An editor script for Unity 3D to collapse all GameObject's in Hierarchy view or to collapse all folders in Project view. See the comments section below for instructions.

EditorCollapseAll.cs

using System.Collections.Generic;
using System.Reflection;
using UnityEditor;
using UnityEditor.IMGUI.Controls;
using UnityEditorInternal;
using UnityEngine.SceneManagement;
#if UNITY_6000_3_OR_NEWER
using EntityId = UnityEngine.EntityId;
#else
using EntityId = System.Int32;
#endif
#if UNITY_6000_3_OR_NEWER
using TreeViewItem = UnityEditor.IMGUI.Controls.TreeViewItem<UnityEngine.EntityId>;
using TreeViewState = UnityEditor.IMGUI.Controls.TreeViewState<UnityEngine.EntityId>;
#elif UNITY_6000_2_OR_NEWER
using TreeViewItem = UnityEditor.IMGUI.Controls.TreeViewItem<int>;
using TreeViewState = UnityEditor.IMGUI.Controls.TreeViewState<int>;
#endif

public static class EditorCollapseAll
{
	private const BindingFlags INSTANCE_FLAGS = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;
	private const BindingFlags STATIC_FLAGS = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static;

	[MenuItem( "Assets/Collapse All", priority = 1000 )]
	private static void CollapseFolders()
	{
		EditorWindow projectWindow = typeof( EditorWindow ).Assembly.GetType( "UnityEditor.ProjectBrowser" ).GetField( "s_LastInteractedProjectBrowser", STATIC_FLAGS ).GetValue( null ) as EditorWindow;
		if( projectWindow )
		{
			object assetTree = projectWindow.GetType().GetField( "m_AssetTree", INSTANCE_FLAGS ).GetValue( projectWindow );
			if( assetTree != null )
				CollapseTreeViewController( projectWindow, assetTree, (TreeViewState) projectWindow.GetType().GetField( "m_AssetTreeState", INSTANCE_FLAGS ).GetValue( projectWindow ) );

			object folderTree = projectWindow.GetType().GetField( "m_FolderTree", INSTANCE_FLAGS ).GetValue( projectWindow );
			if( folderTree != null )
			{
				object treeViewDataSource = folderTree.GetType().GetProperty( "data", INSTANCE_FLAGS ).GetValue( folderTree, null );
				int searchFiltersRootInstanceID = (int) typeof( EditorWindow ).Assembly.GetType( "UnityEditor.SavedSearchFilters" ).GetMethod( "GetRootInstanceID", STATIC_FLAGS ).Invoke( null, null );
                bool isSearchFilterRootExpanded = (bool)treeViewDataSource.GetType().GetMethod("IsExpanded", INSTANCE_FLAGS, null, new System.Type[] { typeof(EntityId) }, null).Invoke(treeViewDataSource, new object[] { searchFiltersRootInstanceID });

                CollapseTreeViewController(projectWindow, folderTree, (TreeViewState)projectWindow.GetType().GetField("m_FolderTreeState", INSTANCE_FLAGS).GetValue(projectWindow), isSearchFilterRootExpanded ? new EntityId[1] { searchFiltersRootInstanceID } : null);

                // Preserve Assets and Packages folders' expanded states because they aren't automatically preserved inside ProjectBrowserColumnOneTreeViewDataSource.SetExpandedIDs
                // https://github.com/Unity-Technologies/UnityCsReference/blob/e740821767d2290238ea7954457333f06e952bad/Editor/Mono/ProjectBrowserColumnOne.cs#L408-L420
#if UNITY_6000_3_OR_NEWER
                InternalEditorUtility.expandedProjectWindowItemIds = (EntityId[])treeViewDataSource.GetType().GetMethod("GetExpandedIDs", INSTANCE_FLAGS).Invoke(treeViewDataSource, null);
#else
                InternalEditorUtility.expandedProjectWindowItems = (int[])treeViewDataSource.GetType().GetMethod("GetExpandedIDs", INSTANCE_FLAGS).Invoke(treeViewDataSource, null);
#endif

				TreeViewItem rootItem = (TreeViewItem) treeViewDataSource.GetType().GetField( "m_RootItem", INSTANCE_FLAGS ).GetValue( treeViewDataSource );
				if( rootItem.hasChildren )
				{
                    foreach (TreeViewItem item in rootItem.children)
                        EditorPrefs.SetBool("ProjectBrowser" + item.displayName, (bool)treeViewDataSource.GetType().GetMethod("IsExpanded", INSTANCE_FLAGS, null, new System.Type[] { typeof(EntityId) }, null).Invoke(treeViewDataSource, new object[] { item.id }));
				}
			}
		}
	}

	[MenuItem( "GameObject/Collapse All", priority = 40 )]
	private static void CollapseGameObjects( MenuCommand command )
	{
		// This happens when this button is clicked while multiple Objects were selected. In this case,
		// this function will be called once for each selected Object. We don't want that, we want
		// the function to be called only once
		if( command.context )
		{
			EditorApplication.update -= CallCollapseGameObjectsOnce;
			EditorApplication.update += CallCollapseGameObjectsOnce;

			return;
		}

		EditorWindow hierarchyWindow = typeof( EditorWindow ).Assembly.GetType( "UnityEditor.SceneHierarchyWindow" ).GetField( "s_LastInteractedHierarchy", STATIC_FLAGS ).GetValue( null ) as EditorWindow;
		if( hierarchyWindow )
		{
			object hierarchyTreeOwner = hierarchyWindow.GetType().GetField( "m_SceneHierarchy", INSTANCE_FLAGS ).GetValue( hierarchyWindow );
			object hierarchyTree = hierarchyTreeOwner.GetType().GetField( "m_TreeView", INSTANCE_FLAGS ).GetValue( hierarchyTreeOwner );
			if( hierarchyTree != null )
			{
				List<EntityId> expandedSceneIDs = new(4);
				foreach( string expandedSceneName in (IEnumerable<string>) hierarchyTreeOwner.GetType().GetMethod( "GetExpandedSceneNames", INSTANCE_FLAGS ).Invoke( hierarchyTreeOwner, null ) )
				{
					Scene scene = SceneManager.GetSceneByName( expandedSceneName );
                    if (scene.IsValid())
#if UNITY_6000_3_OR_NEWER
                        expandedSceneIDs.Add((EntityId)typeof(SceneHandle).GetMethod("ToEntityId", INSTANCE_FLAGS).Invoke(scene.handle, null)); // SceneHandle's EntityId is used by SceneHierarchyWindow
#else
                        expandedSceneIDs.Add(scene.GetHashCode()); // GetHashCode returns m_Handle which in turn is used as the Scene's instanceID by SceneHierarchyWindow
#endif
                }

				CollapseTreeViewController( hierarchyWindow, hierarchyTree, (TreeViewState) hierarchyTreeOwner.GetType().GetField( "m_TreeViewState", INSTANCE_FLAGS ).GetValue( hierarchyTreeOwner ), expandedSceneIDs );
			}
		}
	}

	private static void CallCollapseGameObjectsOnce()
	{
		EditorApplication.update -= CallCollapseGameObjectsOnce;
		CollapseGameObjects( new MenuCommand( null ) );
	}

    private static void CollapseTreeViewController(EditorWindow editorWindow, object treeViewController, TreeViewState treeViewState, IList<EntityId> additionalInstanceIDsToExpand = null)
	{
		object treeViewDataSource = treeViewController.GetType().GetProperty( "data", INSTANCE_FLAGS ).GetValue( treeViewController, null );
        List<EntityId> treeViewSelectedIDs = new(treeViewState.selectedIDs);
        EntityId[] additionalInstanceIDsToExpandArray;
        if (additionalInstanceIDsToExpand != null && additionalInstanceIDsToExpand.Count > 0)
        {
            treeViewSelectedIDs.AddRange(additionalInstanceIDsToExpand);

            additionalInstanceIDsToExpandArray = new EntityId[additionalInstanceIDsToExpand.Count];
            additionalInstanceIDsToExpand.CopyTo(additionalInstanceIDsToExpandArray, 0);
        }
        else
            additionalInstanceIDsToExpandArray = new EntityId[0];

		treeViewDataSource.GetType().GetMethod( "SetExpandedIDs", INSTANCE_FLAGS ).Invoke( treeViewDataSource, new object[] { additionalInstanceIDsToExpandArray } );
		treeViewDataSource.GetType().GetMethod( "RevealItems", INSTANCE_FLAGS ).Invoke( treeViewDataSource, new object[] { treeViewSelectedIDs.ToArray() } );

		editorWindow.Repaint();
	}

	[MenuItem( "CONTEXT/Component/Collapse All", priority = 1400 )]
	private static void CollapseComponents( MenuCommand command )
	{
		// Credit: https://forum.unity.com/threads/is-it-possible-to-fold-a-component-from-script-inspector-view.296333/#post-2353538
		ActiveEditorTracker tracker = ActiveEditorTracker.sharedTracker;
		for( int i = 0, length = tracker.activeEditors.Length; i < length; i++ )
			tracker.SetVisible( i, 0 );

		EditorWindow.focusedWindow.Repaint();
	}
}

Not ideal but I might give it a try. Thanks very much.

thank you

Doesn't work anymore, just switches focus to Packages folder. But Unity does seem to have this function natively now, using Alt + click on the Assets folder.

@milox On which Unity version did it not work as intended?

@milox On which Unity version did it not work as intended?

2020.3.33 and 2021.3.0

I unfortunately couldn't reproduce the issue on 2021.3.0f1:

Well, I get this

@milox I haven't tried it in Two Column Layout before, you're right. I've now fixed the issue. It also works much smoother now ^^

@yasirkula Nice, thanks

Hey, thank you, very useful. I will try to adjust this to do what I long wished for, a "collapse all but one" functionality for components.

@WildRikku You can modify CollapseComponents to achieve that. The right clicked component is stored in command.context.

great script! got my star ❤️
is there any way to implement such option for when exporting a unity package?
it is very annoying when exporting something, unity selects all the assets imported automatically and unselecting them one by one is a nuisance.

@hegworks Thanks! In the export window, deselecting "Include dependencies" should do the trick.