Periodically update silverlight view with MVVM - silverlight

Question

I am trying to use MVVM in Silverlight, but I am quite new to it so I am not quite sure on some things. I have a silverlight page which displays the progress of some server side operation. The current progress comes from a web service and should be refreshed every few seconds (lets say 10 seconds for the sake of the argument).
What is the best way to implement this? The options I could think of was:
Initalize a DispatcherTimer in the Initalize method of my ViewModel and refresh the view from the DispatcherTimer event (putting the timer details in the ViewModel)
Create a wrapper arround DispatcherTimer (e.g. PeriodicCommandExecutor) which would be a Control or resource similar to the Timer control in WindowsForms with a command property that I bind to a Refresh command in the ViewModel (putting the timer details in the View)
I think the second option is preferred, because it makes the ViewModel easier to test and DispatcherTimer is an UI implementation detail which I don't want in my ViewModel propably. Do you agree?
If yes, how would you create such a wrapper. I started doing an DependencyObject with attached properties, but I am not sure how to forward the property values like Interval to the internal DispatcherTimer. Silverlight doesn't seem to provide any events when the dependency properties change and DispatcherTimer is not a DependencyObject so I can't databind directly to its properties.
Thanks!

Answer 1

Why use a DispatcherTimer? Why not use an ordinary System.Threading.Timer, which will fire its callback on a background thread?
If you put your UI progress update somewhere inconspicious (i.e. not in the centre of the UI, maybe in a bottom corner or status bar), then have the background timer chugging away while the user carries on with what they were doing. The progress value can be populated into the viewmodel, and shown on the UI using binding. This way you don't have to tie up the UI thread making web service calls.

Answer 2

At the end I solved my dillema creating a behavior which periodically executes a refresh command on the ViewModel which you can specify.
The code for the behavior is like this
(sorry for VB code):
Option Strict On
Imports System.Windows.Threading
Imports System.Windows.Interactivity
Namespace View.Behaviors
Public Class RefreshBehavior
Inherits Behavior(Of FrameworkElement)
Public Property Command As ICommand
Get
Return DirectCast(GetValue(CommandProperty), ICommand)
End Get
Set(ByVal value As ICommand)
SetValue(CommandProperty, value)
End Set
End Property
Public Shared ReadOnly CommandProperty As DependencyProperty = _
DependencyProperty.Register("Command", _
GetType(ICommand), GetType(RefreshBehavior), _
New PropertyMetadata(Nothing))
Public Property CommandParameter As Object
Get
Return GetValue(CommandParameterProperty)
End Get
Set(ByVal value As Object)
SetValue(CommandParameterProperty, value)
End Set
End Property
Public Shared ReadOnly CommandParameterProperty As DependencyProperty = _
DependencyProperty.Register("CommandParameter", _
GetType(Object), GetType(RefreshBehavior), _
New PropertyMetadata(Nothing))
Public Property Interval As TimeSpan
Get
Return DirectCast(GetValue(IntervalProperty), TimeSpan)
End Get
Set(ByVal value As TimeSpan)
SetValue(IntervalProperty, value)
End Set
End Property
Public Shared ReadOnly IntervalProperty As DependencyProperty = _
DependencyProperty.Register("Interval", _
GetType(TimeSpan), GetType(RefreshBehavior), _
New PropertyMetadata(TimeSpan.Zero, AddressOf OnIntervalUpdate))
Public Property Enabled As Boolean
Get
Return DirectCast(GetValue(EnabledProperty), Boolean)
End Get
Set(ByVal value As Boolean)
SetValue(EnabledProperty, value)
End Set
End Property
Public Shared ReadOnly EnabledProperty As DependencyProperty = _
DependencyProperty.Register("Enabled", _
GetType(Boolean), GetType(RefreshBehavior), _
New PropertyMetadata(False, AddressOf OnEnabledUpdate))
Dim WithEvents timer As New DispatcherTimer()
Private Shared Sub OnEnabledUpdate(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
Dim enable As Boolean = CType(e.NewValue, Boolean)
Dim executor As RefreshBehavior = CType(d, RefreshBehavior)
If Not executor.attached Then Return
Dim timer As DispatcherTimer = executor.timer
If enable AndAlso Not timer.IsEnabled Then
timer.Start()
ElseIf Not enable AndAlso Not timer.IsEnabled Then
timer.Stop()
End If
End Sub
Private Shared Sub OnIntervalUpdate(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
Dim executor As RefreshBehavior = CType(d, RefreshBehavior)
Dim timer As DispatcherTimer = executor.timer
timer.Interval = CType(e.NewValue, TimeSpan)
End Sub
Private WithEvents attachedObject As FrameworkElement
Private Sub OnUnload(ByVal sender As Object, ByVal e As EventArgs) Handles attachedObject.Unloaded
timer.Stop()
End Sub
Private attached As Boolean = False
Protected Overrides Sub OnAttached()
attached = True
attachedObject = AssociatedObject
If Enabled Then timer.Start()
MyBase.OnAttached()
End Sub
Protected Overrides Sub OnDetaching()
timer.Stop()
attached = False
attachedObject = Nothing
MyBase.OnDetaching()
End Sub
Private Sub OnTick(ByVal sender As Object, ByVal e As EventArgs) Handles Timer.Tick
Dim cmd = Command
Dim parameter = CommandParameter
If Interval < TimeSpan.MaxValue AndAlso cmd IsNot Nothing AndAlso cmd.CanExecute(parameter) Then
cmd.Execute(parameter)
End If
End Sub
End Class
End Namespace
You can use it like this:
<i:Interaction.Behaviors>
<Behaviors:RefreshBehavior Enabled="True" Interval="0:0:10" Command="{Binding RefreshPageCommand}" />
</i:Interaction.Behaviors>
I hope it helps someone with a similar problem.