[英]Perform async search that returns ObservableCollection
我正在使用Visual Studio 2015和Entity Framework 6來構建MVVM Light WPF應用程序。 當用戶單擊“搜索”按鈕時,它會調用“ RelayCommand
,該命令已在“視圖模型”的構造函數中定義如下:
SearchEmployeesRelayCommand = new RelayCommand(SearchEmployees);
視圖模型中的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();
}
數據服務的搜索方法如下:
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);
}
}
如果我嘗試使上述方法async
和awaitable
,如下所示:
return await new ObservableCollection<EmployeeViewModel>(query);
它給出了這個錯誤:
'ObservableCollection'不包含'GetAwaiter'的定義,並且找不到擴展方法'GetAwaiter'接受類型為'ObservableCollection'的第一個參數(您是否缺少using指令或程序集引用?)
如果返回了ObservableCollection
如何使搜索async
? 謝謝。
更新 :為了使繁忙的指示器正常工作,我必須進行以下更改:
_worker.DoWork += async (o, ea) =>
{
SearchResults = await dataService
.SearchEmployees(selectedColumnValue, SearchValue);
IsSearching = false;
};
然后,我完全刪除了_worker.RunWorkerCompleted
塊。 可能有更好的方法來做到這一點,但這就是我如何使其工作的方式。
有兩種方法。 首先,您可以保持數據庫訪問同步,並僅在后台線程上運行它。 請注意, Task.Run
是BackgroundWorker
的現代替代品(我有一個博客系列在兩者之間進行了比較 ):
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;
}
}
另外,由於使用的是EF6,因此可以使數據庫查詢異步,並且完全不會與后台線程發生沖突:
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;
}
}
你不應該讓BackgroundWorker.DoWork
異步的; 這將導致它“提早結束”並阻止其正常處理異常。 BGW並非旨在與async
代碼一起使用。
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