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Drag and drop in your controls

The drag and drop of the designer and the field list is built on reusable attached properties (namespace EasyBI.Wpf.DragAndDrop, XAML prefix bi:), usable on any ItemsControl of an application. Being attached properties, they can be set from a style:

<!-- Items can be dragged: a preview follows the pointer, a moved item is dimmed -->
<ListBox ItemsSource="{Binding Fields}" bi:DragSource.IsEnabled="True" bi:DragSource.AllowedEffects="Copy" />

<!-- Drops are decided by an IDropHandler (usually the view-model); an insertion line shows where the item goes -->
<ItemsControl ItemsSource="{Binding Columns}" bi:DropTarget.Handler="{Binding}" bi:DropTarget.Key="Columns" />

public sealed class MyViewModel : IDropHandler
{
    public DragDropEffects DragOver(DropInfo drop) => drop.Item is Field ? DragDropEffects.Copy : DragDropEffects.None;
    public void Drop(DropInfo drop) => Columns.Insert(drop.Index, ((Field)drop.Item!).ToColumn());
    public string? RefusalReason(DropInfo drop) => null;
}
  • DropInfo gives the dragged Item, the target's Key (one handler for several targets), the insertion Index among the target's items and the raw Data for drags coming from elsewhere.
  • DropTarget.IsDragOver is true while an accepted drag is over the target (style triggers); DropTarget.Orientation switches the insertion rules to a vertical stack.
  • A handler may explain a refusal (IDropHandler.RefusalReason, null to say nothing): the reason is shown under the target and set on it as DropTarget.Refusal while the drag is over it.
  • A scrollable target, or its scroll viewer, scrolls when the pointer nears its edges (AutoScroller, also usable on its own); InsertionIndex holds the pure insertion rules.
  • Style keys DragSource.PreviewStyleKey (a ContentControl; DragSource.PreviewTemplate sets the content), DropTarget.InsertionLineStyleKey (a Border) and DropTarget.RefusalStyleKey (a ContentControl holding the reason).

In the designer, fields are dropped between chips, chips are reordered within a shelf, and dimensions move between Axis and Legend keeping their granularity. A refused field says why (DesignerViewModel.RefusalReason): only measures in Values, only dimensions elsewhere, a field already on the shelf, a full axis, a field of another model.