Question

在WPF和MVVM上实现异步操作的最简单方法是什么，让我们说如果用户在某个字段上输入时我想要启动命令然后返回，而线程将执行一些搜索操作然后返回返回并更新属性，以便通知可以更新绑定。

谢谢！

Answer 1

Rob Eisenberg在MIX10 talk期间展示了在MVVM中运行异步操作的非常干净的实现。他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，它将搜索作为后台任务：

    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();
    }

希望有所帮助。

Answer 2

如何在VM上调用命令BackgroundWorker实例？

更新：抓住上面的建议..有一个关于MVVM的在线视频由Jason Dolinger ...我建议你看一下。这是一种更清晰的方式，视图很薄/不包含任何线程代码。

总结：

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

Answer 3

在Shawn Wildermuth的MSDN文章中，他做了类似这样的事情：看看这里的文章： 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;
}

有这样的实现：

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;
    }
  }
}

如何在C＃和MVVM中实现异步操作？

3 个答案: