WP7 MVVM how do I call me.setfocus() from the ViewModel?

Question

I'm attempting to improve my MVVM abilities in my next WP7 App (which is written in Vb.NET). I have a textbox that has been given focus and has a WP7 keyboard displayed. I am using command binding and a xyzzer's bindable Application bar (which is excellent).

http://bindableapplicationb.codeplex.com/

I want to be able to cancel the focus of the TextBox from the ViewModel, by setting focus on the form. Normally (Non MVVM) I would do this from within the form by calling:

  Me.Focus()

But I can't do this from the ViewModel (and I shouldn't). At the moment I am raising an event out of the ViewModel and catching it on the form but its nasty. Is there a MVVM Friendly way of doing it? I haven't been using a Toolkit so far as there are limited examples in vb.net.

I have been using command binding.

Answer 1

Using Pedros example, and the other services I have previously implemented in my App I put together the following solution in vb.net:

Create an IFocus Interface, This Interface can be implemented by the focus service or a Mock

Public Interface IFocusInterface
    Sub Focus()
End Interface

Create a IFocusable Interface. This will be implemented by the ViewModel and accepts an object that implements IFocusInterface.

Public Interface IFocusable
    Property FocusService As IFocusInterface
End Interface

Implement the Focus Interface with a singleton pattern

Imports Microsoft.Phone.Controls

Public NotInheritable Class FocusService
    Implements IFocusInterface

    Private Sub New()
    End Sub

    Private Shared ReadOnly m_instance As New FocusService
    Public Shared ReadOnly Property Instance() As FocusService
        Get
            Return m_instance
        End Get
    End Property

    Public Sub Focus() Implements IFocusInterface.Focus
        Dim rootFrame = TryCast(Application.Current.RootVisual, PhoneApplicationFrame)
        If Not rootFrame Is Nothing Then

            Dim page = TryCast(rootFrame.Content, PhoneApplicationPage)

            If Not page Is Nothing Then
                page.Focus()
            Else
                Throw New Exception("Unable to Cast the Root Frame Content into an Application Page")
            End If

        Else
            Throw New Exception("Unable to Cast the RootVisual into a PhoneApplicationFrame")
        End If

    End Sub

End Class

In your ViewModel Implement IFocusable, and make sure you pass in the Focus Service Singleton into the ViewModel after the ViewModel is Constructed.

Public Class MyViewModel
    Implements INotifyPropertyChanged
    Implements IFocusable

    ' Property for the Focus Service
    <Xml.Serialization.XmlIgnore()> Public Property FocusService As IFocusInterface Implements IFocusable.FocusService

    Public Sub Focus()
        If Not FocusService Is Nothing Then
            FocusService.Focus()
        Else
            Throw New Exception("ViewModel hasn't been passed a Focus Service")
        End If
    End Sub

End Class

Dim tMyViewModel as New MyViewModel
tMyViewModel.FocusService = Vacation_Calc_Model.FocusService.Instance