Added FlowNode abstract classes

Editor/Flow.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 1e14c6504b4b4601bb79db0271af5e61
+timeCreated: 1673299690

Editor/Flow/FlowNodeEditor.cs

@@ -0,0 +1,15 @@
+
+using XNode.Flow;
+
+namespace XNodeEditor.Flow
+{
+    [CustomNodeEditor(typeof(FlowNode))]
+    public class FlowNodeEditor : NodeEditor
+    {
+        public override void OnBodyGUI()
+        {
+            NodeEditorGUILayout.PortPair(target.GetPort("previousNode"), target.GetPort("nextNode"));
+            base.OnBodyGUI();
+        }
+    }
+}

Editor/Flow/FlowNodeEditor.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: efc69c086ec342a8b6aef9ff7ad81f5b
+timeCreated: 1673299707

Editor/NodeEditorGUILayout.cs

@@ -6,6 +6,7 @@ using System.Reflection;
 using UnityEditor;
 using UnityEditorInternal;
 using UnityEngine;
+using XNode.Flow;
 
 namespace XNodeEditor {
     /// <summary> xNode-specific version of <see cref="EditorGUILayout"/> </summary>
@@ -194,7 +195,8 @@ namespace XNodeEditor {
             if (port == null) return;
             if (options == null) options = new GUILayoutOption[] { GUILayout.MinWidth(30) };
             Vector2 position = Vector3.zero;
-            GUIContent content = label != null ? label : new GUIContent(ObjectNames.NicifyVariableName(port.fieldName));
+            GUIContent content = label ?? new GUIContent(ObjectNames.NicifyVariableName(port.fieldName));
+            if (label == null && port.ValueType.IsAssignableFrom(typeof(FlowNode))) content = EditorGUIUtility.TrIconContent("Animation.Play", content.text);
 
             // If property is an input, display a regular property field and put a port handle on the left side
             if (port.direction == XNode.NodePort.IO.Input) {

Runtime/Flow.meta

@@ -0,0 +1,8 @@
+fileFormatVersion: 2
+guid: 835a6229a286e044b87361784c807aef
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Runtime/Flow/FlowNode.cs

@@ -0,0 +1,71 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+namespace XNode.Flow
+{
+    public abstract class FlowNode : Node
+    {
+        public override object GetValue(NodePort port)
+        {
+            if (port.IsConnected && port.Connection.node is FlowNode flowNode) return flowNode;
+            return null;
+        }
+
+        public virtual IEnumerator Perform()
+        {
+            yield break;
+        }
+
+        public virtual IEnumerable<FlowNode> PreviousNodes => Array.Empty<FlowNode>();
+        public virtual IEnumerable<FlowNode> NextNodes => Array.Empty<FlowNode>();
+
+        public FlowNode GetFlowNode(string portName) => GetValue(GetPort(portName)) as FlowNode;
+    }
+
+    public abstract class InFlowNode : FlowNode
+    {
+        [Input(ShowBackingValue.Never, ConnectionType.Multiple, TypeConstraint.Inherited)]
+        [SerializeField, HideInInspector]
+        public FlowNode previousNode;
+
+        public override IEnumerable<FlowNode> PreviousNodes
+        {
+            get { yield return GetFlowNode("previousNodes"); }
+        }
+    }
+
+    public abstract class OutFlowNode : FlowNode
+    {
+        [Output(ShowBackingValue.Never, ConnectionType.Multiple, TypeConstraint.Inherited)]
+        [SerializeField, HideInInspector]
+        public FlowNode nextNode;
+
+        public override IEnumerable<FlowNode> NextNodes
+        {
+            get { yield return GetFlowNode("nextNode"); }
+        }
+    }
+
+    public abstract class InOutFlowNode : FlowNode
+    {
+        [Input(ShowBackingValue.Never, ConnectionType.Multiple, TypeConstraint.Inherited)]
+        [SerializeField, HideInInspector]
+        public FlowNode previousNode;
+
+        [Output(ShowBackingValue.Never, ConnectionType.Multiple, TypeConstraint.Inherited)]
+        [SerializeField, HideInInspector]
+        public FlowNode nextNode;
+
+        public override IEnumerable<FlowNode> PreviousNodes
+        {
+            get { yield return GetFlowNode("previousNodes"); }
+        }
+
+        public override IEnumerable<FlowNode> NextNodes
+        {
+            get { yield return GetFlowNode("nextNode"); }
+        }
+    }
+}

Runtime/Flow/FlowNode.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 19577700addb44bfbe0004ed13078931
+timeCreated: 1673298639