你如何在 C# 和 MVVM 中实现异步操作？

Question

嗨，在 WPF 和 MVVM 上实现异步操作的最简单方法是什么，假设用户在我想启动命令的字段上按 Enter 键，然后返回，同时线程将执行一些搜索操作，然后返回并更新属性，以便通知可以更新绑定。

谢谢！

Answer 1

Rob Eisenberg 在他的MIX10 talk 中展示了在 MVVM 中运行异步操作的非常干净的实现。他已经在他的博客上发布了源代码。

基本思想是将命令实现为返回 IEnumerable 并使用 yield 关键字返回结果。这是他演讲中的一段代码，它作为后台任务执行搜索：

    public IEnumerable<IResult> ExecuteSearch()
    {
        var search = new SearchGames
        {
            SearchText = SearchText
        }.AsResult();

        yield return Show.Busy();
        yield return search;

        var resultCount = search.Response.Count();

        if (resultCount == 0)
            SearchResults = _noResults.WithTitle(SearchText);
        else if (resultCount == 1 && search.Response.First().Title == SearchText)
        {
            var getGame = new GetGame
            {
                Id = search.Response.First().Id
            }.AsResult();

            yield return getGame;
            yield return Show.Screen<ExploreGameViewModel>()
                .Configured(x => x.WithGame(getGame.Response));
        }
        else SearchResults = _results.With(search.Response);

        yield return Show.NotBusy();
    }

希望对您有所帮助。

Answer 2

BackgroundWorker 实例在 VM 上调用您的命令怎么样？

更新： 从上面的建议开始.. Jason Dolinger 有一个关于 MVVM 的在线视频.. 我建议你看看那个。这是一种更简洁的方式，视图很薄/不包含任何线程代码。

总结一下：

• VM ctor 缓存 Dispatcher.CurrentDispatcher 对象（主线程）。
• 更新后备存储（结果）时，使用 _dispatcher.BeginInvoke( () => _results.AddRange( entries) ) 以便正确更新 UI。

Answer 3

在 Shawn Wildermuth 的 MSDN 文章中，他做了这样的事情： 在这里查看文章： http://msdn.microsoft.com/en-us/magazine/dd458800.aspx

以及他最近的博客文章： http://wildermuth.com/2009/12/15/Architecting_Silverlight_4_with_RIA_Services_MEF_and_MVVM_-_Part_1

public interface IGameCatalog
{
  void GetGames();
  void GetGamesByGenre(string genre);
  void SaveChanges();

  event EventHandler<GameLoadingEventArgs> GameLoadingComplete;
  event EventHandler<GameCatalogErrorEventArgs> GameLoadingError;
  event EventHandler GameSavingComplete;
  event EventHandler<GameCatalogErrorEventArgs> GameSavingError;
}

使用这样的实现：

public class GameCatalog : IGameCatalog
{
  Uri theServiceRoot;
  GamesEntities theEntities;
  const int MAX_RESULTS = 50;

  public GameCatalog() : this(new Uri("/Games.svc", UriKind.Relative))
  {
  }

  public GameCatalog(Uri serviceRoot)
  {
    theServiceRoot = serviceRoot;
  }

  public event EventHandler<GameLoadingEventArgs> GameLoadingComplete;
  public event EventHandler<GameCatalogErrorEventArgs> GameLoadingError;
  public event EventHandler GameSavingComplete;
  public event EventHandler<GameCatalogErrorEventArgs> GameSavingError;

  public void GetGames()
  {
    // Get all the games ordered by release date
    var qry = (from g in Entities.Games
               orderby g.ReleaseDate descending
               select g).Take(MAX_RESULTS) as DataServiceQuery<Game>;

    ExecuteGameQuery(qry);
  }

  public void GetGamesByGenre(string genre)
  {
    // Get all the games ordered by release date
    var qry = (from g in Entities.Games
               where g.Genre.ToLower() == genre.ToLower()
               orderby g.ReleaseDate
               select g).Take(MAX_RESULTS) as DataServiceQuery<Game>;

    ExecuteGameQuery(qry);
  }

  public void SaveChanges()
  {
    // Save Not Yet Implemented
    throw new NotImplementedException();
  }

  // Call the query asynchronously and add the results to the collection
  void ExecuteGameQuery(DataServiceQuery<Game> qry)
  {
    // Execute the query
    qry.BeginExecute(new AsyncCallback(a =>
    {
      try
      {
        IEnumerable<Game> results = qry.EndExecute(a);

        if (GameLoadingComplete != null)
        {
          GameLoadingComplete(this, new GameLoadingEventArgs(results));
        }
      }
      catch (Exception ex)
      {
        if (GameLoadingError != null)
        {
          GameLoadingError(this, new GameCatalogErrorEventArgs(ex));
        }
      }

    }), null);
  }

  GamesEntities Entities
  {
    get
    {
      if (theEntities == null)
      {
        theEntities = new GamesEntities(theServiceRoot);
      }
      return theEntities;
    }
  }
}