MVVM Manipulating the view, using code-behind or viewmodel

Question

I'm coding application using MVVM and I have a question, if I should manipulate View using code-behind or I should leave it empty. What I want to do is, when user click a button a hidden view slides out. At the moment I'm doing this in view model. Here is my code:

This is my View:

<Window.DataContext>
        <ViewModels:MainWindowViewModel/>
    </Window.DataContext>
    <Window.Resources>
    <Style x:Key="DockPanelStyle" TargetType="{x:Type DockPanel}" >
        <Setter Property="OverridesDefaultStyle" Value="true"/>
        <Setter Property="Background" Value="AliceBlue"/>
        <Style.Triggers>
            <DataTrigger 
                         Binding="{Binding showView}" 
                         Value="true">
                <DataTrigger.EnterActions>
                    <BeginStoryboard>
                        <Storyboard>
                            <ThicknessAnimation
                                                Storyboard.TargetProperty="Margin" 
                                                To="0,0,0,0" 
                                                AccelerationRatio=".25" 
                                                DecelerationRatio=".25" 
                                                Duration="0:0:0.5" 
                                                 />
                        </Storyboard>
                    </BeginStoryboard>
                </DataTrigger.EnterActions>
                <DataTrigger.ExitActions>
                    <BeginStoryboard>
                        <Storyboard>
                            <ThicknessAnimation
                                                Storyboard.TargetProperty="Margin" 
                                                To="0,0,-400,0" 
                                                DecelerationRatio=".25" 
                                                AccelerationRatio=".25" 
                                                Duration="0:0:0.5" 
                                                />
                        </Storyboard>
                    </BeginStoryboard>
                </DataTrigger.ExitActions>
            </DataTrigger>
        </Style.Triggers>
    </Style>
    </Window.Resources>

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>
        <StackPanel
            Grid.Column="0">
            <Label Content="Main view" HorizontalAlignment="Center" FontSize="42"/>
            <Button Command="{Binding SlideOutCommand}" Content="Show view" FontSize="42"/>
        </StackPanel>
        <DockPanel
            Grid.Column="1"
            Margin="0,0,-400,0"
            Style="{StaticResource DockPanelStyle}">
            <Label Content="Sliding view" HorizontalAlignment="Center" FontSize="42"/>
        </DockPanel>
    </Grid>
</Window>

Code-behind is empty. This is my ViewModel:

public class MainWindowViewModel : INotifyPropertyChanged
    {    
        public MainWindowViewModel()
        {
            showView = false;
        }

        public ICommand SlideOutCommand
        {
            get { return new ActionCommand(action => ShowView()); }
        }

        private void ShowView()
        {
            showView = true;
            //Do some extra logic
        }

        private bool _showView;
        public bool showView
        {
            get { return _showView; }
            set
            {
                _showView = value;
                OnPropertyChanged();
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        virtual protected void OnPropertyChanged([CallerMemberName]string propName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
        }


    }

Or should I go with solution like this:

View:

 <Window.DataContext>
        <ViewModels:MainWindowViewModel/>
    </Window.DataContext>
    <Window.Resources>
    <Style x:Key="DockPanelStyle" TargetType="{x:Type DockPanel}" >
        <Setter Property="OverridesDefaultStyle" Value="true"/>
        <Setter Property="Background" Value="AliceBlue"/>
        <Style.Triggers>
            <DataTrigger 
                         Binding="{Binding Path=Tag, RelativeSource={RelativeSource Self}}" 
                         Value="true">
                <DataTrigger.EnterActions>
                    <BeginStoryboard>
                        <Storyboard>
                            <ThicknessAnimation
                                                Storyboard.TargetProperty="Margin" 
                                                To="0,0,0,0" 
                                                AccelerationRatio=".25" 
                                                DecelerationRatio=".25" 
                                                Duration="0:0:0.5" 
                                                 />
                        </Storyboard>
                    </BeginStoryboard>
                </DataTrigger.EnterActions>
                <DataTrigger.ExitActions>
                    <BeginStoryboard>
                        <Storyboard>
                            <ThicknessAnimation
                                                Storyboard.TargetProperty="Margin" 
                                                To="0,0,-400,0" 
                                                DecelerationRatio=".25" 
                                                AccelerationRatio=".25" 
                                                Duration="0:0:0.5" 
                                                />
                        </Storyboard>
                    </BeginStoryboard>
                </DataTrigger.ExitActions>
            </DataTrigger>
        </Style.Triggers>
    </Style>
    </Window.Resources>

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>
        <StackPanel
            Grid.Column="0">
            <Label Content="Main view" HorizontalAlignment="Center" FontSize="42"/>
            <Button Command="{Binding SlideOutCommand}" Content="Show view" FontSize="42" Click="Button_Click"/>
        </StackPanel>
        <DockPanel
            x:Name="SlideView"
            Grid.Column="1"
            Margin="0,0,-400,0"
            Style="{StaticResource DockPanelStyle}"
            Tag="{Binding Path=showView,RelativeSource={RelativeSource AncestorType=Window}}">
            <Label Content="Sliding view" HorizontalAlignment="Center" FontSize="42"/>
        </DockPanel>
    </Grid>
</Window>

Code-behind:

public partial class MainWindowView : Window, INotifyPropertyChanged
    {
        public MainWindowView()
        {
            InitializeComponent();
            showView = false;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            showView = true;
        }

        private bool _showView;
        public bool showView
        {
            get { return _showView; }
            set
            {
                _showView = value;
                OnPropertyChanged();
            }
        }


        public event PropertyChangedEventHandler PropertyChanged;
        virtual protected void OnPropertyChanged([CallerMemberName]string propName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
        }
    }

And ViewModel:

public class MainWindowViewModel : INotifyPropertyChanged
    {

        public MainWindowViewModel()
        {
        }

        public ICommand SlideOutCommand
        {
            get { return new ActionCommand(action => ShowView()); }
        }

        private void ShowView()
        {
            //Do some extra logic
        }


        public event PropertyChangedEventHandler PropertyChanged;
        virtual protected void OnPropertyChanged([CallerMemberName]string propName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
        }


    }

What is better approach in MVVM pattern?

Answer 1

Some people will tell you that the ViewModel interacts with the model AND the view, but I'm not one of them.

If what you have to do is purely cosmetic and doesn't depend on any model data, I'd say you should handle it solely in the view and its code-behind.

Having these kind of view-only data in the viewmodel clutters it, and if your view is a bit sophisticated, it can become pretty horrible. This, to me, kinda defeats the point of separating all the layers and keeping everything as clean as possible.

Answer 2

For me, the ViewModel should handle any logic connected with the data. For example:

public class OrderViewModel
{
   /*
     Other fields and properties 
   */

   public decimal Total
   {
      get 
      {
         return this.Price * this.Quantity;
      }
   }
}

Here the ViewModel contains a logic for calculating the total price of the order. But any visual representation should NOT be here. The visual representation of the data should be at the View and nowhere else.