.net – Handling Dialogs in WPF with MVVM

design-patternsdialogmvvmnetwpf

In the MVVM pattern for WPF, handling dialogs is one of the more complex operations. As your view model does not know anything about the view, dialog communication can be interesting. I can expose an ICommand that when the view invokes it, a dialog can appear.

Does anyone know of a good way to handle results from dialogs? I am speaking about windows dialogs such as MessageBox.

One of the ways we did this was have an event on the viewmodel that the view would subscribe to when a dialog was required.

public event EventHandler<MyDeleteArgs> RequiresDeleteDialog;

This is OK, but it means that the view requires code which is something I would like to stay away from.

Best Answer

I suggest forgoing the 1990's modal dialogs and instead implementing a control as an overlay (canvas+absolute positioning) with visibility tied to a boolean back in the VM. Closer to an ajax type control.

This is very useful:

<BooleanToVisibilityConverter x:Key="booltoVis" />

as in:

<my:ErrorControl Visibility="{Binding Path=ThereWasAnError, Mode=TwoWay, Converter={StaticResource booltoVis}, UpdateSourceTrigger=PropertyChanged}"/>

Here's how I have one implemented as a user control. Clicking on the 'x' closes the control in a line of code in the usercontrol's code behind. (Since I have my Views in an .exe and ViewModels in a dll, I don't feel bad about code that manipulates UI.)

Wpf dialog

Related Solutions

C# – How should the ViewModel close the form

I was inspired by Thejuan's answer to write a simpler attached property. No styles, no triggers; instead, you can just do this:

<Window ...
        xmlns:xc="clr-namespace:ExCastle.Wpf"
        xc:DialogCloser.DialogResult="{Binding DialogResult}">

This is almost as clean as if the WPF team had gotten it right and made DialogResult a dependency property in the first place. Just put a bool? DialogResult property on your ViewModel and implement INotifyPropertyChanged, and voilà, your ViewModel can close the Window (and set its DialogResult) just by setting a property. MVVM as it should be.

Here's the code for DialogCloser:

using System.Windows;

namespace ExCastle.Wpf
{
    public static class DialogCloser
    {
        public static readonly DependencyProperty DialogResultProperty =
            DependencyProperty.RegisterAttached(
                "DialogResult",
                typeof(bool?),
                typeof(DialogCloser),
                new PropertyMetadata(DialogResultChanged));

        private static void DialogResultChanged(
            DependencyObject d,
            DependencyPropertyChangedEventArgs e)
        {
            var window = d as Window;
            if (window != null)
                window.DialogResult = e.NewValue as bool?;
        }
        public static void SetDialogResult(Window target, bool? value)
        {
            target.SetValue(DialogResultProperty, value);
        }
    }
}

I've also posted this on my blog.

C# – WPF MVVM Focus Field on Load

If it makes you feel better (it makes me feel better) you can do this in Xaml using an attached property:

http://msdn.microsoft.com/en-us/library/system.windows.input.focusmanager.focusedelement.aspx

Anything you can do in codebehind you can do in Xaml if you know the tricks. Fortunately, you didn't have to implement this trick - MS did it for you.

Best Answer

Related Solutions

C# – How should the ViewModel close the form

C# – WPF MVVM Focus Field on Load

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