public class ScriptGraphWindow : EditorWindow
{
    [MenuItem("Tool/ScriptGraph")]
    public static void Open()
    {
        ScriptGraphWindow window = GetWindow<ScriptGraphWindow>();
        window.Show();
    }
}

using UnityEngine.UIElements;
using UnityEditor.Experimental.GraphView;

public class ScriptGraphView : GraphView
{
    public ScriptGraphView() : base()
    {
        // 親のサイズに合わせてサイズを設定
        this.StretchToParentSize();
        // ズームインアウト
        SetupZoom(ContentZoomer.DefaultMinScale, ContentZoomer.DefaultMaxScale);
        // ドラッグで描画範囲を移動
        this.AddManipulator(new ContentDragger());
        // ドラッグで選択した要素を移動
        this.AddManipulator(new SelectionDragger());
        // ドラッグで範囲選択
        this.AddManipulator(new RectangleSelector());
    }
}

private void OnEnable()
{
    var scriptGraph = new ScriptGraphView();
    this.rootVisualElement.Add(scriptGraph);
}

GridBackground {
    --grid-background-color: #282828;
    --line-color: rgba(193,196,192,0.1);
    --tick-line-color: rgba(193,196,192,0.1);
    --spacing: 20
}![Something went wrong]()

public class ScriptGraphView : GraphView
{
    public ScriptGraphView() : base()
    {
        // 省略

        // ussファイルを読み込んでスタイルに追加
        this.styleSheets.Add(Resources.Load<StyleSheet>("GraphViewBackGround"));

        // 背景を一番後ろに追加
        this.Insert(0, new GridBackground());
    }
}

using UnityEngine;
using UnityEngine.UIElements;
using UnityEditor.Experimental.GraphView;

public class MessageNode : Node
{
    private TextField textField;

    public MessageNode()
    {
        // ノードのタイトル設定
        this.title = "Message";

        // ポート（後述）を作成
        var inputPort = Port.Create<Edge>(Orientation.Horizontal, Direction.Input, Port.Capacity.Multi, typeof(Port));
        inputPort.portName = "In";
        inputContainer.Add(inputPort);

        var outputOort = Port.Create<Edge>(Orientation.Horizontal, Direction.Output, Port.Capacity.Single, typeof(Port));
        outputOort.portName = "Out";
        outputContainer.Add(outputOort);

        // メイン部分に入力欄追加
        textField = new TextField();
        // 複数行対応
        textField.multiline = true;
        // 日本語入力対応
        textField.RegisterCallback<FocusInEvent>(evt => { Input.imeCompositionMode = IMECompositionMode.On; });
        textField.RegisterCallback<FocusOutEvent>(evt => { Input.imeCompositionMode = IMECompositionMode.Auto; });

        this.mainContainer.Add(textField);
    }
}

public ScriptGraphView() : base()
{
    // 省略

    this.Add(new MessageNode());
}

using System;
using System.Collections.Generic;
using UnityEditor.Experimental.GraphView;
using UnityEngine;

public class ScriptGraphSearchWindowProvider : ScriptableObject, ISearchWindowProvider
{
    private SctiptGraphWindow _window;
    private ScriptGraphView _graphView;

    public void Init(ScriptGraphView graphView,ScriptGraphWindow window)
    {
        _window = window;
        _graphView = graphView;
    }

    public List<SearchTreeEntry> CreateSearchTree(SearchWindowContext context)
    {
        var entries = new List<SearchTreeEntry>();
        entries.Add(new SearchTreeGroupEntry(new GUIContent("Create Node")));

        entries.Add(new SearchTreeEntry(new GUIContent(nameof(MessageNode))) { level = 1, userData = typeof(MessageNode)});

        return entries;
    }

    public bool OnSelectEntry(SearchTreeEntry SearchTreeEntry, SearchWindowContext context)
    {
        var type = SearchTreeEntry.userData as Type;
        var node = Activator.CreateInstance(type) as Node;

        // ノードの生成位置をマウスの座標にする
        var worldMousePosition = _window.rootVisualElement.ChangeCoordinatesTo(_window.rootVisualElement.parent, context.screenMousePosition - _window.position.position);
        var localMousePosition = _graphView.contentViewContainer.WorldToLocal(worldMousePosition);

        node.SetPosition(new Rect(localMousePosition, new Vector2(100, 100)));

        _scriptGraphView.AddElement(node);
        return true;
    }
}

public ScriptGraphView(ScriptGraphWindow window) : base()
{
    // 省略

    // 右クリックでノード作成するウィンドウ追加
    var searchWindowProvider = ScriptableObject.CreateInstance<ScriptGraphSearchWindowProvider>();
    searchWindowProvider.Init(this, window);
    this.nodeCreationRequest += context =>
    {
        SearchWindow.Open(new SearchWindowContext(context.screenMousePosition), searchWindowProvider);
    };
}

public class ScriptGraphNode : Node
{
}

public class MessageNode : ScriptGraphNode
{
    // 省略
}

public List<SearchTreeEntry> CreateSearchTree(SearchWindowContext context)
{
    var entries = new List<SearchTreeEntry>();
    entries.Add(new SearchTreeGroupEntry(new GUIContent("Create Node")));

    foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies())
    {
        foreach (var type in assembly.GetTypes())
        {
            if (type.IsClass == false) continue;

            if (type.IsAbstract) continue;

            if (type.IsSubclassOf(typeof(ScriptGraphNode)) == false) continue;

            entries.Add(new SearchTreeEntry(new GUIContent(type.Name)) { level = 1, userData = type });
        }
    }

    return entries;
}

public override List<Port> GetCompatiblePorts(Port startPort, NodeAdapter nodeAdapter)
{
    var compatiblePorts = new List<Port>();

    foreach (var port in ports.ToList())
    {
        // 同じノードは繋げない
        if (startPort.node == port.node) continue;

        // Input - Input , Output - Outputは繋げない
        if (startPort.direction == port.direction) continue;

        // ポートタイプが違うものは繋げない
        if (startPort.portType != port.portType) continue;

        compatiblePorts.Add(port);
    }

    return compatiblePorts;
}

[CreateAssetMenu(fileName = "scriptgraph.asset", menuName ="ScriptGraph Asset")]
public class ScriptGraphAsset : ScriptableObject
{
    public List<ScriptNodeData> list = new List<ScriptNodeData>();
}

[Serializable]
public class ScriptNodeData
{
    public int id;

    public NodeType type;

    public Rect rect;

    public int[] outIds;

    public byte[] serialData;
}

public bool OnSelectEntry(SearchTreeEntry SearchTreeEntry, SearchWindowContext context)
{
    // 省略

    node.SetPosition(new Rect(localMousePosition, new Vector2(100, 100)));

    _scriptGraphView.AddElement(node);

    ScriptGraphData data = Serialize(node);

    //　ここで追加しよう！
    _scriptGraphAsset.list.Add(data);

    return true;
}