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.IO;
using UnityEditor;
using UnityEngine;
using UnityEngine.SceneManagement;

public static class EditorCollapseAll
{
	private static int wait = 0;
	private static int undoIndex;

	[MenuItem( "Assets/Collapse All", priority = 1000 )]
	public static void CollapseFolders()
	{
		FileSystemInfo[] rootItems = new DirectoryInfo( Application.dataPath ).GetFileSystemInfos();
		List<Object> rootItemsList = new List<Object>( rootItems.Length );
		for( int i = 0; i < rootItems.Length; i++ )
		{
			Object asset = AssetDatabase.LoadAssetAtPath<Object>( "Assets/" + rootItems[i].Name );
			if( asset != null )
				rootItemsList.Add( asset );
		}

		if( rootItemsList.Count > 0 )
		{
			Undo.IncrementCurrentGroup();
			Selection.objects = Selection.objects;
			undoIndex = Undo.GetCurrentGroup();

			EditorUtility.FocusProjectWindow();

			Selection.objects = rootItemsList.ToArray();

			EditorApplication.update -= CollapseHelper;
			EditorApplication.update += CollapseHelper;
		}
	}

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

	[MenuItem( "GameObject/Collapse All", priority = 40 )]
	private static void CollapseGameObjects( MenuCommand command )
	{
		// This happens when this button is clicked via hierarchy's right click context menu
		// and is called once for each object in the selection. We don't want that, we want
		// the function to be called only once
		if( command.context )
		{
			EditorApplication.update -= CollapseGameObjects;
			EditorApplication.update += CollapseGameObjects;

			return;
		}

		List<GameObject> rootGameObjects = new List<GameObject>();
#if UNITY_2018_3_OR_NEWER
		// Check if a prefab stage is currently open
		var prefabStage = UnityEditor.Experimental.SceneManagement.PrefabStageUtility.GetCurrentPrefabStage();
		if( prefabStage != null && prefabStage.stageHandle.IsValid() )
			rootGameObjects.Add( prefabStage.prefabContentsRoot );
		else
#endif
		{
			int sceneCount = SceneManager.sceneCount;
			for( int i = 0; i < sceneCount; i++ )
				rootGameObjects.AddRange( SceneManager.GetSceneAt( i ).GetRootGameObjects() );
		}

		if( rootGameObjects.Count > 0 )
		{
			Undo.IncrementCurrentGroup();
			Selection.objects = Selection.objects;
			undoIndex = Undo.GetCurrentGroup();

			Selection.objects = rootGameObjects.ToArray();

			EditorApplication.update -= CollapseHelper;
			EditorApplication.update += CollapseHelper;
		}
	}

	[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();
	}

	private static void CollapseHelper()
	{
		if( wait < 1 ) // Increase the number if script doesn't always work
			wait++;
		else
		{
			EditorApplication.update -= CollapseHelper;
			wait = 0;

			EditorWindow focusedWindow = EditorWindow.focusedWindow;
			if( focusedWindow != null )
				focusedWindow.SendEvent( new Event { keyCode = KeyCode.LeftArrow, type = EventType.KeyDown, alt = true } );

			Undo.RevertAllDownToGroup( undoIndex );
		}
	}
}

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".

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.