C# – Opening new window in MVVM WPF

cmvvmwpf

I have a Button and I bind this button to a command in ViewModel say OpenWindowCommand. When I click on the button I want to open a new window. But creating a window instance and showing a window from view model is a violation of MVVM. I have created interface like

interface IWindowService
{
    void showWindow(object dataContext);
}

and WindowService implements this interface like

class WindowService : IWindowService
{
    public void showWindow(object dataContext)
    {
        ChildWindow window=new ChildWindow();
        window.DataContext=dataContext;
        window.Show();
    }
}

In this class I have specified ChildWindow. So this class is tightly coupled with showing ChildWindow. When I want to show another window, I have to implement another class with the same interface and logic. How can I make this class generic so that I can just pass an instance of any window and the class will be able to open any window?

I am not using any built MVVM frameworks. I have read many articles on StackOverflow but I could not found any solution for this.

Best Answer

You say "creating window instance and showing window from view model is violation of MVVM". This is correct.

You are now trying to create an interface that takes a type of view specified by the VM. This is just as much of a violation. You may have abstracted away the creation logic behind an interface, but you are still requesting view creations from within the VM.

VM's should only care about creating VM's. If you really need a new window to host the new VM, then provide an interface as you have done, but one that does NOT take a view. Why do you need the view? Most (VM first) MVVM projects use implicit datatemplates to associate a view with a particular VM. The VM knows nothing about them.

Like this:

class WindowService:IWindowService
{
    public void ShowWindow(object viewModel)
    {
        var win = new Window();
        win.Content = viewModel;
        win.Show();
    }
}

Obviously you need to make sure you have your VM->View implicit templates set up in app.xaml for this to work. This is just standard VM first MVVM.

eg:

<Application x:Class="My.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="clr-namespace:My.App.ViewModels"
             xmlns:vw="clr-namespace:My.App.Views"
             StartupUri="MainWindow.xaml">
    <Application.Resources>

        <DataTemplate DataType="{x:Type vm:MyVM}">
            <vw:MyView/>
        </DataTemplate>

    </Application.Resources>
</Application>

Related Solutions

C# – Binding a WPF ComboBox to a custom list

You set the DisplayMemberPath and the SelectedValuePath to "Name", so I assume that you have a class PhoneBookEntry with a public property Name.

Have you set the DataContext to your ConnectionViewModel object?

I copied you code and made some minor modifications, and it seems to work fine. I can set the viewmodels PhoneBookEnty property and the selected item in the combobox changes, and I can change the selected item in the combobox and the view models PhoneBookEntry property is set correctly.

Here is my XAML content:

<Window x:Class="WpfApplication6.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window1" Height="300" Width="300">
<Grid>
    <StackPanel>
        <Button Click="Button_Click">asdf</Button>
        <ComboBox ItemsSource="{Binding Path=PhonebookEntries}"
                  DisplayMemberPath="Name"
                  SelectedValuePath="Name"
                  SelectedValue="{Binding Path=PhonebookEntry}" />
    </StackPanel>
</Grid>
</Window>

And here is my code-behind:

namespace WpfApplication6
{

    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
            ConnectionViewModel vm = new ConnectionViewModel();
            DataContext = vm;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            ((ConnectionViewModel)DataContext).PhonebookEntry = "test";
        }
    }

    public class PhoneBookEntry
    {
        public string Name { get; set; }

        public PhoneBookEntry(string name)
        {
            Name = name;
        }

        public override string ToString()
        {
            return Name;
        }
    }

    public class ConnectionViewModel : INotifyPropertyChanged
    {
        public ConnectionViewModel()
        {
            IList<PhoneBookEntry> list = new List<PhoneBookEntry>();
            list.Add(new PhoneBookEntry("test"));
            list.Add(new PhoneBookEntry("test2"));
            _phonebookEntries = new CollectionView(list);
        }

        private readonly CollectionView _phonebookEntries;
        private string _phonebookEntry;

        public CollectionView PhonebookEntries
        {
            get { return _phonebookEntries; }
        }

        public string PhonebookEntry
        {
            get { return _phonebookEntry; }
            set
            {
                if (_phonebookEntry == value) return;
                _phonebookEntry = value;
                OnPropertyChanged("PhonebookEntry");
            }
        }

        private void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        public event PropertyChangedEventHandler PropertyChanged;
    }
}

Edit: Geoffs second example does not seem to work, which seems a bit odd to me. If I change the PhonebookEntries property on the ConnectionViewModel to be of type ReadOnlyCollection, the TwoWay binding of the SelectedValue property on the combobox works fine.

Maybe there is an issue with the CollectionView? I noticed a warning in the output console:

System.Windows.Data Warning: 50 : Using CollectionView directly is not fully supported. The basic features work, although with some inefficiencies, but advanced features may encounter known bugs. Consider using a derived class to avoid these problems.

Edit2 (.NET 4.5): The content of the DropDownList can be based on ToString() and not of DisplayMemberPath, while DisplayMemberPath specifies the member for the selected and displayed item only.

Wpf – Good examples of MVVM Template

Unfortunately there is no one great MVVM example app that does everything, and there are a lot of different approaches to doing things. First, you might want to get familiar with one of the app frameworks out there (Prism is a decent choice), because they provide you with convenient tools like dependency injection, commanding, event aggregation, etc to easily try out different patterns that suit you.

The prism release:
http://www.codeplex.com/CompositeWPF

It includes a pretty decent example app (the stock trader) along with a lot of smaller examples and how to's. At the very least it's a good demonstration of several common sub-patterns people use to make MVVM actually work. They have examples for both CRUD and dialogs, I believe.

Prism isn't necessarily for every project, but it's a good thing to get familiar with.

CRUD: This part is pretty easy, WPF two way bindings make it really easy to edit most data. The real trick is to provide a model that makes it easy to set up the UI. At the very least you want to make sure that your ViewModel (or business object) implements INotifyPropertyChanged to support binding and you can bind properties straight to UI controls, but you may also want to implement IDataErrorInfo for validation. Typically, if you use some sort of an ORM solution setting up CRUD is a snap.

This article demonstrates simple crud operations: http://dotnetslackers.com/articles/wpf/WPFDataBindingWithLINQ.aspx

It is built on LinqToSql, but that is irrelevant to the example - all that is important is that your business objects implement INotifyPropertyChanged (which classes generated by LinqToSql do). MVVM is not the point of that example, but I don't think it matters in this case.

This article demonstrates data validation
http://blogs.msdn.com/wpfsdk/archive/2007/10/02/data-validation-in-3-5.aspx

Again, most ORM solutions generate classes that already implement IDataErrorInfo and typically provide a mechanism to make it easy to add custom validation rules.

Most of the time you can take an object(model) created by some ORM and wrap it in a ViewModel that holds it and commands for save/delete - and you're ready to bind UI straight to the model's properties.

The view would look like something like this (ViewModel has a property Item that holds the model, like a class created in the ORM):

<StackPanel>
   <StackPanel DataContext=Item>
      <TextBox Text="{Binding FirstName, Mode=TwoWay, ValidatesOnDataErrors=True}" />
      <TextBox Text="{Binding LastName, Mode=TwoWay, ValidatesOnDataErrors=True}" />
   </StackPanel>
   <Button Command="{Binding SaveCommand}" />
   <Button Command="{Binding CancelCommand}" />
</StackPanel>

Dialogs: Dialogs and MVVM are a bit tricky. I prefer to use a flavor of the Mediator approach with dialogs, you can read a little more about it in this StackOverflow question:
WPF MVVM dialog example

My usual approach, which is not quite classic MVVM, can be summarized as follows:

A base class for a dialog ViewModel that exposes commands for commit and cancel actions, an event to lets the view know that a dialog is ready to be closed, and whatever else you will need in all of your dialogs.

A generic view for your dialog - this can be a window, or a custom "modal" overlay type control. At its heart it is a content presenter that we dump the viewmodel into, and it handles the wiring for closing the window - for example on data context change you can check if the new ViewModel is inherited from your base class, and if it is, subscribe to the relevant close event (the handler will assign the dialog result). If you provide alternative universal close functionality (the X button, for instance), you should make sure to run the relevant close command on the ViewModel as well.

Somewhere you need to provide data templates for your ViewModels, they can be very simple especially since you probably have a view for each dialog encapsulated in a separate control. The default data template for a ViewModel would then look something like this:

<DataTemplate DataType="{x:Type vmodels:AddressEditViewModel}">
   <views:AddressEditView DataContext="{Binding}" />
</DataTemplate>

The dialog view needs to have access to these, because otherwise it won't know how to show the ViewModel, aside from the shared dialog UI its contents are basically this:

<ContentControl Content="{Binding}" />

The implicit data template will map the view to the model, but who launches it?

This is the not-so-mvvm part. One way to do it is to use a global event. What I think is a better thing to do is to use an event aggregator type setup, provided through dependency injection - this way the event is global to a container, not the whole app. Prism uses the unity framework for container semantics and dependency injection, and overall I like Unity quite a bit.

Usually, it makes sense for the root window to subscribe to this event - it can open the dialog and set its data context to the ViewModel that gets passed in with a raised event.

Setting this up in this way lets ViewModels ask the application to open a dialog and respond to user actions there without knowing anything about the UI so for the most part the MVVM-ness remains complete.

There are times, however, where the UI has to raise the dialogs, which can make things a bit trickier. Consider for example, if the dialog position depends on the location of the button that opens it. In this case you need to have some UI specific info when you request a dialog open. I generally create a separate class that holds a ViewModel and some relevant UI info. Unfortunately some coupling seems unavoidable there.

Pseudo code of a button handler that raises a dialog which needs element position data:

ButtonClickHandler(sender, args){
    var vm = DataContext as ISomeDialogProvider; // check for null
    var ui_vm = new ViewModelContainer();
    // assign margin, width, or anything else that your custom dialog might require
    ...
    ui_vm.ViewModel = vm.SomeDialogViewModel; // or .GetSomeDialogViewModel()
    // raise the dialog show event
}

The dialog view will bind to position data, and pass the contained ViewModel to the inner ContentControl. The ViewModel itself still doesn't know anything about the UI.

In general I don't make use of the DialogResult return property of the ShowDialog() method or expect the thread to block until the dialog is closed. A non-standard modal dialog doesn't always work like that, and in a composite environment you often don't really want an event handler to block like that anyhow. I prefer to let the ViewModels deal with this - the creator of a ViewModel can subscribe to its relevant events, set commit/cancel methods, etc, so there is no need to rely on this UI mechanism.

So instead of this flow:

// in code behind
var result = somedialog.ShowDialog();
if (result == ...

I use:

// in view model
var vm = new SomeDialogViewModel(); // child view model
vm.CommitAction = delegate { this.DoSomething(vm); } // what happens on commit 
vm.CancelAction = delegate { this.DoNothing(vm); } // what happens on cancel/close (optional)
// raise dialog request event on the container

I prefer it this way because most of my dialogs are non-blocking pseudo-modal controls and doing it this way seems more straightforward than working around it. Easy to unit test as well.

Best Answer

Related Solutions

C# – Binding a WPF ComboBox to a custom list

Wpf – Good examples of MVVM Template

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