WPF: Limiting Valid Control Part for Dragging Operation via DependencyObject

Question

I'm asking this question after reading this answer on StackOverflow.

The supplied answer works great for allow you to move a whole control. But supposed I have a Usercontrol I've created like so:

<UserControl x:Class="WpfTest.UserControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300" Width="300">
    <Border Background="Blue">
        <Grid>
            <Grid.ColumnDefinitions>

            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="26"></RowDefinition>
                <RowDefinition Height="*"></RowDefinition>
            </Grid.RowDefinitions>
            <Canvas Grid.Row="0" Background="orange"></Canvas>
            <Canvas Grid.Row="1" Background="Purple"></Canvas>
        </Grid>
    </Border>
</UserControl>

I only want the control to move when I'm mouse clicking on the header part of the control, Grid.Row[0] (orange part). I don't want the control to respond to drag operations when clicking on the rest of the control.

How can I accomplish this?

Answer 1

I found a really great example on CodeProject called DiagramDesigner. This should get you started in the right direction. CodeProject, DiagramDesigner4

Answer 2

Assuming you're using the DraggableExtender.CanDrag attached property, you can set up a trigger to turn this on and off depending on whether the mouse is over your orange header:

<UserControl x:Class="WpfTest.UserControl1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Width="300" Height="300">
    <UserControl.Style>
        <Style>
            <Setter Property="(DraggableExtender.CanDrag)" Value="False"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding IsMouseOver, ElementName=headerCanvas}" Value="True">
                    <Setter Property="(drag:DraggableExtender.CanDrag)" Value="True"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </UserControl.Style>
    <Border Background="Blue">
        <Grid>
            <Grid.ColumnDefinitions>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="26"></RowDefinition>
                <RowDefinition Height="*"></RowDefinition>
            </Grid.RowDefinitions>
            <Canvas x:Name="headerCanvas" Grid.Row="0" Background="orange"></Canvas>
            <Canvas Grid.Row="1" Background="Purple"></Canvas>
        </Grid>
    </Border>
</UserControl>

Answer 3

So, after some digging around, I solved my problem. The solution was to utilize a Thumb.

My UserControl XAML looks like this:

<UserControl x:Class="WpfTest.UserControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300" Width="300">
    <Border Background="Blue">
        <Grid>
            <Grid.ColumnDefinitions>

            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="26"></RowDefinition>
                <RowDefinition Height="*"></RowDefinition>
            </Grid.RowDefinitions>
            <Canvas Grid.Row="0" Background="orange"></Canvas>
            <Thumb Grid.Row="0" Background="Cyan" DragCompleted="OnDragComplete" DragStarted="OnDragStarted" DragDelta="OnDragDelta" ></Thumb>
            <Canvas Grid.Row="1" Background="Purple"></Canvas>
        </Grid>
    </Border>
</UserControl>

We can see the addition of the Thumb with a different color, which covers up the orange header (letting me know the thumb is actually there).

Code behind for the Thumb events looks like this:

public partial class UserControl1 : UserControl
    {
        private bool _isDragging;
        private double _x;
        private double _y;

        public UserControl1()
        {
            InitializeComponent();
        }

        private void OnDragComplete(object sender, DragCompletedEventArgs e)
        {
            _isDragging = false;

        }

        private void OnDragStarted(object sender, DragStartedEventArgs e)
        {
            _isDragging = true;

            _x = (double)GetValue(Canvas.LeftProperty);
            _y = (double)GetValue(Canvas.TopProperty);

        }

        private void OnDragDelta(object sender, DragDeltaEventArgs e)
        {
            if (!_isDragging) return;

            _x += e.HorizontalChange;
            _y += e.VerticalChange;


            SetValue(Canvas.LeftProperty,_x );
            SetValue(Canvas.TopProperty,_y );
        }
    }

And this got me exactly what I wanted. One question did remain: I see some tearing of the usercontrol when I move it around. It's just not a smooth drawing effect, and I can't seem to find a way to eliminate that. Any clues?