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What is the process for implementing auto scrolling via drag operation in .NET MAUI?

asked 2021-10-01 11:00:00 +0000

bukephalos gravatar image

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answered 2022-07-12 02:00:00 +0000

djk gravatar image

The process for implementing auto scrolling via drag operation in .NET MAUI involves the following steps:

  1. Create a custom control that inherits from ScrollView.

  2. Add a PanGestureRecognizer to the custom control.

  3. Handle the PanGestureRecognizer's PanUpdated event to determine the drag direction and speed.

  4. Calculate the position of the scroll content based on the drag direction and speed.

  5. Start a timer to continuously update the scroll position until the drag operation is complete.

  6. Stop the timer when the drag operation is complete.

  7. Handle other pan-related events, such as PanStarted and PanCompleted, to enable/disable the auto scrolling feature as needed.

Here is sample code that demonstrates this process:

public class AutoScrollView : ScrollView
{
    private readonly PanGestureRecognizer _panGesture;

    private Timer _scrollTimer;
    private double _scrollOffset;

    public AutoScrollView()
    {
        _panGesture = new PanGestureRecognizer();
        _panGesture.PanUpdated += OnPanUpdated;
        GestureRecognizers.Add(_panGesture);
    }

    private void OnPanUpdated(object sender, PanUpdatedEventArgs e)
    {
        switch (e.StatusType)
        {
            case GestureStatus.Running:
                var speed = CalculateScrollSpeed(e);
                _scrollOffset = CalculateScrollOffset(speed);
                StartScrollTimer();
                break;
            case GestureStatus.Completed:
            case GestureStatus.Canceled:
                StopScrollTimer();
                break;
        }
    }

    private double CalculateScrollSpeed(PanUpdatedEventArgs e)
    {
        var x = Math.Abs(e.TotalX);
        var y = Math.Abs(e.TotalY);
        return Math.Max(x, y);
    }

    private double CalculateScrollOffset(double speed)
    {
        var direction = _panGesture.PanOffset.Normalize();
        var distance = speed / 60.0; // 60 fps
        var offset = direction * distance * 2.0; // 2x factor for speed
        return offset;
    }

    private void StartScrollTimer()
    {
        if (_scrollTimer == null)
        {
            _scrollTimer = new Timer(16); // 60 fps
            _scrollTimer.Elapsed += OnScrollTimerElapsed;
        }
        if (!_scrollTimer.Enabled)
        {
            _scrollTimer.Start();
        }
    }

    private void StopScrollTimer()
    {
        if (_scrollTimer != null && _scrollTimer.Enabled)
        {
            _scrollTimer.Stop();
        }
    }

    private void OnScrollTimerElapsed(object sender, ElapsedEventArgs e)
    {
        Dispatcher.BeginInvokeOnMainThread(() =>
        {
            ScrollToAsync(Content.X + _scrollOffset, Content.Y + _scrollOffset, false);
        });
    }
}

This code creates a custom control called AutoScrollView that adds a PanGestureRecognizer to enable drag operation. The OnPanUpdated event handler calculates the drag direction and speed using the CalculateScrollSpeed and CalculateScrollOffset methods. It then starts a timer to continuously update the scroll position until the drag operation is complete. The OnScrollTimerElapsed event handler updates the scroll position using the ScrollToAsync method.

To use this control, simply replace the ScrollView control with AutoScrollView in your XAML code. You can customize the drag speed and direction by changing the calculations in the CalculateScrollSpeed and CalculateScrollOffset methods. You can also enable/disable the auto scrolling feature using other pan-related events, such as PanStarted and PanCompleted.

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Asked: 2021-10-01 11:00:00 +0000

Seen: 2 times

Last updated: Jul 12 '22