particles/MainMenu/MainMenu.cs

using System.Globalization;
using Godot;

public class MainMenu : Control
{

    // Nodes for menu items
    private Main _main;
    private Button _simulateButton;
    private Button _toggleFullScreenButton;
    private TextEdit _seedText;
    private TextEdit _particleCountText;
    private Button _randomizeButton;
    private Label _zoomValue;
    private HSlider _zoomSlider;

    private Label _invalidLabel;

    private string _prevSeedText;
    private string _prevParticleCountText;

    public override void _Ready()
    {
        // Init nodes
        _main = GetTree().Root.GetNode<Main>("Main");
        _simulateButton = GetNode("MenuButtons").GetNode<Button>("SimulateButton");
        _toggleFullScreenButton = GetNode("MenuButtons").GetNode<Button>("ToggleFullscreenButton");
        _seedText = GetNode("MenuButtons").GetNode("Inputs").GetNode("Seed").GetNode<TextEdit>("SeedText");
        _particleCountText = GetNode("MenuButtons").GetNode("Inputs").GetNode("ParticleCount").GetNode<TextEdit>("ParticleCountText");
        _randomizeButton = GetNode("MenuButtons").GetNode("Inputs").GetNode("Seed").GetNode<Button>("RandomizeButton");
        _zoomValue = GetNode("MenuButtons").GetNode("Inputs").GetNode("Zoom").GetNode<Label>("ZoomValue");
        _zoomSlider = GetNode("MenuButtons").GetNode("Inputs").GetNode("Zoom").GetNode<HSlider>("ZoomSlider");
        _invalidLabel = GetNode<Label>("InvalidLabel");
        
        // Connect signals
        _simulateButton.Connect("pressed", this, nameof(_OnSimulatePressed));
        _toggleFullScreenButton.Connect("pressed", this, nameof(_OnToggleFullscreenPressed));
        _randomizeButton.Connect("pressed", this, nameof(_OnRandomizePressed));
        _zoomSlider.Connect("value_changed", this, nameof(_OnZoomSliderChange));
        _seedText.Connect("text_changed", this, nameof(_OnSeedTextChange));
        _particleCountText.Connect("text_changed", this, nameof(_OnParticleCountTextChange));
        
        // Random seed
        GD.Randomize();
        var randomSeed = Mathf.Abs((int)GD.Randi()); 
        _main.Seed = randomSeed;
        RefreshSeedText();
        
        // Set default previous values
        _prevSeedText = _seedText.Text;
        _prevParticleCountText = _particleCountText.Text;
    }

    public void _OnParticleCountTextChange()
    {
        var particleCountCheck = int.TryParse(_particleCountText.Text, out var nParticles);
        if (!particleCountCheck && !_particleCountText.Text.Empty())
        {
            ShowInvalid();
            _particleCountText.Text = _prevParticleCountText;
        }
        else if (!_particleCountText.Text.Empty())
        {
            _prevParticleCountText = _particleCountText.Text;
        }
    }

    public void _OnSeedTextChange()
    {
        var seedCheck = int.TryParse(_seedText.Text, out var seed);
        if (!seedCheck && !_seedText.Text.Empty())
        {
            ShowInvalid();
            _seedText.Text = _prevSeedText;
        }
        else if (!_seedText.Text.Empty())
        {
            _prevSeedText = _seedText.Text;
        }
    }

    public void _OnZoomSliderChange(float value)
    {
        _zoomValue.Text = value.ToString(CultureInfo.InvariantCulture);
        _main.Zoom = value;
    }

    public void RefreshSeedText()
    {
        _seedText.Text = _main.Seed.ToString();
        _prevSeedText = _seedText.Text;
    }

    public void _OnSimulatePressed()
    {
        // Start simulation with seed and number of particles
        if (_seedText.Text.Empty() || _particleCountText.Text.Empty())
        {
            ShowInvalid();
            return;
        }
        _main.Zoom = (float)_zoomSlider.Value;
        _main.Seed = int.Parse(_seedText.Text);
        _main.StartSimulation(int.Parse(_particleCountText.Text));
    }

    public void _OnToggleFullscreenPressed()
    {
        OS.WindowFullscreen = !OS.WindowFullscreen;
    }

    public void _OnRandomizePressed()
    {
        var randomSeed = Mathf.Abs((int)GD.Randi());
        _main.Seed = randomSeed;
        RefreshSeedText();
    }

    public override void _Input(InputEvent @event)
    {
        // Quit only if menu is visible
        if (Visible && @event.IsActionPressed("quit"))
        {
            GetTree().Quit();
        }
    }

    private async void ShowInvalid()
    {
        GetNode<Tween>("InvalidTween").ResetAll();
        GetNode<Timer>("InvalidTimer").Stop();
        GetNode<Timer>("InvalidTimer").WaitTime = 3.0f;
        _invalidLabel.Modulate = new Color(1, 1, 1, 1);
        var invalidTimer = GetNode<Timer>("InvalidTimer");
        invalidTimer.Start();
        await ToSignal(invalidTimer, "timeout");
        var invalidTween = GetNode<Tween>("InvalidTween");
        invalidTween.InterpolateProperty(_invalidLabel, "modulate:a", 1, 0, 1);
        invalidTween.Start();
        await ToSignal(invalidTween, "tween_all_completed");
    }
}