Feature: Add `CanAttachFrom` Behavior

While CanAttachTo is really helpful, sometimes it’s beneficial to have the inverse, i.e. CanAttachFrom when you want to permit/deny attachment from a port.

It’s the same logic (as CanAttachTo) but swapping the ports.

//PortModel.cs
public virtual bool CanAttachFrom(PortModel port)
    => port != this && !port.Locked && Parent != port.Parent;

//DragNewLinkBehavior.cs
if (!(model is PortModel port) || !port.CanAttachFrom(_ongoingLink.SourcePort!) || !_ongoingLink.SourcePort!.CanAttachTo(port))
{
    Diagram.Links.Remove(_ongoingLink);
    _ongoingLink = null;
    return;
}

[zHaytam]

In 3.0.0, another option was added for the links factory to return null in order to not create an ongoing link. This can also be used to do any checks from the source port or node and decide whether to create a link or not.

[zHaytam]

Very sorry for the late reply, I thought I did a long time ago…

In your example (picture), how can the user connect red to blue? In CanAttachTo, you have both ports, so you should be able to test on both of them no?

public override bool CanAttachTo(PortModel port)
{
            // Checks for same-node/port attachments
            if (!base.CanAttachTo(port))
                return false;

            // Only able to attach to the same port type
            if (!(port is ColoredPort cp))
                return false;

            return IsRed == cp.IsRed || IsBlue == cp.IsBlue || IsGreen == cp.IsGreen;
}

This would work for all cases no? Do you have a running example that shows the issue?