MVVM. Can i implement logic code in behavior or should i put it whole to binded command in VM?

Issue

I have behavior for entry completed command, that validates data inside of entry, logic for that i keep in ViewModel. But also, when i complete entry i need to change the focus to the next entry (next entry does not have a x:Name, because it is inside of the DataTemplate). To change focus i need object of the next entry, and i finding that object via his parent right in the Behavior file, not in VM. It is correctly way to do this in MVVM, or i should pass entry object via parameter to VM and find his parent and then next entry in VM?

Behavior:

using System.Windows.Input;
using Xamarin.Forms;

namespace RollPositioningMobile.ViewModels.Behaviors
{
    public class EntryCompletedBehavior : Behavior<Entry>
    {
        protected override void OnAttachedTo(Entry bindable)
        {
            base.OnAttachedTo(bindable);

            AssociatedObject = bindable;

            bindable.BindingContextChanged += Bindable_BindingContextChanged;
            bindable.Completed += Bindable_Completed;
        }
        protected override void OnDetachingFrom(Entry bindable)
        {
            base.OnDetachingFrom(bindable);

            bindable.BindingContextChanged -= Bindable_BindingContextChanged;
            bindable.Completed -= Bindable_Completed;

            AssociatedObject = null;
        }

        private void Bindable_BindingContextChanged(object sender, System.EventArgs e)
        {
            OnBindingContextChanged();
        }
        private void Bindable_Completed(object sender, System.EventArgs e)
        {
            if (Command == null) return;

            Entry entry = (Entry)sender;
            Grid circularEntriesGrid;

            //If this handler was invoked from the RequiredValueEntry,
            //focus on the next Entry mechanic is different
            if (entry.TabIndex == 0)
            {
                //First we need to define the main Grid,
                //and then find subgrid inside of the main
                Grid mainGrid = (Grid)entry.Parent.Parent;
                circularEntriesGrid = mainGrid.FindByName<Grid>("CircularEntriesGrid");
            }
            else
            {
                //Get the Frame parent and then the Grid parent of the Entry
                circularEntriesGrid = (Grid)entry.Parent.Parent;

                //Check for last entry
                if (circularEntriesGrid.Children.Count == entry.TabIndex)
                    return;
            }

            //Execution of the input data validation commands
            if (Command.CanExecute(entry.TabIndex))
                Command.Execute(entry.TabIndex);

            //Set focus on the next Entry
            if ((bool)CommandParameter)
            {
                var nextEntryFrame = (Frame)circularEntriesGrid.Children[entry.TabIndex];
                var nextEntry = (Entry)nextEntryFrame.Content;
                nextEntry.Focus();
            }
            else
            {
                entry.Focus();
            }
        }

        protected override void OnBindingContextChanged()
        {
            base.OnBindingContextChanged();
            BindingContext = AssociatedObject.BindingContext;
        }

        public Entry AssociatedObject { get; private set; }

        public static readonly BindableProperty CommandProperty =
                BindableProperty.Create("Command", typeof(ICommand), typeof(EntryCompletedBehavior), null);
        public ICommand Command
        {
            get { return (ICommand)GetValue(CommandProperty); }
            set { SetValue(CommandProperty, value); }
        }

        public static readonly BindableProperty CommandParameterProperty =
            BindableProperty.Create("CommandParameter", typeof(object), typeof(EntryCompletedBehavior), null);
        public object CommandParameter
        {
            get { return GetValue(CommandParameterProperty); }
            set { SetValue(CommandParameterProperty, value); }
        }
    }
}

VM part:

public ICommand EntryCompletedCommand { get; protected set; }
    void EntryCompleted(object param)
    {
        var entryId = (int)param;

        //Validating required value
        if (entryId == 0)
        {
            RequiredValueIsValid = true;
            if (RequiredValue <= 0)
            {
                Application.Current.MainPage.DisplayAlert("Error", $"Required value cannot be equals to {RequiredValue}", "OK");
                RequiredValueIsValid = false;
            }
            return;
        }

        //Validating actual values
        var entry = ActualValues[entryId - 1];
        var value = entry.Value;
        if (value < 0 || Math.Abs(value - RequiredValue) > Measurment.MaxOffset)
        {
            entry.Value = entry.OldValue;
            Application.Current.MainPage.DisplayAlert("Error", $"The actual and the required value should not differ more than {Measurment.MaxOffset}", "OK");
            entry.IsValid = false;
            return;
        }
        entry.IsValid = true;
        RequiredValueIsValid = true;
        entry.OldValue = entry.Value;
    }

Solution

Can i implement logic code in behavior or should i put it whole to binded command in VM?

Since you had used MVVM , it would better to handle the logic in ViewModel . We always do some customization of the element itself .

In your case , it is necessary to pass the entry as params to ViewModel . You could binding the value of IsFocused to the property of Model and handle it directly in VM .

Here is a similar thread and I had provided a whole solution .Hope it will be helpful to you .

Answered By - Lucas Zhang - MSFT