Create diagram connectors using drag and drop

In this post we show how to create diagram links via drag-and-drop operation from a NodeListView used as a palette. Since the NodeListView control can contain only DiagramNode objects, we will create a custom node shape representing a connector. Once the dummy connector node is created, the NodeCreated event handler will replace it with a DiagramLink. In addition, the handler will find nearby nodes and automatically connect the link to them.

Let’s start with a new WPF project. Add a reference to mindfusion.diagramming.dll, and add Diagram and NodeListView controls to the window:

<ScrollViewer
	Grid.Column="0"
	Focusable="False"
	HorizontalScrollBarVisibility="Visible">
	
	<diag:Diagram
		x:Name="diagram"
		AllowDrop="True"
		NodeCreated="OnNodeCreated"
		NodeModified="OnNodeModified">
	</diag:Diagram>
	
</ScrollViewer>

<diag:NodeListView
	x:Name="nodeList"
	Grid.Column="1">
</diag:NodeListView>

In the Window constructor, create a custom node shape that will represent connectors:

var connectorShape = new Shape(
	null, // no borders
	new[] // decorations
	{
		new LineTemplate(10, 10, 10, 50),
		new LineTemplate(10, 50, 90, 50),
		new LineTemplate(90, 50, 90, 90)
	},
	null,
	FillRule.Nonzero, "Connector");

Add a few nodes to the NodeListView, along with the dummy connector node:

var item1 = new ShapeNode { Shape = Shapes.Rectangle };
NodeListView.SetLabel(item1, "Activity");
nodeList.Items.Add(item1);

var item2 = new ShapeNode { Shape = Shapes.Decision };
NodeListView.SetLabel(item2, "Decision");
nodeList.Items.Add(item2);

var item3 = new ShapeNode { Shape = connectorShape };
NodeListView.SetLabel(item3, "Connector");
nodeList.Items.Add(item3);

In the NodeCreated event handler, check if the newly created node represents a connector, and replace it with a DiagramLink:

var node = e.Node as ShapeNode;
if (node != null)
{
	if (node.Shape.Id == "Connector")
	{
		// replace the dummy connector node with a DiagramLink
		var bounds = node.Bounds;
		diagram.Items.Remove(node);

		var link = diagram.Factory.CreateDiagramLink(
			bounds.TopLeft, bounds.BottomRight);
		link.SegmentCount = 3;
		link.Shape = LinkShape.Cascading;

		ConnectToNearbyNode(link);
	}
	...
}

Otherwise if it is a regular node, set its anchor points and connect it to nearby unconnected links, if there are any:

else
{
	node.AnchorPattern = AnchorPattern.Decision2In2Out;
	node.Effects.Add(new GlassEffect());
	ConnectToNearbyLink(node);
}

The ConnectToNearbyNode method uses a LINQ query to find nodes in the vicinity of the link’s start or end point. If one is found at a distance shorter than 120 points, it is set as the link’s Origin or Destination.

private void ConnectToNearbyNode(DiagramLink link)
{
	// connect to a nearby origin node
	var origin = diagram.Nodes.OrderBy(n =>
		Utilities.Distance(n.GetCenter(), link.StartPoint)).FirstOrDefault();

	if (origin != null)
	{
		var distance = Utilities.Distance(origin.GetCenter(), link.StartPoint);
		if (distance < 120)
		{
			link.Origin = origin;
			link.Route();
		}
	}

	// connect to a nearby destination node
	var destination = diagram.Nodes.Where(n => n != origin).OrderBy(n =>
		Utilities.Distance(n.GetCenter(), link.EndPoint)).FirstOrDefault();

	if (destination != null)
	{
		var distance = Utilities.Distance(destination.GetCenter(), link.EndPoint);
		if (distance < 120)
		{
			link.Destination = destination;
			link.Route();
		}
	}
}

Similarly, the ConnectToNearbyLink method finds a nearby unconnected link and sets the specified node as the link’s Origin or Destination.

private void ConnectToNearbyLink(DiagramNode node)
{
	var outLink = diagram.Links.Where(l => l.Origin is DummyNode).OrderBy(l =>
		Utilities.Distance(node.GetCenter(), l.StartPoint)).FirstOrDefault();

	if (outLink != null)
	{
		var distance = Utilities.Distance(node.GetCenter(), outLink.StartPoint);
		if (distance < 90)
		{
			outLink.Origin = node;
			outLink.Route();
			return;
		}
	}

	var inLink = diagram.Links.Where(l => l.Destination is DummyNode).OrderBy(l =>
		Utilities.Distance(node.GetCenter(), l.EndPoint)).FirstOrDefault();

	if (inLink != null)
	{
		var distance = Utilities.Distance(node.GetCenter(), inLink.EndPoint);
		if (distance < 90)
		{
			inLink.Destination = node;
			inLink.Route();
			return;
		}
	}
}

The result of several drag and drop operation is displayed below.

The complete sample project is available for download here:
http://mindfusion.eu/_samples/LinkDragDrop.zip

Enjoy!