Xamarin Form picker using EventToCommandBahavior

Question

I have a custom picker. I need to call an API on selection changed of the picker and the implementation must be only in MVVM. DistanceDropdown is the x:Name of picker.

      <custom:MyPicker.Behaviors>
             <behaviors:EventToCommandBehavior EventName="SelectedIndexChanged"
                                               Command="{Binding Path=BindingContext.DistanceChangedCommand,Source={x:Reference DistanceDropdown}}"></behaviors:EventToCommandBehavior>
   </ccustom:MyPicker.Behaviors>

And DistanceChangedCommand has a method

      DistanceChangedCommand = new Command<object>(async (x) => await DistanceChangedAction(x as SelectionChangedEventArgs));

The method gets hit, but the args are null

      private async Task DistanceChangedAction(SelectionChangedEventArgs selectionChangedEventArgs)
        {
            await Task.CompletedTask;
            
        }

Where am I going wrong? I tried with CommandParameter={Binding} also

Answer 1

The event SelectedIndexChanged of the Picker View has a delegate handler signature as follow: private void Picker_SelectedIndexChanged(object sender, EventArgs e). So your command should cast to EventArgs and not SelectionChangedEventArgs.

A better way is to take advantage of CommndParameter

      <Picker x:Name="DistanceDropdown" ...>
 <Picker.Behaviors>
          <xct:EventToCommandBehavior
                  EventName="SelectedIndexChanged"
                  Command="{Binding ItemTappedCommand}"
                  CommandParameter="{Binding Source={x:Reference DistanceDropdown},
                                             Path=SelectedIndex}"/>
    </Picker.Behaviors>

and you will expect a parameter of type int:

      DistanceChangedCommand = new Command<int>(async (x) => await DistanceChangedAction(x);

private async Task DistanceChangedAction(int selectedIndex)
        {
            await Task.CompletedTask;
            
        }

Better yet, you can use AsyncCommand (also comes with Xamarin-Community-Toolkit):

      public ICommand DistanceChangedCommand { get; private set; } 
                                      = new AsyncCommand<int>(selectedIndex
                                            => await Task.CompletedTask);