Avalonia/src/Avalonia.Controls/Presenters/ItemVirtualizer.cs

// Copyright (c) The Avalonia Project. All rights reserved.
// Licensed under the MIT license. See licence.md file in the project root for full license information.

using System;
using System.Collections;
using System.Collections.Specialized;
using Avalonia.Controls.Primitives;
using Avalonia.Controls.Utils;
using Avalonia.Input;
using Avalonia.VisualTree;

namespace Avalonia.Controls.Presenters
{
    /// <summary>
    /// Base class for classes which handle virtualization for an <see cref="ItemsPresenter"/>.
    /// </summary>
    internal abstract class ItemVirtualizer : IVirtualizingController, IDisposable
    {
        private bool disposedValue;

        /// <summary>
        /// Initializes a new instance of the <see cref="ItemVirtualizer"/> class.
        /// </summary>
        /// <param name="owner"></param>
        public ItemVirtualizer(ItemsPresenter owner)
        {
            Owner = owner;
            Items = owner.Items;
            ItemCount = owner.Items.Count();
        }

        /// <summary>
        /// Gets the <see cref="ItemsPresenter"/> which owns the virtualizer.
        /// </summary>
        public ItemsPresenter Owner { get; }

        /// <summary>
        /// Gets the <see cref="IVirtualizingPanel"/> which will host the items.
        /// </summary>
        public IVirtualizingPanel VirtualizingPanel => Owner.Panel as IVirtualizingPanel;

        /// <summary>
        /// Gets the items to display.
        /// </summary>
        public IEnumerable Items { get; private set; }

        /// <summary>
        /// Gets the number of items in <see cref="Items"/>.
        /// </summary>
        public int ItemCount { get; private set; }

        /// <summary>
        /// Gets or sets the index of the first item displayed in the panel.
        /// </summary>
        public int FirstIndex { get; protected set; }

        /// <summary>
        /// Gets or sets the index of the first item beyond those displayed in the panel.
        /// </summary>
        public int NextIndex { get; protected set; }

        /// <summary>
        /// Gets a value indicating whether the items should be scroll horizontally or vertically.
        /// </summary>
        public bool Vertical => VirtualizingPanel.ScrollDirection == Orientation.Vertical;

        /// <summary>
        /// Gets a value indicating whether logical scrolling is enabled.
        /// </summary>
        public abstract bool IsLogicalScrollEnabled { get; }

        /// <summary>
        /// Gets the value of the scroll extent.
        /// </summary>
        public abstract double ExtentValue { get; }

        /// <summary>
        /// Gets or sets the value of the current scroll offset.
        /// </summary>
        public abstract double OffsetValue { get; set; }

        /// <summary>
        /// Gets the value of the scrollable viewport.
        /// </summary>
        public abstract double ViewportValue { get; }

        /// <summary>
        /// Gets the <see cref="ExtentValue"/> as a <see cref="Size"/>.
        /// </summary>
        public Size Extent => Vertical ? new Size(0, ExtentValue) : new Size(ExtentValue, 0);

        /// <summary>
        /// Gets the <see cref="ViewportValue"/> as a <see cref="Size"/>.
        /// </summary>
        public Size Viewport => Vertical ? new Size(0, ViewportValue) : new Size(ViewportValue, 0);

        /// <summary>
        /// Gets or sets the <see cref="OffsetValue"/> as a <see cref="Vector"/>.
        /// </summary>
        public Vector Offset
        {
            get
            {
                return Vertical ? new Vector(0, OffsetValue) : new Vector(OffsetValue, 0);
            }

            set
            {
                OffsetValue = Vertical ? value.Y : value.X;
            }
        }
        
        /// <summary>
        /// Creates an <see cref="ItemVirtualizer"/> based on an item presenter's 
        /// <see cref="ItemVirtualizationMode"/>.
        /// </summary>
        /// <param name="owner">The items presenter.</param>
        /// <returns>An <see cref="ItemVirtualizer"/>.</returns>
        public static ItemVirtualizer Create(ItemsPresenter owner)
        {
            var virtualizingPanel = owner.Panel as IVirtualizingPanel;
            var scrollable = (ILogicalScrollable)owner;
            ItemVirtualizer result = null;

            if (virtualizingPanel != null && scrollable.InvalidateScroll != null)
            {
                switch (owner.VirtualizationMode)
                {
                    case ItemVirtualizationMode.Simple:
                        result = new ItemVirtualizerSimple(owner);
                        break;
                }
            }

            if (result == null)
            {
                result = new ItemVirtualizerNone(owner);
            }

            if (virtualizingPanel != null)
            {
                virtualizingPanel.Controller = result;
            }

            return result;
        }

        /// <inheritdoc/>
        public virtual void UpdateControls()
        {
        }

        /// <summary>
        /// Gets the next control in the specified direction.
        /// </summary>
        /// <param name="direction">The movement direction.</param>
        /// <param name="from">The control from which movement begins.</param>
        /// <returns>The control.</returns>
        public virtual IControl GetControlInDirection(NavigationDirection direction, IControl from)
        {
            return null;
        }

        /// <summary>
        /// Called when the items for the presenter change, either because 
        /// <see cref="ItemsPresenterBase.Items"/> has been set, the items collection has been
        /// modified, or the panel has been created.
        /// </summary>
        /// <param name="items">The items.</param>
        /// <param name="e">A description of the change.</param>
        public virtual void ItemsChanged(IEnumerable items, NotifyCollectionChangedEventArgs e)
        {
            Items = items;
            ItemCount = items.Count();
        }

        /// <summary>
        /// Scrolls the specified item into view.
        /// </summary>
        /// <param name="item">The item.</param>
        public virtual void ScrollIntoView(object item)
        {
        }

        /// <inheritdoc/>
        public virtual void Dispose()
        {
            if (VirtualizingPanel != null)
            {
                VirtualizingPanel.Controller = null;
                VirtualizingPanel.Children.Clear();
            }

            Owner.ItemContainerGenerator.Clear();
        }

        /// <summary>
        /// Invalidates the current scroll.
        /// </summary>
        protected void InvalidateScroll() => ((ILogicalScrollable)Owner).InvalidateScroll();
    }
}