【Unity】Inspector に GetComponentInChildren を行うボタンを追加する

Inspector にオブジェクトをアタッチするのが面倒なことがあるので、
ボタンひとつで GetComponentInChildren してくれる拡張を作成

コード

using System.Linq;
using System.Reflection;
using UnityEditor;
using UnityEditor.IMGUI.Controls;
using UnityEngine;
 
public class GetComponentAttribute : PropertyAttribute
{
}
  
[CustomPropertyDrawer(typeof(GetComponentAttribute))]
public class GetComponentDrawer : PropertyDrawer
{
    private const float ButtonWidth = 32;
    
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        if (property.propertyType != SerializedPropertyType.ObjectReference)
        {
            EditorGUI.PropertyField(position, property, label);
            return;
        }
 
        if (property.objectReferenceValue != null)
        {
            EditorGUI.PropertyField(position, property, label);
            return;
        }
 
        position.width -= ButtonWidth;
        EditorGUI.PropertyField(position, property, label);
        position.x += position.width;
        position.width = ButtonWidth;
        if (GUI.Button(position, "Get"))
        {
            GetComponent(position, property);
        }
    }
 
    private void GetComponent(Rect position, SerializedProperty property)
    {
        var obj = property.serializedObject.targetObject as Component;
        if (obj == null)
        {
            Debug.LogError("GameObject Not Found");
            return;
        }
 
        var type = property.serializedObject.targetObject.GetType();
        var fieldInfo = type.GetField(property.propertyPath, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static);
        if (fieldInfo == null)
            return;
        
        var fieldType = fieldInfo.FieldType;
        
        var components = obj.GetComponentsInChildren(fieldType);
        if (components.Length == 0)
            Debug.Log("Component Not Found");
        else if (components.Length == 1)
            property.objectReferenceValue = components[0];
        else
            PopupWindow.Show(position, new Popup(property, components));
    }
 
    private class Popup : PopupWindowContent
    {
        private readonly TreeView _treeView;
        
        public Popup(SerializedProperty property, Component[] components)
        {
            _treeView = new PopupTreeView(new TreeViewState(), this, property, components);
            GUIUtility.keyboardControl = _treeView.treeViewControlID;
        }
        
        public override void OnGUI(Rect rect)
        {
            _treeView.OnGUI(rect);
        }
        
        private class PopupTreeView : TreeView
        {
            private readonly SerializedProperty _property;
            private readonly Popup _popup;
            private readonly Component[] _components;
 
            public PopupTreeView(TreeViewState state, Popup popup, SerializedProperty property, Component[] components) : base(state)
            {
                _property = property;
                _popup = popup;
                _components = components;
                Reload();
            }
 
            public PopupTreeView(TreeViewState state, MultiColumnHeader multiColumnHeader) : base(state, multiColumnHeader)
            {
            }
             
            protected override TreeViewItem BuildRoot()
            {
                var root = new TreeViewItem(-1, -1, "root");
                for (var i = 0; i < _components.Length; ++i)
                    root.AddChild(new TreeViewItem(i, 0, _components[i].name));

                return root;
            }
 
            protected override void SingleClickedItem(int id)
            {
                DecideObject();
            }
            
            private void DecideObject()
            {
                if (state.selectedIDs.Count <= 0)
                    return;

                _property.serializedObject.Update();
                _property.objectReferenceValue = _components[state.selectedIDs.First()];
                _property.serializedObject.ApplyModifiedProperties();
                _popup.editorWindow.Close();
            }
        }
    }
}

サンプル

これを利用することで ボタンが表示される

このボタンを押すと、コンポーネントが一つしかない場合は自動で参照が入り
複数ある場合はリストを表示する

using UnityEngine;
using UnityEngine.UI;
 
public class Sample : MonoBehaviour
{
    [SerializeField, GetComponent]
    private Image _image;
    
    [SerializeField, GetComponent]
    private Button _button;
}