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Archive for September, 2011

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.

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