異步命令Wpf C＃

Question

該程序具有一個更新按鈕，該按鈕將請求發送到服務器。 我的任務是，當您單擊該按鈕時，將在單獨的異步線程中啟動對服務器的請求，並且在用戶界面（wpf）上會出現“旋轉加載”圖像。 我收到一個錯誤：

“無法將lambda表達式轉換為預期的委托類型，因為該塊中的某些返回類型不能隱式轉換為委托返回類型”。

如何將async與void結合起來，或者我可以做得更好？

ViewModel：

public CarasViewModel()
    {
      ...
     AddCommand = new AsyncCommand<Task>(() => Add());
      ...
    }

public IAsyncCommand AddCommand { get; private set; }
    async Task Add()
    {
        await Task.Run(() => OnAdd());
    }

void OnAdd()
    {
        var Result = Helper.Get(Configuration.Settings);
        if (Result != null)
        {
            var SelectionViewModel = new SelectionViewModel(Result);
            if (DialogService.ShowModalWindow(selectionViewModel))
            {
                ...
            }
        }
        else
        {
            MessageBoxService.ShowError("Check your connection settings.");
        }
    }

代碼段AsyncCommand：

public class AsyncCommand<TResult> : AsyncCommandBase, INotifyPropertyChanged
{
    private readonly Func<Task<TResult>> _command;
    private NotifyTaskCompletion<TResult> _execution;

    public AsyncCommand(Func<Task<TResult>> command)
    {
        _command = command;
    }
...

Answer 1

您對TResult和Task感到困惑。 由於您已經有了AsyncCommand<TResult>

public class AsyncCommand<TResult> : AsyncCommandBase, INotifyPropertyChanged
{
}

一個像工廠一樣的靜態AsyncCommand類來創建命令

    public static class AsyncCommand
    {
        public static AsyncCommand<object> Create(Func<Task> command)
        {
            return new AsyncCommand<object>(async () => { await command(); return null; });
        }

        public static AsyncCommand<TResult> Create<TResult>(Func<Task<TResult>> command)
        {
            return new AsyncCommand<TResult>(command);
        }
    }

接下來，通過工廠AsyncCommand創建命令，如下所示：

public async Task<string> Add()
{
    //you can put whatever you want, in my project, it's string containing the message
    //your stuff
    var Result = Helper.Get(Configuration.Settings);
}

private AsyncCommand<string> _addCommand;
public AsyncCommand<string> AddCommand
        {
            get
            {
                if (_addCommand!= null)
                    return _addCommand;

                _addCommand= AsyncCommand.Create(Add);
                return _addCommand;
            }
        }

XAML

如果任務正在執行，我會顯示進度欄

<Grid VerticalAlignment="Center" Visibility="{Binding AddCommand.Execution, Converter={StaticResource NullToVisibilityConverter}}">
                <!--Busy indicator-->
                <ProgressBar HorizontalAlignment="Stretch" Foreground="{Binding AccentBaseColor}" 
                                           Visibility="{Binding AddCommand.Execution.IsNotCompleted,
                             Converter={StaticResource BooleanToVisibilityConverter}}"
                             IsIndeterminate="True"
                             Value="100" />
</Grid>

Answer 2

我們認為這可能是語義模型錯誤。 對於所有lambda表達式，語義模型都應顯示null類型。

您應該在調用命令時使用調度程序。

public CarasViewModel()
{
 AddCommand = new ICommand<Task>(() => Add());
}

public ICommand AddCommand { get; private set; }

async Task Add()
{
   Thread longRunningThread = new Thread(new ThreadStart(delegate (){ Thread.Sleep(10000); Application.Current.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal, new CalculateTimeElapsed(OnAdd)); }));
longRunningThread.Start();}
void OnAdd{}