Custom editor tool

LookAtTool

using UnityEngine;
using UnityEditor;
using UnityEditor.EditorTools;

[EditorTool( "Look at Tool" )]
public sealed class LookAtTool : EditorTool {
    private const string ICON_NAME = "ViewToolOrbit";

    private GUIContent cachedIcon;

    public override GUIContent toolbarIcon {
        get
        {
            if ( cachedIcon == null )
                cachedIcon = EditorGUIUtility.IconContent( ICON_NAME, "Look at Tool" );

            return cachedIcon;
        }
    }

    public override void OnToolGUI( EditorWindow window ) {
        var view = SceneView.lastActiveSceneView;
        if ( Selection.transforms.Length > 1 ) {
            Vector3 focusPoint = default;
            foreach ( var transform in Selection.transforms ) {
                focusPoint += transform.position;
            }

            focusPoint /= Selection.transforms.Length;
            view.LookAt( focusPoint );
            Handles.Label( focusPoint, "Focus point" );
            Handles.SphereHandleCap( 0, focusPoint, Quaternion.identity, 0.1f, EventType.Repaint );
        }

        else if ( Selection.activeTransform != null ) {
            view.LookAt( Selection.activeTransform.position );
        }
    }
}