执行返回 ObservableCollection 的异步搜索

Question

我正在使用Visual Studio 2015和Entity Framework 6来构建MVVM Light WPF应用程序。当用户单击"搜索"按钮时，它会调用在视图模型的构造函数中定义如下的RelayCommand：

SearchEmployeesRelayCommand = new RelayCommand(SearchEmployees);

视图模型中的SearchEmployees方法如下所示：

private BackgroundWorker _worker;
public void SearchEmployees()
{
    _worker = new BackgroundWorker(); // use this to show busy indicator
    var dataService = new EmployeeDataService();
    _worker.DoWork += (o, ea) =>
    {
        SearchResults = dataService.SearchEmployees(SelectedColumn, SearchValue);
    };
    _worker.RunWorkerCompleted += (o, ea) =>
    {
        IsSearching = false;
    };
    IsSearching = true;
    _worker.RunWorkerAsync();
}

数据服务的搜索方法如下所示：

public ObservableCollection<EmployeeViewModel> 
    SearchEmployees(string selectedColumn, string searchValue)
{
    var paramEmployee = Expression.Parameter(typeof(Employee), "e");
    var comparison = Expression.Lambda<Func<Employee, bool>>(
        Expression.Equal(
            Expression.Property(paramEmployee, selectedColumn),
            Expression.Constant(searchValue)),
            paramEmployee).Compile();
    using (var context = new MyEntities())
    {
        var query = (from e in context.Employees
                     .Where(comparison)
                     select new EmployeeViewModel
                     {
                         // Various EF model properties...
                     });
        return new ObservableCollection<EmployeeViewModel>(query);
    }
}

如果我尝试使上述方法async和awaitable，如下所示：

return await new ObservableCollection<EmployeeViewModel>(query);

它给出了此错误：

"ObservableCollection"不包含"GetAwaiter">

的定义，并且找不到接受"ObservableCollection"类型的第一个参数的扩展方法"GetAwaiter"(您是否缺少使用指令或程序集引用？

如果返回ObservableCollection，如何使搜索async？谢谢。

更新：要使繁忙指示器正常工作，我必须进行以下更改：

_worker.DoWork += async (o, ea) =>
{
    SearchResults = await dataService
        .SearchEmployees(selectedColumnValue, SearchValue);
    IsSearching = false;
};

我完全删除了_worker.RunWorkerCompleted块。可能有更好的方法可以做到这一点，但这就是我让它工作的方式。

Answer 1

有几种方法。首先，您可以保持数据库访问同步，并在后台线程上运行它。请注意，Task.Run是BackgroundWorker的现代替代品(我有一个博客系列将两者相提并论(：

public async Task SearchEmployeesAsync()
{
  var dataService = new EmployeeDataService();
  var selectedColumn = SelectedColumn;
  var searchValue = searchValue;
  IsSearching = true;
  try
  {
    SearchResults = await Task.Run(() => dataService.SearchEmployees(selectedColumn, searchValue));
  }
  finally
  {
    IsSearching = false;
  }
}

或者，由于使用的是 EF6，因此可以使数据库查询异步，而根本不会弄乱后台线程：

public async Task<ObservableCollection<EmployeeViewModel>> 
    SearchEmployeesAsync(string selectedColumn, string searchValue)
{
  var paramEmployee = Expression.Parameter(typeof(Employee), "e");
  var comparison = Expression.Lambda<Func<Employee, bool>>(
    Expression.Equal(
      Expression.Property(paramEmployee, selectedColumn),
      Expression.Constant(searchValue)),
      paramEmployee).Compile();
  using (var context = new MyEntities())
  {
    var query = (from e in context.Employees
                 .Where(comparison)
                 select new EmployeeViewModel
                 {
                     // Various EF model properties...
                 });
    var data = await query.ToListAsync();
    return new ObservableCollection<EmployeeViewModel>(data);
  }
}
public async Task SearchEmployeesAsync()
{
  var dataService = new EmployeeDataService();
  IsSearching = true;
  try
  {
    SearchResults = await dataService.SearchEmployeesAsync(SelectedColumn, SearchValue);
  }
  finally
  {
    IsSearching = false;
  }
}

您不应该使BackgroundWorker.DoWork异步;这将导致它"提前结束"并阻止它正常处理异常。BGW 根本不是为使用async代码而设计的。