WPF：窗口上的异步操作关闭

Question

我需要在关闭窗口时执行异步操作（基本上是为了保存所有数据）。问题在于程序和UI线程死亡太早了，任务永远不会完成。

这是代码示例：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        Closed += MainWindow_Closed;
        _items = new(Enumerable.Range(0,10));
    }

    private ObservableCollection<int> _items;

    private async void MainWindow_Closed(object? sender, EventArgs e)
    {
        await LongTask();
    }

    private async Task LongTask()
    {
        await Task.Run(() =>
        {
            Trace.WriteLine("LongTask Start");
            Application.Current.Dispatcher.BeginInvoke(() =>
            {
                // do some action in the UI thread
                _items.Clear();
                Trace.WriteLine("Cleared elements");
            });

            Thread.Sleep(5000);
            Trace.WriteLine("LongTask End");
        });
    }
}

控制台输出切勿打印“已清除元素”或“ longtask end”。
我还在异步任务中提出了一个操作，它需要UI线程来完成操作，这在我的用例中需要。 （因此，一个人不能在主线程中使用longtask（）。getawaiter（）。getResult（），因为它会导致死锁）。
thanks寻求帮助!!

作为概念证明，这是您可以做到这一点的一种方法。这里的关键是

_confirmClosure

Answer 1

UI可以防止任何重新进入的机会

同时执行保存。

，然后在等待保存之后，再次致电

Close

，这次是
• _confirmClosure
设置为`false。

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        Loaded += (sender, e) =>
        {
            if(dataGrid.Columns
                .OfType<DataGridTextColumn>()
                .FirstOrDefault(c => c.Header.ToString() == "Name") is { } column)
            {
                // Autosize "Fill" for item name
                column.Width = new DataGridLength(1, DataGridLengthUnitType.Star);
            }
        };
        Closing += async (sender, e) =>
        {
            if (_confirmClosure)
            {
                switch (
                    MessageBox.Show(
                        "Do you want to save before closing?",
                        "Confirm Exit",
                        MessageBoxButton.YesNoCancel,
                        MessageBoxImage.Question))
                {
                    case MessageBoxResult.Yes:
                        e.Cancel = true;
                        await Dispatcher.BeginInvoke(async () =>
                        {
                            IsEnabled = false;  // Prevent any more calls.
                            await DataContext.Save();
                            _confirmClosure = false;
                            Close();
                        });
                        break;

                    case MessageBoxResult.No:
                        break;

                    case MessageBoxResult.Cancel:
                        e.Cancel = true;
                        break;
                }
            }
        };
    }
    bool _confirmClosure = true;
    new MainPageViewModel DataContext => (MainPageViewModel)base.DataContext;
}

• test测试的少量VM
  

  我们将列出10000个项目的“大”列表，然后在关闭时将其保存到文件中。

  public class MainPageViewModel : INotifyPropertyChanged
  {
      public ObservableCollection<Item> Items { get; } = new ObservableCollection<Item>(
              Enumerable.Range(1, 10000)
              .Select(_=>new Item { Id = _, Name = $"Item {_}" }));
  
      public event PropertyChangedEventHandler PropertyChanged;
  
      internal async Task Save()
      {
          try
          {
              Mouse.OverrideCursor = Cursors.Wait;
              await Task.Run(() =>
              {
                  var path = Path.Combine(
                      Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
                      "StackOverflow",
                      Assembly.GetEntryAssembly()?.GetName()?.Name ?? "SaveThenClose");
                      var json = JsonConvert.SerializeObject(Items);
                  File.WriteAllText(path, json);
              });
              // Add a few seconds for good measure, just for demo purposes.
              await Task.Delay(TimeSpan.FromSeconds(2.5));
          }
          finally
          {
              // Reset the cursor to default
              Mouse.OverrideCursor = null;
          }
      }
  }
  
  public class Item
  {
      public int Id { get; set; }
      public string? Name { get; set; }
  }
  

<Window x:Class="save_list_then_close.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:save_list_then_close"
        mc:Ignorable="d"
        Title="MainWindow" Height="250" Width="400"
        WindowStartupLocation="CenterScreen">
    <Window.DataContext>
        <local:MainPageViewModel/>
    </Window.DataContext>
    <Grid>
        <DataGrid
            Name="dataGrid"
            ItemsSource="{Binding Items}" 
            AutoGenerateColumns="True" 
            IsReadOnly="True" 
            AlternatingRowBackground="LightGray"/>
    </Grid>
</Window>

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