Caliburn.Micro EventAggregator.PublishOnUIThreadAsync "The tasks argument included a null value. Parameter name: tasks"

Question

我正在尝试使用 EventAggregators 进行跨 VM 通信。

在我的 ChildViewModel 中，我做了类似的事情：

public async void ThisMethodIsCalledByUI()
{
    // ShowMessageEvent is a simple class with with only 1 string property and a MessageDialogResult enum
    ShowMessageEvent msg = new ShowMessageEvent("This is the message from ChildVM");

    // doing this works, but MessageDialogResult will be false below since no await happens
    //_eventAggregator.PublishOnUIThreadAsync(msg);

    // doing this triggers exception
    // however msg.DialogResult is already Affirmative by the time 
    // the exception is thrown, so it's almost as intended here
    await _eventAggregator.PublishOnUIThreadAsync(msg);

    // expected: Affirmative
    Debug.WriteLine(msg.DialogResult);

}

但是我需要等待，因为我想使用 MessageDialogResult。在 MainViewModel 中：

public class MainViewModel : Conductor<IScreen>.Collection.OneActive, IHandle<object>
{
    private readonly IEventAggregator _eventAggregator;

    public MainViewModel()
    {
        _eventAggregator = new EventAggregator();
        _events.SubscribeOnPublishedThread(this);
    }

    public async Task HandleAsync(object message, CancellationToken cancellationToken)
    {
        if (message is ShowMessageEvent msg)
        {
            // Simulating work, show dialog, etc.
            await Task.Delay(500);
            // Implementation irrelevant, assume the dialog returns Affirmative
            msg.DialogResult = await _dialogs.ShowMessageAsync(msg.Message);

        }
    }
}

所以目标是通知 MainViewModel 显示一个对话框，然后在继续之前等待并在 ChildViewModel 中检索对话框结果。是的，作为解决方法，我可以直接从 ChildViewModel 调用对话框，但这不是重点，我想避免这种情况。

我的实现基本上已经按预期完成了所有事情，我唯一需要解决的是抛出的异常。

有人知道我做错了什么吗？

编辑

我实际上可以让它像这样工作：

public async void ThisMethodIsCalledByUI()
{
    // ShowMessageEvent is a simple class with with only 1 string property and a MessageDialogResult enum
    ShowMessageEvent msg = new ShowMessageEvent("This is the message from ChildVM");

    try
    {
        await _eventAggregator.PublishOnUIThreadAsync(msg);
    }
    // suppress the problematic exception
    catch (ArgumentException e)
    {
        Console.WriteLine(e);
    }

    // works as expected: Affirmative
    Debug.WriteLine(msg.DialogResult);

}

虽然这是可行的，但我更喜欢不需要抑制异常的设计...

Answer 1

现在我完成了自己的扩展实现：

/// <summary>
/// EventAggregatorExtensions
/// </summary>
public static class EventAggregatorExtensions
{
    /// <summary>
    /// Publishes a message to the UI thread 
    /// </summary>
    /// <param name="eventAggregator"></param>
    /// <param name="message"></param>
    /// <param name="cancellationToken"></param>
    /// <returns></returns>
    public static async Task PublishOnUIThreadAsyncCustom(this IEventAggregator eventAggregator, object message,
                                              CancellationToken cancellationToken)
    {
        try
        {
            await eventAggregator.PublishOnUIThreadAsync(message, cancellationToken);
        }
        // Suppress a specific exception that is always caused when above line is awaited
        catch (ArgumentException e)
        {
            if (!e.Message.Contains("The tasks argument included a null value.") | !e.ParamName.Equals("tasks"))
            {
                throw;
            }
        }
    }

    /// <summary>
    /// Publishes a message to the UI thread 
    /// </summary>
    /// <param name="eventAggregator"></param>
    /// <param name="message"></param>
    /// <returns></returns>
    public static async Task PublishOnUIThreadAsyncCustom(this IEventAggregator eventAggregator, object message)
    {
        await PublishOnUIThreadAsyncCustom(eventAggregator, message, CancellationToken.None);
    }
}

Caliburn.Micro EventAggregator.PublishOnUIThreadAsync "The tasks argument included a null value. Parameter name: tasks"

Caliburn.Micro EventAggregator.PublishOnUIThreadAsync "The tasks argument included a null value. Parameter name: tasks"

c#

mvvm

async-await

caliburn.micro

eventaggregator