EinmaleinsTrainer/ViewModels/MainWindowViewModel.cs

using ReactiveUI;
using System;
using System.Reactive.Linq;

namespace EinmaleinsTrainer.ViewModels;

public class MainWindowViewModel : ViewModelBase
{
    private ViewModelBase _current;
    public ViewModelBase Current
    {
        get => _current;
        set => SetProperty(ref _current, value);
    }

    public readonly MainViewModel MainModel = new(); 
    public readonly QuizViewModel QuizModel = new();
    public readonly ResultViewModel ResultModel = new();
    private readonly ViewModelBase _dummy = new();

    public MainWindowViewModel()
    {
        MainModel.Init();
        _current = MainModel;

        ResultModel.RestartCommand
            .ObserveOn(RxApp.MainThreadScheduler)
            .Subscribe(result =>
            {
                // In order to keep the last value if required
                int secondsPerQuestion = MainModel.SecondsPerQuestion;
                MainModel.Init();
                if (result.TimeRequired != null)
                {
                    MainModel.SecondsPerQuestion = (result.Score >= 10) ?
                        (int)Math.Round(0.15 * result.TimeRequired.Value.TotalSeconds, 0, MidpointRounding.AwayFromZero) :
                        secondsPerQuestion;
                    MainModel.UseTimer = true;
                }
                Current = MainModel;
            });

        QuizModel.Finished
            .ObserveOn(RxApp.MainThreadScheduler)
            .Subscribe(result =>
            {
                ResultModel.Init(result);
                Current = ResultModel;
            });

        QuizModel.RedrawRequired += (s, a) =>
        {
            Current = _dummy;
            Current = QuizModel;
        };

        MainModel.StartCommand
            .ObserveOn(RxApp.MainThreadScheduler)
            .Subscribe(quiz =>
            {
                if (quiz.Questions.Count == 0) return;
                QuizModel.Init(quiz);
                Current = QuizModel;
            });

        MainModel.RedrawRequired += (s, a) =>
        {
            Current = _dummy;
            Current = MainModel;
        };

    }
}