Avalonia/src/Avalonia.Controls/TransitioningContentControl.cs

using System;
using System.Threading;
using System.Threading.Tasks;
using Avalonia.Animation;
using Avalonia.Controls.Presenters;
using Avalonia.Controls.Templates;
using Avalonia.Data;

namespace Avalonia.Controls;

/// <summary>
/// Displays <see cref="ContentControl.Content"/> according to an <see cref="IDataTemplate"/>,
/// using a <see cref="PageTransition"/> to move between the old and new content. 
/// </summary>
public class TransitioningContentControl : ContentControl
{
    private CancellationTokenSource? _currentTransition;
    private ContentPresenter? _presenter2;
    private bool _isFirstFull;
    private bool _shouldAnimate;

    /// <summary>
    /// Defines the <see cref="PageTransition"/> property.
    /// </summary>
    public static readonly StyledProperty<IPageTransition?> PageTransitionProperty =
        AvaloniaProperty.Register<TransitioningContentControl, IPageTransition?>(
            nameof(PageTransition),
            defaultValue: new ImmutableCrossFade(TimeSpan.FromMilliseconds(125)));

    /// <summary>
    /// Gets or sets the animation played when content appears and disappears.
    /// </summary>
    public IPageTransition? PageTransition
    {
        get => GetValue(PageTransitionProperty);
        set => SetValue(PageTransitionProperty, value);
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        var result = base.ArrangeOverride(finalSize);

        if (_shouldAnimate)
        {
            _currentTransition?.Cancel();

            if (_presenter2 is not null &&
                Presenter is Visual presenter &&
                PageTransition is { } transition)
            {   
                _shouldAnimate = false;
                
                var cancel = new CancellationTokenSource();
                _currentTransition = cancel;

                var from = _isFirstFull ? _presenter2 : presenter;
                var to = _isFirstFull ? presenter : _presenter2;

                transition.Start(from, to, true, cancel.Token).ContinueWith(x =>
                {
                    if (!cancel.IsCancellationRequested)
                    {
                        HideOldPresenter();
                    }
                }, TaskScheduler.FromCurrentSynchronizationContext());
            }

            _shouldAnimate = false;
        }

        return result;
    }

    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
    {
        base.OnAttachedToVisualTree(e);
        UpdateContent(false);
    }

    protected override bool RegisterContentPresenter(ContentPresenter presenter)
    {
        if (base.RegisterContentPresenter(presenter))
        {
            return true;
        }

        if (presenter is ContentPresenter p &&
            p.Name == "PART_ContentPresenter2")
        {
            _presenter2 = p;
            _presenter2.IsVisible = false;
            UpdateContent(false);
            return true;
        }

        return false;
    }

    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
    {
        if (change.Property == ContentProperty)
        {
            UpdateContent(true);
            return;
        }

        base.OnPropertyChanged(change);
    }

    private void UpdateContent(bool withTransition)
    {
        if (VisualRoot is null || _presenter2 is null || Presenter is null)
        {
            return;
        }

        var currentPresenter = _isFirstFull ? _presenter2 : Presenter;
        currentPresenter.Content = Content;
        currentPresenter.IsVisible = true;

        _isFirstFull = !_isFirstFull;

        if (PageTransition is not null && withTransition)
        {
            _shouldAnimate = true;
            InvalidateArrange();
        }
        else
        {
            HideOldPresenter();
        }
    }

    private void HideOldPresenter()
    {
        var oldPresenter = _isFirstFull ? _presenter2 : Presenter;
        if (oldPresenter is not null)
        {
            oldPresenter.Content = null;
            oldPresenter.IsVisible = false;
        }
    }

    private class ImmutableCrossFade : IPageTransition
    {
        private readonly CrossFade _inner;

        public ImmutableCrossFade(TimeSpan duration) => _inner = new CrossFade(duration);

        public Task Start(Visual? from, Visual? to, bool forward, CancellationToken cancellationToken)
        {
            return _inner.Start(from, to, cancellationToken);
        }
    }
}