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;

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( int ) }, null ).Invoke( treeViewDataSource, new object[] { searchFiltersRootInstanceID } );

				CollapseTreeViewController( projectWindow, folderTree, (TreeViewState) projectWindow.GetType().GetField( "m_FolderTreeState", INSTANCE_FLAGS ).GetValue( projectWindow ), isSearchFilterRootExpanded ? new int[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
				InternalEditorUtility.expandedProjectWindowItems = (int[]) treeViewDataSource.GetType().GetMethod( "GetExpandedIDs", INSTANCE_FLAGS ).Invoke( treeViewDataSource, null );

				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( int ) }, 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 )
		{
#if UNITY_2018_3_OR_NEWER
			object hierarchyTreeOwner = hierarchyWindow.GetType().GetField( "m_SceneHierarchy", INSTANCE_FLAGS ).GetValue( hierarchyWindow );
#else
			object hierarchyTreeOwner = hierarchyWindow;
#endif
			object hierarchyTree = hierarchyTreeOwner.GetType().GetField( "m_TreeView", INSTANCE_FLAGS ).GetValue( hierarchyTreeOwner );
			if( hierarchyTree != null )
			{
				List<int> expandedSceneIDs = new List<int>( 4 );
				foreach( string expandedSceneName in (IEnumerable<string>) hierarchyTreeOwner.GetType().GetMethod( "GetExpandedSceneNames", INSTANCE_FLAGS ).Invoke( hierarchyTreeOwner, null ) )
				{
					Scene scene = SceneManager.GetSceneByName( expandedSceneName );
					if( scene.IsValid() )
						expandedSceneIDs.Add( scene.GetHashCode() ); // GetHashCode returns m_Handle which in turn is used as the Scene's instanceID by SceneHierarchyWindow
				}

				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<int> additionalInstanceIDsToExpand = null )
	{
		object treeViewDataSource = treeViewController.GetType().GetProperty( "data", INSTANCE_FLAGS ).GetValue( treeViewController, null );
		List<int> treeViewSelectedIDs = new List<int>( treeViewState.selectedIDs );
		int[] additionalInstanceIDsToExpandArray;
		if( additionalInstanceIDsToExpand != null && additionalInstanceIDsToExpand.Count > 0 )
		{
			treeViewSelectedIDs.AddRange( additionalInstanceIDsToExpand );

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

		treeViewDataSource.GetType().GetMethod( "SetExpandedIDs", INSTANCE_FLAGS ).Invoke( treeViewDataSource, new object[] { additionalInstanceIDsToExpandArray } );
#if UNITY_2019_1_OR_NEWER
		treeViewDataSource.GetType().GetMethod( "RevealItems", INSTANCE_FLAGS ).Invoke( treeViewDataSource, new object[] { treeViewSelectedIDs.ToArray() } );
#else
		foreach( int treeViewSelectedID in treeViewSelectedIDs )
			treeViewDataSource.GetType().GetMethod( "RevealItem", INSTANCE_FLAGS ).Invoke( treeViewDataSource, new object[] { treeViewSelectedID } );
#endif

		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();
	}
}

How To
Just put it in Assets/Editor folder of your project.

Collapsing Hierarchy View
To collapse all GameObjects in the currently active Hierarchy window, either use the GameObject menu or right click a GameObject and select "Collapse All". Note that parent GameObjects containing the selected GameObjects will not be collapsed.

Collapsing Project View
To collapse all folders in Project window, either use the Assets menu or right click an asset and select "Collapse All". Note that folders containing the selected assets will not be collapsed.

Collapsing Components
To collapse all components in the Inspector window, right click a component and select "Collapse All".

Hey this is cool! Thankyou!
I combined it with this to have collapse on components also. https://forum.unity.com/threads/expand-collapse-all-components-in-a-gameobject-refresh-issue.517866/#post-3471049

FYI the same functionality is achieved by simply holding Alt when clicking a foldout 😃

That's right Alt + LMB on the disclosing arrow achieves the exact same thing. Came here 'cause I'm looking for collapse all components in inspector.

Added! Simply right click a component and select "Collapse All".

Tried using the updated code just now and Inspector Collapse All does not work, at least not on Unity 2018.4.0f1

Edit: It has started working after a little while ( after restarting Unity I think). But there is still a problem: When navigating away and back from a collapsed game object the inspector shows the object expanded, in other words it does not seem to remember the collapsed state of the object.

Is there any way to fix this issue?

You can add this code to the top of CollapseComponents function:

if( Selection.activeGameObject )
{
	foreach( Component component in Selection.activeGameObject.GetComponents<Component>() )
		UnityEditorInternal.InternalEditorUtility.SetIsInspectorExpanded( component, false );
}

But be aware that this will affect that component type (e.g. Transform) on ALL objects. There is no way to set the collapsed state of a component permanantly object-wise.

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