Question

I am trying to implement a snap grid using WPF and a canvas. I am thinking my math is off because the UIElement won’t snap to the grid in the background. Below is the xaml I use to create the grid and the method I use to try and snap the UIElement to the closest grid line. The method used is fired as soon as the mouse button up event is fired. If this not the correct approach for WPF can someone point me in the right direction?

XAML

 <Border x:Name="dragBorder" 
            BorderBrush="Black"
            BorderThickness="1"
            Margin="5"
            CornerRadius="3">
        <Canvas x:Name="dragCanvas"
                            AllowDragging="true"
                            AllowDragOutOfView="False"
                            Height="{Binding ElementName=dragBorder, Path=ActualHeight}"
                            Width="{Binding ElementName=dragBorder, Path=ActualWidth}">
            <Canvas.Background>
                <VisualBrush TileMode="Tile"
                             Viewport="0,0,16,16"
                             ViewportUnits="Absolute"
                             Viewbox="0,0,16,16"
                             ViewboxUnits="Absolute">
                    <VisualBrush.Visual>
                        <Ellipse Fill="#FF000000"
                                 Width="2"
                                 Height="2" />
                    </VisualBrush.Visual>
                </VisualBrush>
            </Canvas.Background>
        </Canvas>
    </Border>

Method

private void SnapToGrid(UIElement element)
    {
        double xSnap = (Canvas.GetLeft(element) / gridWidth) * gridWidth;
        double ySnap = (Canvas.GetTop(element) / gridWidth) * gridWidth;

        Canvas.SetLeft(element, xSnap);
        Canvas.SetTop(element, ySnap);

        double tempX = Canvas.GetLeft(element);
        double tempY = Canvas.GetTop(element);
    }   

Answer 1

Your problem is your function doesn’t actually do anything. You divide by the grid size then multiply by the grid size, so in effect you’re doing nothing (2 * 16 / 16 = 2). What you need to use is the modulus % operator and adjust the x/y position based on the distance from your grid size.

Here is a working function that snaps left/top if closer to the left/top grid line or right/down otherwise:

private void SnapToGrid( UIElement element ) {
    double xSnap = Canvas.GetLeft( element ) % GRID_SIZE;
    double ySnap = Canvas.GetTop( element ) % GRID_SIZE;

    // If it's less than half the grid size, snap left/up 
    // (by subtracting the remainder), 
    // otherwise move it the remaining distance of the grid size right/down
    // (by adding the remaining distance to the next grid point).
    if( xSnap <= GRID_SIZE / 2.0 )
        xSnap *= -1;
    else
        xSnap = GRID_SIZE - xSnap;
    if( ySnap <= GRID_SIZE / 2.0 )
        ySnap *= -1;
    else
        ySnap = GRID_SIZE - ySnap;

    xSnap += Canvas.GetLeft( element );
    ySnap += Canvas.GetTop( element );

    Canvas.SetLeft( element, xSnap );
    Canvas.SetTop( element, ySnap );
}