CustomHierarchyView.cs

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

[InitializeOnLoad]
public class CustomHierarchyView
{
	private static GUIContent toggledOnIcon;
	private static GUIContent toggledOffIcon;
	private static GUIStyle customButton;
	private static GUIStyle customLabel;

	static CustomHierarchyView()
	{
		EditorApplication.hierarchyWindowItemOnGUI += HierarchyWindowItemOnGUI;
	}

	~CustomHierarchyView()
	{
		EditorApplication.hierarchyWindowItemOnGUI -= HierarchyWindowItemOnGUI;
	}

	private static void Setup()
	{
		if (customButton != null)
		{
			return;
		}

		toggledOnIcon = EditorGUIUtility.IconContent("animationvisibilitytoggleon");
		toggledOffIcon = EditorGUIUtility.IconContent("animationvisibilitytoggleoff");
		
		customButton = new GUIStyle(GUI.skin.button);
		customLabel = new GUIStyle(GUI.skin.label);		

		customButton.border.bottom = 0;
		customButton.border.top = 0;
		customButton.border.right = 0;
		customButton.border.left = 0;
		customButton.normal.background = null;
		customButton.active.background = null;
		customButton.hover.background = null;
		customButton.focused.background = null;
		customButton.alignment = TextAnchor.MiddleCenter;

		customLabel.alignment = TextAnchor.MiddleRight;
	}

	private static void HierarchyWindowItemOnGUI(int instanceID, Rect selectionRect)
	{
		Setup();

		var instanceObject = EditorUtility.InstanceIDToObject(instanceID);

		if (instanceObject == null)
		{
			return;
		}

		if ((instanceObject is GameObject) == false)
		{
			return;
		}

		var gameObject = instanceObject as GameObject;
		var transform = gameObject.transform;

		if (transform.childCount == 0)
		{
			return;
		}

		var firstChild = transform.GetChild(0);
		bool isHidden = ((int)firstChild.hideFlags & (int)HideFlags.HideInHierarchy) == (int)HideFlags.HideInHierarchy;
		
		float width = 25f;
		var rect = selectionRect;
		rect.x = selectionRect.x + selectionRect.width - width;
		rect.width = width;

		if (isHidden)
		{
			if (GUI.Button(rect, toggledOffIcon, customButton))
			{
				ToggleHighlightInChildren(gameObject, false);
			}

			var labelRect = selectionRect;
			labelRect.width = selectionRect.width - width;
			GUI.Label(labelRect, transform.childCount.ToString("N0"), customLabel);
		}
		else
		{
			if (GUI.Button(rect, toggledOnIcon, customButton))
			{
				ToggleHighlightInChildren(gameObject, true);
			}
		}
	}

	private static void ToggleHighlightInChildren(GameObject gameObject, bool toggleOn)
	{
		foreach (Transform child in gameObject.transform)
		{
			HideFlags hideFlags = child.hideFlags;
			if( toggleOn )
			{
				hideFlags = (HideFlags)((int)hideFlags | (int)HideFlags.HideInHierarchy);
			}
			else
			{
				hideFlags = (HideFlags)((int)hideFlags & (~(int)HideFlags.HideInHierarchy));
			}

			child.hideFlags = hideFlags;
		}
	}
}