BackgroundTask with VB.NET

Question

Hello.

I am trying to create a background task with VB. This is what my code looks like :

Imports Windows.ApplicationModel.Background
 Imports Windows.Storage
 Imports Windows.UI.Core
 Imports Windows.UI.Core.CoreWindow
 Imports System.Runtime.InteropServices
 Public NotInheritable Class clsBack 
Implements IBackgroundTask 
<DllImport("user32", setlasterror:=True)> _
 Public Shared Function SystemParametersInfo( _
 ByVal intAction As Integer, _
 ByVal intParam As Integer, _
 ByVal strParam As String, _
 ByVal intWinIniFlag As Integer) As Integer 
' returns non-zero value if function succeeds 
End Function 
Const SPIF_UPDATEINIFILE = &H1 
Const SPI_SETDESKWALLPAPER = 20 
Const SPIF_SENDWININICHANGE = &H2 
Private arrFileNames() As String 
Public Cancelled As Boolean 
Public Sub Run(TaskInstance As IBackgroundTaskInstance) 
Dim Deferral As BackgroundTaskDeferral = taskInstance.GetDeferral() 
'Await blahbla 
Deferral.Complete() 
End Sub 
Public Shared Function RegisterBackgroundTask(TaskEntryPoint As String, TaskName As String, Trigger As IBackgroundTrigger, Condition As IBackgroundCondition) As BackgroundTaskRegistration 
For Each cur In BackgroundTaskRegistration.AllTasks 
If cur.Value.Name = TaskName Then 
Return DirectCast(cur.Value, BackgroundTaskRegistration) 
End If 
Next 
Dim Builder As New BackgroundTaskBuilder 
Builder.Name = TaskName 
Builder.TaskEntryPoint = TaskEntryPoint 
Builder.SetTrigger(Trigger) 
If Condition IsNot Nothing Then 
Builder.AddCondition(Condition) 
End If 
Dim task As BackgroundTaskRegistration = Builder.Register() 
 Return task 
End Function 
Private Sub OnCompleted(Task As IBackgroundTaskRegistration, args As BackgroundTaskCompletedEventArgs) 
Dim Settings = ApplicationData.Current.LocalSettings 
Dim Key = Task.TaskId.ToString() 
Dim Message = Settings.Values(Key).ToString() 
' UpdateUI() 
End Sub 
Private Async Function UpdateUI() As Task 
Dim MyDispatcher = GetForCurrentThread().Dispatcher 
Await MyDispatcher.RunAsync(CoreDispatcherPriority.Normal, Function() 
' Dim strPicFileName As String = arrFileNames(lstFiles.SelectedIndex) 
''Try 'Dim x As Integer 
'x = SystemParametersInfo(SPI_SETDESKWALLPAPER, 0&, strPicFileName, SPIF_UPDATEINIFILE Or SPIF_SENDWININICHANGE) 
''Catch ex As Exception 
'Dim errorval As Integer 
'errorval = Marshal.GetLastWin32Error() 
'Debug.WriteLine(errorval) 
'Debug.WriteLine(x) 
'End Try 
End Function) 
End Function 
Public Shared Sub UnregisterBackgroundTasks(TaskName As String) 
For Each cur In BackgroundTaskRegistration.AllTasks 
If cur.Value.Name = TaskName Then 
cur.Value.Unregister(True)
 End If 
Next 
End Sub 
Public Sub OnCanceled(sender As IBackgroundTaskInstance, Reason As BackgroundTaskCancellationReason) 
Cancelled = True 
End Sub 
End Class

It keeps on telling my that my class need to have Sub Run(TaskInstance As IBackgroundTaskInstance)  as I have.

Any suggestions? Am I on the wrong path?

Answer 1

Your background task registration and task builder code need to be in your main app class with the background task as a seperate class.

Have a look here http://msdn.microsoft.com/en-us/library/windows/apps/hh977055.aspx

Answer 2

Hi. Thanks for your interest!

OK, so I should move the RegisterBackgroundTask to my main class. I'll give that a shot.

The link you have provided is what I am trying to implement, I guess my English is bad or I read wrong.

I'll keep you up to date!

Answer 3

OK, I moved everything except for the Run sub out of my BackgroundTask, and still I get the same error

Any help?

Answer 4

In your example the run sub is commented out.

Here is an example of  background task in vb

Public NotInheritable Class BackgroundTask
    Implements IBackgroundTask
    
    Public Sub Run(taskInstance As IBackgroundTaskInstance) Implements IBackgroundTask.Run

        'Do some stuff here
        
    End Sub
    
End Class

That is the basics of it.

Answer 5

Thanks again.

It seems as if the code error messed up the code colours. It is indeed not commented, but here it displays green.

I think, as there is no VB sample code given, that I'd have to do my Background task in C#, and the rest of my app in VB. This really sucks.

Why does VB always have to play second fiddle to C#, especially when examples on Windows 8 are concerned?!

If I hadn't known C#, this would have been really problematic and I hope that VB guys can eventually figure this out. It seems simple enough, but nothing! I have been working with VB for years, and I have always converted C# code to VB without issues, except for now, because there is lack of documentation.

I'll let you know if my C# workaround ( which is unecessary in any logic sense ) works.

Answer 6

Can you repost your code as it is now. Also have you registered the background task in the app manifest.

Everything your background task needs to do must happen or be called from within the run sub. It also seems that you're trying to interact with windows functions which aren't available to windows apps.   

Your background class also needs to be added to your solution as a new project

Windows.ApplicationModel.Background namespace.

1. Create a new project for background tasks and add it to your solution. To do this, right-click on your solution and select Add->New Project. Then select the Windows Runtime Component project type, name the project, and click OK.
2. Reference the background tasks project from your Windows Store app project. First, right-click on your Windows Store app project and select Properties. Then go to Common Properties and click Add New Reference, check the box next to your background tasks project, and click OK on both dialogs.

3. Create a new class that implements the IBackgroundTask interface. The Runmethod is a required entry point that will be called when the specified event is triggered; this method is required in every background task.

Answer 7

Here is vb code for adding background task and task triggers as well as background task.

In mainpage class :

Public Async Function CheckIfBackgroundTaskExists() As Task

        Dim status As BackgroundAccessStatus = Await BackgroundExecutionManager.RequestAccessAsync()

        Dim taskStart As Boolean = BackgroundTaskRegistration.AllTasks.Any(Function(x) x.Value.Name = "MosaicBackgroundTask")
        If Not taskStart Then
            RegisterStartTask()
        End If

        Dim taskTimer As Boolean = BackgroundTaskRegistration.AllTasks.Any(Function(x) x.Value.Name = "MosaicBackgroundTaskTimer")
        If Not taskStart Then
            RegisterTimerTask()
        End If

    End Function

    Public Shared Sub RegisterStartTask()

        Dim builder As New BackgroundTaskBuilder()
        builder.Name = "MosaicBackgroundTask"
        builder.TaskEntryPoint = "MosaicTileClockUpdater.MosaicTileClockBackgroundTask"
        builder.SetTrigger(New SystemTrigger(SystemTriggerType.UserPresent, False))
        Dim task As BackgroundTaskRegistration = builder.Register()

    End Sub

    Public Shared Sub RegisterTimerTask()

        Dim builder2 As New BackgroundTaskBuilder()
        builder2.Name = "MosaicBackgroundTaskTimer"
        builder2.TaskEntryPoint = "MosaicTileClockUpdater.MosaicTileClockBackgroundTask"
        builder2.SetTrigger(New TimeTrigger(60, False))
        Dim task2 As BackgroundTaskRegistration = builder2.Register()

    End Sub

Background task class :

Imports Windows.ApplicationModel.Background
Imports Windows.Data.Xml.Dom
Imports Windows.UI.Notifications
Imports Windows.UI.StartScreen

Public NotInheritable Class MosaicTileClockBackgroundTask
    Implements IBackgroundTask

    Dim datetimenow As DateTime

    Public Sub Run(taskInstance As IBackgroundTaskInstance) Implements IBackgroundTask.Run

        datetimenow = DateTime.Now
        
        InitialUpdate()
        CreateSchedule()

    End Sub

In the background tasks run sub are 2 methods which run and and make subsequent method and function calls.

This vb code works so try adapting it to your needs and check that you have followed all the steps in the example correctly. Here is another link to check http://blogs.msdn.com/b/windowsappdev/archive/2012/05/24/being-productive-in-the-background-background-tasks.aspx

Answer 8

Wow, thanks for your continuous effort to help an old burnt out programmer like me.

I'll try to implement your code, then take it from there. I will let you know how it goes!

Thanks once again, and have a blessed night

Hannes

Answer 9

There is no limit to my stupidity and how thankful I ma for you pointing it out :)

Thanks for all your help

Answer 10

You're welcome , please bear in mind that I'm only able to help because I've had the same issues.