ShrlAlgoToolkit/ShrlAlgoToolkit.RevitAddins/Common/Assists/SingletonViewAssist.cs

using System.Windows;

namespace ShrlAlgoToolkit.RevitAddins.Common.Assists;

public sealed record SingletonViewAssist<T>
    where T : Window, new()
{
    private static T _instance;
    private static readonly object Padlock = new();

    private SingletonViewAssist() { }

    public static T GetInstance(out bool isNewCreate)
    {
        isNewCreate = false;
        //double-check locking
        if (_instance == null)
        {
            isNewCreate = true;
            lock (Padlock)
            {
                _instance ??= new T();
                _instance.Closed += OnWindowClosed;
            }
        }
        return _instance;
    }

    private static void OnWindowClosed(object sender, EventArgs e)
    {
        _instance = null;
    }
}