Question

I basically have a simple problem in my program that I just want to make sure goes right. It should on the click of the mouse button add the MouseEventHandler and then move the circle along with the mouse until the event handler gets removed. I simplified the code to the very basics:

XAML:

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
    <Grid Name="grid1" Background="White" MouseLeftButtonUp="grid_MouseUp">
        <Ellipse Height="50" HorizontalAlignment="Left" Margin="12,12,0,0" Name="ellipse1" Stroke="{x:Null}" VerticalAlignment="Top" Width="50" Fill="Black" MouseLeftButtonDown="ellipse1_MouseDown" />
    </Grid>
</Window>

C#:

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

    private static Point _oldPoint = new Point(), _newPoint = new Point();

    private void ellipse1_MouseDown(object sender, MouseButtonEventArgs e)
    {
        _oldPoint = e.GetPosition(grid1);
        grid1.MouseMove += new MouseEventHandler(grid_MouseMove);
    }

    private void grid_MouseUp(object sender, MouseButtonEventArgs e)
    {
        grid1.MouseMove -= new MouseEventHandler(grid_MouseMove);
    }

    private void grid_MouseMove(object sender, MouseEventArgs e)
    {
        _newPoint = e.GetPosition(grid1);
        ellipse1.Margin = new Thickness(ellipse1.Margin.Left - _oldPoint.X + _newPoint.X, ellipse1.Margin.Top - _oldPoint.Y + _newPoint.Y, 0, 0);
        _oldPoint = _newPoint;
    }
}

Now in general this code works fine and I think is quite neat as it doesn’t check the movement of the mouse until one actually presses the button. However, my question is as follows:

I had to add the MouseMove event to the grid rather than to the circle, because once the mouse pointer loses focus of the circle (by moving the mouse too fast) it doesn’t trigger the MouseMove event anymore. But why exactly does that happen? At the beginning of the event the mouse was definitely above the circle and then it moved. Yes, it moved away from the circle but shouldn’t that still trigger the event?

Answer 1

You can capture the mouse and handle all events in your ellipse.

<Grid Name="grid1" Background="White">
    <Ellipse Height="50" HorizontalAlignment="Left" Margin="12,12,0,0" Name="ellipse1" Stroke="{x:Null}" VerticalAlignment="Top" Width="50" Fill="Black"
             MouseLeftButtonDown="ellipse1_MouseDown" MouseLeftButtonUp="ellipse1_MouseUp" />
</Grid>

with this code behind

private void ellipse1_MouseDown(object sender, MouseButtonEventArgs e)
{
    Mouse.Capture(ellipse1);
    _oldPoint = e.GetPosition(grid1);
    ellipse1.MouseMove += new MouseEventHandler(ellipse1_MouseMove);
}

private void ellipse1_MouseUp(object sender, MouseButtonEventArgs e)
{
    Mouse.Capture(null);
    ellipse1.MouseMove -= new MouseEventHandler(ellipse1_MouseMove);
}

I’ve moved and renamed grid_MouseMove to ellipse1_MouseMove.