That's a nice addition for BP nodes, but I noticed that this is for UFlowNode (as previous validation logic). I propose to move it to UFlowNodeBase, it will give users ability to implement validation logic for AddOns also, because currently we can't do that, only runtime message logging is supported.

That's a nice addition for BP nodes, but I noticed that this is for UFlowNode (as previous validation logic). I propose to move it to UFlowNodeBase, it will give users ability to implement validation logic for AddOns also, because currently we can't do that, only runtime message logging is supported.

I've taken the feedback into account, and AddOns validation is now possible :)
However, I have an unresolved issue at this stage : when you add a validation log and pass the AddOn reference in the message, the clickable link doesn't redirect to the node in the graph.

Do you have any ideas on the ideal way to do this?
I thought about referencing the Flow Node owner, but it's only valued at runtime in the AddOn.

Unfortunately, SGraphPanel does not know anything about AddOns. When SGraphPanel::Update is called, it loops over top level nodes only, creates their respective SWidgets and stores them in a map NodeToWidgetLookup. When you want to select some node, JumpToNode is called and, eventually, in SNodePanel::Tick it tries to find respective SWidget for UEdGraphNode in NodeToWidgetLookup map (in our case, this is AddOn node) and it is not there :)

We could try to add UEdGraphNodes created for AddOns to Nodes array in UEdGraph, but I don't know if this will work) Simpler way would be just to pass owning FlowNode, but as you said, it is available at runtime only.

I think we can add ParentNode property to AddOns (it will contain pointer to owning UFlowNode). We can set it in UFlowGraphNode::AddSubNode or UFlowGraphNode::SetParentNodeForSubNode (currently only graph node knows about its parent, but it is direct parent, so it can be another AddOn). Then we have to have getter function for ParentNode in UFlowNodeBase, that will return this for UFlowNode and actual parent UFlowNode for UFlowNodeAddOn. And change this

FFlowGraphToken::FFlowGraphToken(const UFlowNodeBase* InFlowNodeBase)
	: GraphNode(InFlowNodeBase->GetGraphNode())
{
	CachedText = InFlowNodeBase->GetNodeTitle();
}

to this

FFlowGraphToken::FFlowGraphToken(const UFlowNodeBase* InFlowNodeBase)
	: GraphNode(InFlowNodeBase->GetParentNode()->GetGraphNode())
{
	CachedText = InFlowNodeBase->GetNodeTitle();
}