Avalonia/src/Avalonia.Controls/ContentControl.cs

using Avalonia.Collections;
using Avalonia.Controls.Metadata;
using Avalonia.Controls.Mixins;
using Avalonia.Controls.Presenters;
using Avalonia.Controls.Primitives;
using Avalonia.Controls.Templates;
using Avalonia.Layout;
using Avalonia.LogicalTree;
using Avalonia.Metadata;

namespace Avalonia.Controls
{
    /// <summary>
    /// Displays <see cref="Content"/> according to an <see cref="IDataTemplate"/>.
    /// </summary>
    [TemplatePart("PART_ContentPresenter", typeof(IContentPresenter))]
    public class ContentControl : TemplatedControl, IContentControl, IContentPresenterHost
    {
        /// <summary>
        /// Defines the <see cref="Content"/> property.
        /// </summary>
        public static readonly StyledProperty<object?> ContentProperty =
            AvaloniaProperty.Register<ContentControl, object?>(nameof(Content));

        /// <summary>
        /// Defines the <see cref="ContentTemplate"/> property.
        /// </summary>
        public static readonly StyledProperty<IDataTemplate?> ContentTemplateProperty =
            AvaloniaProperty.Register<ContentControl, IDataTemplate?>(nameof(ContentTemplate));

        /// <summary>
        /// Defines the <see cref="HorizontalContentAlignment"/> property.
        /// </summary>
        public static readonly StyledProperty<HorizontalAlignment> HorizontalContentAlignmentProperty =
            AvaloniaProperty.Register<ContentControl, HorizontalAlignment>(nameof(HorizontalContentAlignment));

        /// <summary>
        /// Defines the <see cref="VerticalContentAlignment"/> property.
        /// </summary>
        public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
            AvaloniaProperty.Register<ContentControl, VerticalAlignment>(nameof(VerticalContentAlignment));

        static ContentControl()
        {
            ContentProperty.Changed.AddClassHandler<ContentControl>((x, e) => x.ContentChanged(e));
        }

        /// <summary>
        /// Gets or sets the content to display.
        /// </summary>
        [Content]
        [DependsOn(nameof(ContentTemplate))]
        public object? Content
        {
            get { return GetValue(ContentProperty); }
            set { SetValue(ContentProperty, value); }
        }

        /// <summary>
        /// Gets or sets the data template used to display the content of the control.
        /// </summary>
        public IDataTemplate? ContentTemplate
        {
            get { return GetValue(ContentTemplateProperty); }
            set { SetValue(ContentTemplateProperty, value); }
        }

        /// <summary>
        /// Gets the presenter from the control's template.
        /// </summary>
        public IContentPresenter? Presenter
        {
            get;
            private set;
        }

        /// <summary>
        /// Gets or sets the horizontal alignment of the content within the control.
        /// </summary>
        public HorizontalAlignment HorizontalContentAlignment
        {
            get { return GetValue(HorizontalContentAlignmentProperty); }
            set { SetValue(HorizontalContentAlignmentProperty, value); }
        }

        /// <summary>
        /// Gets or sets the vertical alignment of the content within the control.
        /// </summary>
        public VerticalAlignment VerticalContentAlignment
        {
            get { return GetValue(VerticalContentAlignmentProperty); }
            set { SetValue(VerticalContentAlignmentProperty, value); }
        }

        /// <inheritdoc/>
        IAvaloniaList<ILogical> IContentPresenterHost.LogicalChildren => LogicalChildren;

        /// <inheritdoc/>
        bool IContentPresenterHost.RegisterContentPresenter(IContentPresenter presenter)
        {
            return RegisterContentPresenter(presenter);
        }

        /// <summary>
        /// Called when an <see cref="IContentPresenter"/> is registered with the control.
        /// </summary>
        /// <param name="presenter">The presenter.</param>
        protected virtual bool RegisterContentPresenter(IContentPresenter presenter)
        {
            if (presenter.Name == "PART_ContentPresenter")
            {
                Presenter = presenter;
                return true;
            }

            return false;
        }

        protected virtual void ContentChanged(AvaloniaPropertyChangedEventArgs e)
        {
            UpdateLogicalTree(e.OldValue, e.NewValue);
        }

        protected void UpdateLogicalTree(object? toRemove, object? toAdd)
        {
            if (toRemove is ILogical oldChild)
            {
                LogicalChildren.Remove(oldChild);
            }

            if (toAdd is ILogical newChild)
            {
                LogicalChildren.Add(newChild);
            }
        }
    }
}