c# - Give some command to View in MVVM

Question

Let's imagine I have some user control. The user control has some child windows. And user control user wants to close child windows of some type. There is a method in user control code behind:

public void CloseChildWindows(ChildWindowType type)
{
   ...
}

But I can't call this method as I don't have direct access to the view.

Another solution I think about is to somehow expose user control ViewModel as one of its properties (so I can bind it and give command directly to ViewModel). But I don't want user control users to know anything about user control ViewModel.

So what is the right way to solve this problem?

Answer 1

I feel I just found a rather nice MVVM solution to this problem. I wrote a behavior that is exposing a type property WindowType and a boolean property Open. DataBinding the latter allows the ViewModel to open and close the windows easily, without knowing anything about the View.

Gotta love behaviors... :)

Xaml:

<UserControl x:Class="WpfApplication1.OpenCloseWindowDemo"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:WpfApplication1"
             xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">

    <UserControl.DataContext>
        <local:ViewModel />
    </UserControl.DataContext>
    <i:Interaction.Behaviors>
        <!-- TwoWay binding is necessary, otherwise after user closed a window directly, it cannot be opened again -->
        <local:OpenCloseWindowBehavior WindowType="local:BlackWindow" Open="{Binding BlackOpen, Mode=TwoWay}" />
        <local:OpenCloseWindowBehavior WindowType="local:YellowWindow" Open="{Binding YellowOpen, Mode=TwoWay}" />
        <local:OpenCloseWindowBehavior WindowType="local:PurpleWindow" Open="{Binding PurpleOpen, Mode=TwoWay}" />
    </i:Interaction.Behaviors>
    <UserControl.Resources>
        <Thickness x:Key="StdMargin">5</Thickness>
        <Style TargetType="Button" >
            <Setter Property="MinWidth" Value="60" />
            <Setter Property="Margin" Value="{StaticResource StdMargin}" />
        </Style>
        <Style TargetType="Border" >
            <Setter Property="Margin" Value="{StaticResource StdMargin}" />
        </Style>
    </UserControl.Resources>

    <Grid>
        <StackPanel>
            <StackPanel Orientation="Horizontal">
                <Border Background="Black" Width="30" />
                <Button Content="Open" Command="{Binding OpenBlackCommand}" CommandParameter="True" />
                <Button Content="Close" Command="{Binding OpenBlackCommand}" CommandParameter="False" />
            </StackPanel>
            <StackPanel Orientation="Horizontal">
                <Border Background="Yellow" Width="30" />
                <Button Content="Open" Command="{Binding OpenYellowCommand}" CommandParameter="True" />
                <Button Content="Close" Command="{Binding OpenYellowCommand}" CommandParameter="False" />
            </StackPanel>
            <StackPanel Orientation="Horizontal">
                <Border Background="Purple" Width="30" />
                <Button Content="Open" Command="{Binding OpenPurpleCommand}" CommandParameter="True" />
                <Button Content="Close" Command="{Binding OpenPurpleCommand}" CommandParameter="False" />
            </StackPanel>
        </StackPanel>
    </Grid>
</UserControl>

YellowWindow (Black/Purple alike):

<Window x:Class="WpfApplication1.YellowWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="YellowWindow" Height="300" Width="300">
    <Grid Background="Yellow" />
</Window>

ViewModel, ActionCommand:

using System;
using System.ComponentModel;
using System.Windows.Input;

namespace WpfApplication1
{
    public class ViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            if (this.PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }

        private bool _blackOpen;
        public bool BlackOpen { get { return _blackOpen; } set { _blackOpen = value; OnPropertyChanged("BlackOpen"); } }

        private bool _yellowOpen;
        public bool YellowOpen { get { return _yellowOpen; } set { _yellowOpen = value; OnPropertyChanged("YellowOpen"); } }

        private bool _purpleOpen;
        public bool PurpleOpen { get { return _purpleOpen; } set { _purpleOpen = value; OnPropertyChanged("PurpleOpen"); } }

        public ICommand OpenBlackCommand { get; private set; }
        public ICommand OpenYellowCommand { get; private set; }
        public ICommand OpenPurpleCommand { get; private set; }


        public ViewModel()
        {
            this.OpenBlackCommand = new ActionCommand<bool>(OpenBlack);
            this.OpenYellowCommand = new ActionCommand<bool>(OpenYellow);
            this.OpenPurpleCommand = new ActionCommand<bool>(OpenPurple);
        }

        private void OpenBlack(bool open) { this.BlackOpen = open; }
        private void OpenYellow(bool open) { this.YellowOpen = open; }
        private void OpenPurple(bool open) { this.PurpleOpen = open; }

    }

    public class ActionCommand<T> : ICommand
    {
        public event EventHandler CanExecuteChanged;
        private Action<T> _action;

        public ActionCommand(Action<T> action)
        {
            _action = action;
        }

        public bool CanExecute(object parameter) { return true; }

        public void Execute(object parameter)
        {
            if (_action != null)
            {
                var castParameter = (T)Convert.ChangeType(parameter, typeof(T));
                _action(castParameter);
            }
        }
    }
}

OpenCloseWindowBehavior:

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;

namespace WpfApplication1
{
    public class OpenCloseWindowBehavior : Behavior<UserControl>
    {
        private Window _windowInstance;

        public Type WindowType { get { return (Type)GetValue(WindowTypeProperty); } set { SetValue(WindowTypeProperty, value); } }
        public static readonly DependencyProperty WindowTypeProperty = DependencyProperty.Register("WindowType", typeof(Type), typeof(OpenCloseWindowBehavior), new PropertyMetadata(null));

        public bool Open { get { return (bool)GetValue(OpenProperty); } set { SetValue(OpenProperty, value); } }
        public static readonly DependencyProperty OpenProperty = DependencyProperty.Register("Open", typeof(bool), typeof(OpenCloseWindowBehavior), new PropertyMetadata(false, OnOpenChanged));

        /// <summary>
        /// Opens or closes a window of type 'WindowType'.
        /// </summary>
        private static void OnOpenChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var me = (OpenCloseWindowBehavior)d;
            if ((bool)e.NewValue)
            {
                object instance = Activator.CreateInstance(me.WindowType);
                if (instance is Window)
                {
                    Window window = (Window)instance;
                    window.Closing += (s, ev) => 
                    {
                        if (me.Open) // window closed directly by user
                        {
                            me._windowInstance = null; // prevents repeated Close call
                            me.Open = false; // set to false, so next time Open is set to true, OnOpenChanged is triggered again
                        }
                    }; 
                    window.Show();
                    me._windowInstance = window;
                }
                else
                {
                    // could check this already in PropertyChangedCallback of WindowType - but doesn't matter until someone actually tries to open it.
                    throw new ArgumentException(string.Format("Type '{0}' does not derive from System.Windows.Window.", me.WindowType));
                }
            }
            else 
            {
                if (me._windowInstance != null)
                    me._windowInstance.Close(); // closed by viewmodel
            }
        }
    }
}