MetroThumbContentControl is a Thumb that can hold arbitrary content. WPF's own Thumb is a Control with a template, so wrapping content in it is awkward; this one derives from ContentControlEx instead and raises the same three drag events.

<mah:MetroThumbContentControl DragDelta="OnDragDelta">
    <Border Background="{DynamicResource MahApps.Brushes.Accent}" Padding="12">
        <TextBlock Text="Drag me" />
    </Border>
</mah:MetroThumbContentControl>

private void OnDragDelta(object sender, DragDeltaEventArgs e)
{
    Canvas.SetLeft(this.card, Canvas.GetLeft(this.card) + e.HorizontalChange);
    Canvas.SetTop(this.card, Canvas.GetTop(this.card) + e.VerticalChange);
}

The control has no appearance of its own. Its style is

<Style x:Key="MahApps.Styles.MetroThumbContentControl"
       BasedOn="{StaticResource MahApps.Styles.ContentControlEx}"
       TargetType="{x:Type mah:MetroThumbContentControl}" />

— the ContentControlEx style with nothing added. What you see is entirely your content.

You are probably already using it

This is not a control most applications place by hand. It is what makes two familiar things work:

• MetroWindow's title bar. PART_TitleBar in the window template is a MetroThumbContentControl, and dragging it is what moves the window. The handler checks IsWindowDraggable, ignores movements of 2px or less, and refuses to drag a maximized window unless the pointer is actually within TitleBarHeight.
• Flyout headers, which use the same IMetroThumb interface to let a flyout be dragged by its header.

Both go through IMetroThumb, a small interface carrying just the three drag events and MouseDoubleClick. MetroThumb — a plain Thumb implementing the same interface — is the other implementation.

Events and state

All three are bubbling routed events, unlike Thumb's, so a parent can handle them without attaching to the thumb itself.

A drag is abandoned automatically if the control loses mouse capture — a class handler on LostMouseCapture calls CancelDragAction(), which releases capture, clears IsDragging and raises DragCompleted with Canceled set.

Touch

OnPreviewTouchDown captures the touch device, OnPreviewTouchUp releases it, and OnLostTouchCapture takes it back if it is lost while still down. That last part is what keeps a drag alive when a finger slides outside the control — without it a touch drag stops the moment the contact leaves the bounds.

Inherited behaviour

Because it derives from ContentControlEx, ControlsHelper.ContentCharacterCasing and ControlsHelper.RecognizesAccessKey apply to its content, and a string child is upper- or lower-cased by the same converters.

Focusable is overridden to False in the static constructor, so the control never takes focus — appropriate for something whose whole job is to be dragged.

It also supplies a MetroThumbContentControlAutomationPeer.

Related

ContentControlEx is the base and supplies the style. MetroWindow and Flyout are where the library uses it.