(MVVM) ListBox Binding 和 实时刷新

当需要用Lisbbox 来log 一些记录的时候，ObservableCollection 并不可以是记录实时的反应在WPF 的UI上面。

这个时候就需要用一个异步collection 来完成。

    /// <summary>
    /// Represents the asynchronous observable collection.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class AsyncObservableCollection<T> : ObservableCollection<T>
    {
        /// <summary>
        /// The _synchronization context
        /// </summary>
        private readonly SynchronizationContext synchronizationContext = SynchronizationContext.Current;

        /// <summary>
        /// Initializes a new instance of the <see cref="AsyncObservableCollection{T}"/> class.
        /// </summary>
        public AsyncObservableCollection()
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="AsyncObservableCollection{T}"/> class.
        /// </summary>
        /// <param name="list">The list.</param>
        public AsyncObservableCollection(IEnumerable<T> list)
            : base(list)
        {
        }

        /// <summary>
        /// Inserts the item.
        /// </summary>
        /// <param name="index">The index.</param>
        /// <param name="item">The item.</param>
        protected override void InsertItem(int index, T item)
        {
            this.ExecuteOnSyncContext(() => base.InsertItem(index, item));
        }

        /// <summary>
        /// Removes the item.
        /// </summary>
        /// <param name="index">The index.</param>
        protected override void RemoveItem(int index)
        {
            this.ExecuteOnSyncContext(() => base.RemoveItem(index));
        }

        /// <summary>
        /// Sets the item.
        /// </summary>
        /// <param name="index">The index.</param>
        /// <param name="item">The item.</param>
        protected override void SetItem(int index, T item)
        {
            this.ExecuteOnSyncContext(() => base.SetItem(index, item));
        }

        /// <summary>
        /// Moves the item.
        /// </summary>
        /// <param name="oldIndex">The old index.</param>
        /// <param name="newIndex">The new index.</param>
        protected override void MoveItem(int oldIndex, int newIndex)
        {
            this.ExecuteOnSyncContext(() => base.MoveItem(oldIndex, newIndex));
        }

        /// <summary>
        /// Clears the items.
        /// </summary>
        protected override void ClearItems()
        {
            this.ExecuteOnSyncContext(() => base.ClearItems());
        }

        /// <summary>
        /// Executes the on synchronize context.
        /// </summary>
        /// <param name="action">The action.</param>
        private void ExecuteOnSyncContext(Action action)
        {
            if (SynchronizationContext.Current == this.synchronizationContext)
            {
                action();
            }
            else
            {
                this.synchronizationContext.Send(_ => action(), null);
            }
        }
    } 

 另外还需要启用一个新的线程来更新collection

后续补充:

UI 编程中只要搞清楚如下两点：

1. UI 线程（主线程，入Dispatcher队，只用于更新UI）， 在UI线程里面不要做耗时的操作，所有UI线程里面都是用于更新UI的。

2. 所有的耗时操作全部放到另外的线程里去做。

搞清楚如上两点，这个Listbox的更新可以通过如下代码来实现了。

           // Start a non-UI thread to do some works without blocking UI thread.
            Task.Factory.StartNew(() =>
            {
                for (int i = 0; i < 5; i++)
                {
                    // Put UI change enter dispatcher queue.
                    this.Dispatcher.Invoke(() =>
                    {
                        ((LoggerListBoxViewModel)(this.DataContext)).Logger.Add("Log~~~~.." + i);
                    });

                    // Other works.
                    Thread.Sleep(1000); 
                }
            });

在ViewModel裏面可以如下更新：

  private void LogMessage(string message)
        {
            // Start a non-UI thread to do some works without blocking UI thread.
            Task.Factory.StartNew(() =>
            {
                // Put UI change enter dispatcher queue.
                Application.Current.Dispatcher.Invoke(() =>
                {
                    this.Logger.Add(message);
                }); 
            }); 
        }

注意，如上，Disparter.Invoke是UI线程，里面不能做耗时的工作。仅仅用于更新 UI。