Updating Tiles Using a Scheduled Task Agent

The background agent for the sample is the TaskScheduler class, which is a subclass of the ScheduledTaskAgent class. TaskScheduler is registered using the WMAppManifest.xml file.

As you saw earlier, a periodic task is registered in the TodoItemViewModel class. This causes the TaskScheduler.OnInvoke method to be called periodically. The OnInvoke method determines what kind of task is being launched by the OS, either periodic or resource intensive. For the to-do sample, it is always a periodic task (see Listing 32.9).

LISTING 32.9. TaskScheduler OnInvoke Method (excerpt)

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protected override void OnInvoke(ScheduledTask task)
{
    /* Detect if the task is periodic or resource intensive. */
    if (task is PeriodicTask)
    {
        ProcessPeriodicTask();
    }
    else
    {
        ProcessResourceIntensiveTask();
    }
...
    NotifyComplete();
}

The main purpose of the TaskScheduler is to refresh the shell tiles on the Start Experience, updating the overdue status and changing the background image as required (see Listing 32.10).

The ProcessPeriodicTask method retrieves all tiles for the app containing the to-do item query string parameter. Each tile is processed by extracting the Id of the to-do item, and then by retrieving the actual TodoItem object using the to-do service. A new StandardTileData object is created for the TodoItem using the TodoTileDataCreator, and the tile is updated.

LISTING 32.10. TaskScheduler ProcessPeriodicTask Method

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void ProcessPeriodicTask()
{
    TodoService todoService = new TodoService();

    string queryWithEquals = TodoItemIdQueryKey + "=";
    IEnumerable<ShellTile> tiles = ShellTile.ActiveTiles.Where(
            tile => tile.NavigationUri.ToString().Contains(queryWithEquals));

    foreach (var tile in tiles)
    {
        /* NavigationUri.Query raises an exception on relative URIs. */
        Dictionary<string, string> queryPairs
            = UrlUtility.ParseQueryString(tile.NavigationUri.ToString());
        string idString = queryPairs[TodoItemIdQueryKey];
        int todoItemId;
        if (!int.TryParse(idString, out todoItemId))
        {
            Debug.Assert(false, "Invalid id " + idString);
            continue;
        }

        TodoItem todoItem;
        if (!todoService.TryGetTodoItem(todoItemId, out todoItem))
        {
            Debug.Assert(false, "Unknown item " + idString);
            continue;
        }

        StandardTileData tileData = TodoTileDataCreator.CreateTile(
                                                 todoItem.Description,
                                                 todoItem.DueDate);
        tile.Update(tileData);
    }
}

Using a scheduled task agent to update shell tiles provides the means to engage with your users even when your app is not running. It should be remembered, however, that there is no guarantee that a scheduled task will run; therefore, it is wise not to depend solely on a background task for activities critical to the proper functioning of your app. Indeed, updating of shell tiles should also be done within your foreground app, which is not done in the sample.