WPF - Moving a Canvas - wpf

Question

I have trouble with moving a canvas in a Window.
<Canvas x:Name="Canvas1" Height="200" Grid.Column="1" Margin="10,15,92,54" Grid.Row="1" Background="#FFECECEC" Grid.RowSpan="2" MouseDown="Canvas_MouseDown" MouseUp="Canvas_MouseUp" MouseMove="Canvas_MouseMove" >
<TextBox Height="100" Width="100" Margin="50,50,327,65" Background="Red"/>
<TextBox Height="100" Width="100" Margin="10,15,327,65" Background="Blue" />
<Canvas.RenderTransform>
<TransformGroup>
<TranslateTransform x:Name="translate" />
</TransformGroup>
</Canvas.RenderTransform>
</Canvas>
This is code behind:
private void Canvas_MouseDown(object sender, MouseButtonEventArgs e)
{
Canvas1.CaptureMouse();
point = Mouse.GetPosition(Grid1);
}
private void Canvas_MouseMove(object sender, MouseEventArgs e)
{
if (Canvas1.IsMouseCaptured)
{
translate.X = e.GetPosition(Grid1).X - point.X;
translate.Y = e.GetPosition(Grid1).Y - point.Y;
}
}
private void Canvas_MouseUp(object sender, MouseButtonEventArgs e)
{
Canvas1.ReleaseMouseCapture();
}
First time, it seem woork fine.
But second time i try to click on my canvas, this move but from initial position.
How can i fix it?

Answer 1

When initializing point on mouse down, subtract the current translation:
private void Canvas1_MouseDown(object sender, MouseButtonEventArgs e)
{
Canvas1.CaptureMouse();
point = Mouse.GetPosition(Grid1);
point.X -= translate.X;
point.Y -= translate.Y;
}
Alternatively, always only add the difference vector:
private void Canvas1_MouseDown(object sender, MouseButtonEventArgs e)
{
Canvas1.CaptureMouse();
point = Mouse.GetPosition(Grid1);
}
private void Canvas1_MouseMove(object sender, MouseEventArgs e)
{
if (Canvas1.IsMouseCaptured)
{
var p = e.GetPosition(Grid1);
var diff = p - point;
point = p;
translate.X += diff.X;
translate.Y += diff.Y;
}
}
Also make sure the Grid doesn't position the Canvas, by setting the Canvas alignment to Top/Left:
<Canvas x:Name="Canvas1" HorizontalAlignment="Left" VerticalAlignment="Top" ...>
And probably don't set a Margin, but instead initialize the TranslateTransform appropriately.