MVVM在选择更改时调用异步方法

Question

I have a simple list box with selected Item binded to ViewModel's property

<ListBox SelectedItem="{Binding SelectedItem, Mode=TwoWay}">

and I have an async method that I want to run when the selection changed

async Task foo();

I could put this call inside the setter for SelectedItem but that's a code smell to me.

public object SelectedItem
{
    get{...}
    set
    {
         foo(); //Compiler warning and blocks UI
         ...
    }
}

Task.Run inside set is an option, but still looks bad to me and exception thrown by foo will be swollen

Code behind SelectionChanged event might be another option but whats the most MVVM way to solve this problem?

Answer 1

If you want the approach with zero code behind, here is what I would do.

Convert the SelectedItemChanged event to a command using Interaction.

Here is code on how you can convert an event to a command

   <i:Interaction.Triggers>
        <i:EventTrigger EventName="SomeEvent">
            <i:InvokeCommandAction Command="{Binding Path=SomeCommand, Mode=OneWay}"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>

Your command in the view model will then block the UI and invoke the async functionality.

Answer 2

I would set up an event handler.

something like

 public class ViewModel:INotifyPropertyChanged
{

    public ViewModel()
    {
        PropertyChanged += SelectedItemChanged;
    }

    private async void SelectedItemChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == nameof(SelectedItem))
        {
            await Foo();
        }
    }

    public Task Foo()
    {
        return Task.Run(() =>
        {
            var a = "B";
        });
    }

    private object _selectedItem;

    public object SelectedItem
    {
        get=> _selectedItem;
        set
        {
            if (value != _selectedItem)
            {
                _selectedItem = value;
                OnPropertyChanged();
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}