A UI container for async loading.

AsyncBox.cs

using System;
using System.Collections;
using System.ComponentModel;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Markup;
using System.Windows.Media;
using System.Windows.Threading;
using Walterlv.Demo;
using Walterlv.Demo.Utils.Threading;
using DispatcherDictionary = System.Collections.Concurrent.ConcurrentDictionary<System.Windows.Threading.Dispatcher, Walterlv.Demo.Utils.Threading.DispatcherAsyncOperation<System.Windows.Threading.Dispatcher>>;

namespace Walterlv.Windows
{
    [ContentProperty(nameof(Child))]
    public class AsyncBox : FrameworkElement
    {
        /// <summary>
        /// 保存外部 UI 线程和与其关联的异步 UI 线程。
        /// 例如主 UI 线程对应一个 AsyncBox 专用的 UI 线程；外面可能有另一个 UI 线程，那么对应另一个 AsyncBox 专用的 UI 线程。
        /// </summary>
        private static readonly DispatcherDictionary RelatedAsyncDispatchers = new DispatcherDictionary();

        private UIElement _child;

        private readonly HostVisual _hostVisual;

        private VisualTargetPresentationSource _targetSource;

        private UIElement _loadingView;

        private readonly ContentPresenter _contentPresenter;

        private bool _isChildReadyToLoad;

        private Type _loadingViewType;

        public AsyncBox()
        {
            _hostVisual = new HostVisual();
            _contentPresenter = new ContentPresenter();
            Loaded += OnLoaded;
        }

        public UIElement Child
        {
            get => _child;
            set
            {
                if (Equals(_child, value)) return;

                if (value != null)
                {
                    RemoveLogicalChild(value);
                }

                _child = value;

                if (_isChildReadyToLoad)
                {
                    ActivateChild();
                }
            }
        }

        public Type LoadingViewType
        {
            get
            {
                if (_loadingViewType == null)
                {
                    throw new InvalidOperationException(
                        $"在 {nameof(AsyncBox)} 显示之前，必须先为 {nameof(LoadingViewType)} 设置一个 {nameof(UIElement)} 作为 Loading 视图。");
                }

                return _loadingViewType;
            }
            set
            {
                if (value == null)
                {
                    throw new ArgumentNullException(nameof(LoadingViewType));
                }

                if (_loadingViewType != null)
                {
                    throw new ArgumentException($"{nameof(LoadingViewType)} 只允许被设置一次。", nameof(value));
                }

                _loadingViewType = value;
            }
        }

        /// <summary>
        /// 返回一个可等待的用于显示异步 UI 的后台 UI 线程调度器。
        /// </summary>
        private DispatcherAsyncOperation<Dispatcher> GetAsyncDispatcherAsync() => RelatedAsyncDispatchers.GetOrAdd(
            Dispatcher, dispatcher => UIDispatcher.RunNewAsync("AsyncBox"));

        private UIElement CreateLoadingView()
        {
            var instance = Activator.CreateInstance(LoadingViewType);
            if (instance is UIElement element)
            {
                return element;
            }

            throw new InvalidOperationException($"{LoadingViewType} 必须是 {nameof(UIElement)} 类型");
        }

        private async void OnLoaded(object sender, RoutedEventArgs e)
        {
            if (DesignerProperties.GetIsInDesignMode(this)) return;

            var dispatcher = await GetAsyncDispatcherAsync();
            _loadingView = await dispatcher.InvokeAsync(() =>
            {
                var loadingView = CreateLoadingView();
                _targetSource = new VisualTargetPresentationSource(_hostVisual)
                {
                    RootVisual = loadingView
                };
                return loadingView;
            });
            AddVisualChild(_contentPresenter);
            AddVisualChild(_hostVisual);

            await LayoutAsync();
            await Dispatcher.Yield(DispatcherPriority.Background);

            _isChildReadyToLoad = true;
            ActivateChild();
        }

        private void ActivateChild()
        {
            var child = Child;
            if (child != null)
            {
                _contentPresenter.Content = child;
                AddLogicalChild(child);
                InvalidateMeasure();
            }
        }

        private async Task LayoutAsync()
        {
            var dispatcher = await GetAsyncDispatcherAsync();
            await dispatcher.InvokeAsync(() =>
            {
                if (_loadingView != null)
                {
                    _loadingView.Measure(RenderSize);
                    _loadingView.Arrange(new Rect(RenderSize));
                }
            });
        }

        protected override int VisualChildrenCount => _loadingView != null ? 2 : 0;

        protected override Visual GetVisualChild(int index)
        {
            switch (index)
            {
                case 0:
                    return _contentPresenter;
                case 1:
                    return _hostVisual;
                default:
                    return null;
            }
        }

        protected override IEnumerator LogicalChildren
        {
            get
            {
                if (_isChildReadyToLoad)
                {
                    yield return _contentPresenter;
                }
            }
        }

        protected override Size MeasureOverride(Size availableSize)
        {
            if (_isChildReadyToLoad)
            {
                _contentPresenter.Measure(availableSize);
                return _contentPresenter.DesiredSize;
            }

            var size = base.MeasureOverride(availableSize);
            return size;
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            if (_isChildReadyToLoad)
            {
                _contentPresenter.Arrange(new Rect(finalSize));
                var renderSize = _contentPresenter.RenderSize;
                LayoutAsync().ConfigureAwait(false);
                return renderSize;
            }

            var size = base.ArrangeOverride(finalSize);
            LayoutAsync().ConfigureAwait(false);
            return size;
        }
    }
}

是因为 AsyncBox 如果要写到 XAML 里面，要求卡只能卡在构造函数之后。如果构造函数卡了，就是卡在 XAML 上，救不回来的。解决方法有二：

1. 将卡顿代码从构造函数移至后续（如 Loaded 事件中）
2. 不在 XAML 中初始化，改用 C# 初始化

如果你试验成功，那么我更新下我的博客。