SwipeGestureRecognizer.cs

using System.Drawing;
using Gondwana.Timers;
 
namespace Gondwana.Input.Touch.Gestures;

public sealed class SwipeGestureRecognizer : IDisposable
{
    private readonly ITouchInput _touchInput;
    private readonly Dictionary<int, SwipeState> _activeSwipes = new();
    private readonly HashSet<int> _activeContacts = new();
    private bool _isMultiTouchSequence;
    private bool _isDisposed;

    public double MinimumSwipeSpeedPixelsPerSecond { get; set; } = 200;

    public double MinimumSwipeDistancePixels { get; set; } = 30;

    public event EventHandler<SwipedEventArgs>? Swiped;

    public SwipeGestureRecognizer(ITouchInput touchInput)
    {
        _touchInput = touchInput ?? throw new ArgumentNullException(nameof(touchInput));
        _touchInput.TouchBegan += OnTouchBegan;
        _touchInput.TouchEnded += OnTouchEnded;
    }
 
    private void OnTouchBegan(object? sender, TouchEventArgs e)
    {
        _activeContacts.Add(e.Touch.Id);
 
        if (_activeContacts.Count > 1)
        {
            _isMultiTouchSequence = true;
            _activeSwipes.Clear();
            return;
        }
 
        if (!_isMultiTouchSequence)
            _activeSwipes[e.Touch.Id] = new SwipeState(e.Touch.Position, e.Tick);
    }
 
    private void OnTouchEnded(object? sender, TouchEventArgs e)
    {
        _activeContacts.Remove(e.Touch.Id);
 
        if (_isMultiTouchSequence)
        {
            _activeSwipes.Remove(e.Touch.Id);
            if (_activeContacts.Count == 0)
                _isMultiTouchSequence = false;
            return;
        }
 
        if (!_activeSwipes.TryGetValue(e.Touch.Id, out var state))
            return;
 
        _activeSwipes.Remove(e.Touch.Id);
 
        // A system-cancelled contact must never produce a swipe.
        if (e.Touch.Phase == TouchPhase.Cancelled)
            return;
 
        var endPos = e.Touch.Position;
        var dx = endPos.X - state.StartPosition.X;
        var dy = endPos.Y - state.StartPosition.Y;
 
        var distance = Math.Sqrt(dx * dx + dy * dy);
        var elapsed = HighResTimer.GetDuration(state.StartTick, e.Tick);
 
        if (elapsed <= 0)
            return;
 
        var speed = distance / elapsed;
 
        if (!WouldRecognize(distance, elapsed))
            return;
 
        var direction = Math.Abs(dx) >= Math.Abs(dy)
            ? (dx >= 0 ? SwipeDirection.Right : SwipeDirection.Left)
            : (dy >= 0 ? SwipeDirection.Down : SwipeDirection.Up);
 
        Swiped?.Invoke(this, new SwipedEventArgs(direction, state.StartPosition, endPos, speed));
    }

    public void Dispose()
    {
        if (_isDisposed) return;
        _isDisposed = true;
 
        _touchInput.TouchBegan -= OnTouchBegan;
        _touchInput.TouchEnded -= OnTouchEnded;
        Reset();
    }
 
    internal void Reset()
    {
        _activeSwipes.Clear();
        _activeContacts.Clear();
        _isMultiTouchSequence = false;
    }
 
    internal bool WouldRecognize(double distance, double elapsedSeconds)
        => elapsedSeconds > 0 &&
           distance >= MinimumSwipeDistancePixels &&
           distance / elapsedSeconds >= MinimumSwipeSpeedPixelsPerSecond;
 
    private readonly record struct SwipeState(Point StartPosition, long StartTick);
}