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...

Please elaborate what you mean by "storage folder issue". Do you get an exception or something else? — Stefan Wick MSFT
If you want to answer please help..Please dont down vote..There is nothing wrong with question..Nothing happens on that line It just stops executing and does not proceed to next line..no exception nothing...in output windows backgroundhosts exits(already mentioned) — uncle_scrooge
The question doesn't have enough info on the problem, so please update it so others know what you are facing. I don't see where you mention in the question that the background task exits without exception. Trying to help you articulate good questions, so you get good answers from the community. — Stefan Wick MSFT
Did you declare the "Picture Library" capability in your app's manifest? — Stefan Wick MSFT
Sorry, missed the comment in the code snippet. My bad! (edited the post and the vote now) — Stefan Wick MSFT

Kevin Gosse Kevin Gosse · Accepted Answer · 2017-06-17T14:33:45

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()
{
    ...
}

Setting a wallpaper in background task

1 Answers