Style.Line = new System.Drawing.Pen(System.Drawing.Brushes.Black);
      myMap.Layers.Add(myLayer);
      myMap.ZoomToExtents();
      DrawImage();
}
private void DrawImage(){
      // Convert System.Drawing.Image to ImageSource to wich an image is bound
      using (Bitmap bitmap = new Bitmap(myMap.GetMap())){
             IntPtr hBitmap = bitmap.GetHbitmap();
             // Use System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap
             BitmapSource bitmapSource = CreateBitmapSourceFromHBitmap(
                hBitmap,
                IntPtr.Zero,
                Int32Rect.Empty,
                BitmapSizeOptions.FromEmptyOptions());
             // Set image1 control data source
             image1.Source = bitmapSource;
      }
}

This project is not about rewriting a new SharpMap library for WPF but rather build up a WPF application from scratch based on SharpMap which I strongly believe will give me a great insight of the WPF technology and how GIS applications work.

In this little introduction, we demonstrated how easy it is to create a basic GIS application using WPF.

To be continued…

Read Full Post »