Setting a wallpaper in background task

Question

I want to get all images from a storage folder in background task. Firstly registered a background task in app_entering background method. I am also able to debug Run method,but none of the await methods are working-

public void Run(IBackgroundTaskInstance taskInstance)
    {

        var differal = taskInstance.GetDeferral();
         UpdateUI();
        differal.Complete();

    }

public async void UpdateUI()
    {

            StorageFolder folder = await KnownFolders.PicturesLibrary.GetFolderAsync("Wall_e_photos")//here execution stops and backgroundtaskhost exits.
            var files = await GetFilesAsync();
            foreach (StorageFile file in files)
            {
                if (file.Name.Contains("wall_e"))
                {
                    }

  }
       }

Struggling from long time..Initially background tasks was not working,after it started working..now storage folder issue (backgroundtask exits when getting the folder).

Also for a note I followed this link- http://www.codeguru.com/win_mobile/win_store_apps/setting-wallpapers-in-a-windows-8-store-app-with-vb.htm

There they have used dispatcher,If I use var dispatcher=MyDispatcher = GetForCurrentThread().Dispatcher,then It gives null reference exception

IF I use Windows.ApplicationModel.Core.CoreApplication.MainView ,then it gives could not create a new view exception..

Please help me...

Answer 1

You have an issue here:

var differal = taskInstance.GetDeferral();
UpdateUI();
differal.Complete();

UpdateUI is an async method, so the method call will end immediately (while the method continues executing in the background). Therefore, you're calling differal.Complete(); before the end of the work.

A simple way to solve that is to pass the deferral as parameter to the UpdateUI method, and complete it at the end:

public async void UpdateUI(BackgroundTaskDeferral deferral)
{
    StorageFolder folder = await KnownFolders.PicturesLibrary.GetFolderAsync("Wall_e_photos")//here execution stops and backgroundtaskhost exits.    
    var files = await GetFilesAsync();
    foreach (StorageFile file in files)
    {
        if (file.Name.Contains("wall_e"))
        {
        }
    }

    deferral.Complete();
}

An alternative is to change UpdateUI to be async Task, then wait for its continuation:

public async void Run(IBackgroundTaskInstance taskInstance)
{
    var differal = taskInstance.GetDeferral();
    await UpdateUI();
    differal.Complete();
}

public async Task UpdateUI()
{
    ...
}