ReactiveUI“引人注目的示例”搜索与Caliburn.Micro.ReactiveUI

Question

我已经使用Caliburn.Micro构建了一个现有的Wpf应用程序，我希望开始集成ReactiveUI。首先，我从Caliburn.Micro切换到了Caliburn.Micro.ReactiveUI库。

我有一个ListBox，它使用卡利伯恩公司的约定绑定到一个BindableCollection。我使用CM来定位MyViewModel的视图，BindableCollection是从EntityFramework执行的搜索中填充的。

我正在尝试实现所示的异步“引人注目的示例”，但是每当执行搜索时，ReactiveUI.dll都会抛出和未处理的InvalidCastException。我无法看到任何细节，因为我没有ReactiveUI.pdb符号文件。我正在将我的BindableCollection属性包装在一个ObservableAsPropertyHelper中，以便在用户执行搜索时，ReactiveUI将更新BindableCollection，以便Caliburn.Micro可以像往常一样执行它。

有人知道什么会导致无效的强制转换异常吗？我对CM有一定的经验，但对于Rx和RxUI，我完全是新手。所以，我可能完全搞错了。

以下是相关代码：

public class MainViewModel: ReactiveScreen
{
        private ObservableAsPropertyHelper<BindableCollection<MyViewModel>> _myViewModels;
        public BindableCollection<MyViewModel> MyViewModels
        {
            get { return _myViewModels.Value; }
        }

        [ImportingConstructor]
        public MainViewModel(IEventAggregator eventAggregator)
        {

            _myEntities = new myEntities();
            var searchService = new SearchService(_myEntities);
            Items.CollectionChanged += delegate
            {
                NotifyOfPropertyChange(() => Status);
            };
            var searchTerms = this
                .ObservableForProperty(x => x.SearchTerm)
                .Value()
                .Throttle(TimeSpan.FromSeconds(.5));
            var searchResults = searchTerms.SelectMany(searchTerm => searchService.SearchAsync(searchTerm));
            var latestMatches = searchTerms
               .CombineLatest(searchResults,
                   (searchTerm, searchResult) =>
                       searchResult.SearchTerm != searchTerm
                           ? null
                           : searchResult.Matches)
               .Where(matches => matches != null);
            _myViewModels= latestMatches.ToProperty(this, x => x.MyViewModels);

        }
}

SearchService看起来是这样的：

public class SearchService
    {
        private MyEntities _myEntities;
        public SearchService(MyEntities myEntities)
        {
            _myEntities= myEntities;
        }
        public async Task<SearchResult> SearchAsync(string searchTerm)
        {
            var query = await (from m in _myEntities.Employees select m).ToListAsync();
            if (!String.IsNullOrEmpty(searchTerm))
            {
                searchTerm = searchTerm.Trim();
                query = (List<Employee>) query.Where(
                    x => x.LastName.Contains(searchTerm) || x.FirstName.Contains(searchTerm))
                    .Select(x => x);
            }
            return new SearchResult
            {
                SearchTerm = searchTerm,
                Matches = new BindableCollection<MyViewModel>(query.Select(x => new MyViewModel{ Employee = x }))
            };

        }
    }

SearchResult：

public class SearchResult
    {
        public string SearchTerm { get; set; }
        public BindableCollection<MyViewModel> Matches { get; set; }
    }

谢谢,

肖恩

编辑：

在将断点设置在不同的位置之后，我得到了异常的详细信息。

消息：

{“OnError发生在将中断绑定或命令的对象(通常为ObservableAsPropertyHelper)上。为此，请订阅对象的ThrownExceptions属性”}

内部例外：

{“无法强制转换WhereSelectListIterator2[HR.DataContext.Models.Employee,HR.DataContext.Models.Employee]' to type System.Collections.Generic.List1HR.DataContext.Models.Employee'."}类型的对象”

StackTrace：

at ReactiveUI.RxApp.<>c__DisplayClass5.<.cctor>b__2()
   at System.Reactive.Concurrency.Scheduler.Invoke(IScheduler scheduler, Action action)
   at System.Reactive.Concurrency.DispatcherScheduler.<>c__DisplayClass1`1.<Schedule>b__0()
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)

Answer 1

你的问题就在眼前：

query = (List<Employee>)
    query
        .Where(x => x.LastName.Contains(searchTerm)
            || x.FirstName.Contains(searchTerm))
        .Select(x => x);

查询的类型似乎是List<Employee>，但query.Where(...).Select(...)返回的类型不是。

您需要在末尾添加一个.ToList()。它应该是这样的：

query = //(List<Employee>) no need to cast now
    query
        .Where(x => x.LastName.Contains(searchTerm)
            || x.FirstName.Contains(searchTerm))
        .ToList();

我删除了.Select(x => x)，因为它什么也没做。