Xamarin: Updating an ObservableCollection in the Background - xamarin

Question

I have a Xamarin.Forms Application based on a Listview populated by an ObservableCollection<Item> and bound to a SQLite model exposed as List<Item>.
Now I have difficulties figuring out how I can update the data via the web.
The whole process is supposed to run in the background. If everything runs as desired, the update process would be based on async ... await tasks and new items would appear one by one on the screen as they are pulled in.
Would anyone please guide me how to lay out my application and how to implement such a background update task?
Notes:
More experienced colleagues warned me that such a concept cannot be done in Xamarin at all, since, so they say, ObservableCollection "does not support to be updated by a background task". I did some research on this, and found indeed some indication that this could be true, but the infos were long outdated (from 2008), things have very likely changed since then.
For performance reasons I cannot simply pull in the complete list and throw away the existing list, but I need to implement a record based update looking at the items one by one. To accomplish this records have an unique Id, and are timestamped. My app knows when it has last seen the web service, and can query all items which have changed since then. I already have a REST service pulling in the changed Items data from the backend as a List, but cannot figure out how to refresh the ObservableCollection and the underlying database.

Answer 1

I do all my updates on change to the ListView. Here I have a button in the a list view which when clicked updates a property which persists by saving it to the sql database. It assumes you have your database set up.
Database:
Function which updates item if exist or saves new. This is a Task so can be called asynchronously.
public Task<int> SaveItemAsync(Item item)
{
if(item.ItemId != 0)
{
return database.UpdateAsync(item);
}
else
{
return database.InsertAsync(itme);
}
}
Xaml
List View which binds to an Observable collection created from the item database.
GestureRecognizers is set up on the image and is bound to a tapCommand in the ViewModel - The code behind the Xaml defines the binding context
Code behind
ItemViewModel viewModel;
public MessagePage ()
{
InitializeComponent ();
viewModel = new ItemViewModel();
BindingContext = viewModel;
}
And then the Xaml
Bind to the ObsevableCollection "ItemsPassed" and this set as the binding context within it. As a result you need to go back to the
page BindingContext so note the binding path for the TapCommand.
Pass the ItemId through as a parameter
<ListView ItemsSource="{Binding ItemsPassed}"
HasUnevenRows="True"
x:Name="ItemListView">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<ContentView>
<StackLayout Padding="20">
<StackLayout Orientation="Horizontal">
<StackLayout Orientation="Vertical" HorizontalOptions="Start">
<Label Text="{Binding ItemText}"
FontAttributes="Bold"/>
</StackLayout>
<Image Source="{Binding Favourited, HeightRequest="12" HorizontalOptions="EndAndExpand">
<Image.GestureRecognizers>
<TapGestureRecognizer
Command="{Binding Path=BindingContext.TapCommand, Source={x:Reference ItemListView}}"
CommandParameter="{Binding ItemId}"/>
</Image.GestureRecognizers>
</Image>
</StackLayout>
</StackLayout>
</ContentView>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
ViewModel
Define the ICommand and assign a function to it
Find the item in the observable collection and change the property
This also needs to be changed in the database using the await App.Database.SaveItemAsync(item) - because of this the function called of the command must be async
public class ItemsViewModel : INotifyPropertyChanged
{
public ObservableCollection<Item> ItemsPassed { get; set; } = new ObservableCollection<Item>();
// Get items out of yourdatabase
public async void GetItems()
{
List<Item> itemsOut = await App.Database.GetItemsAsync();
foreach (Item i in itemsOut)
{
ItemsPassed.Add(i);
}
}
ICommand tapCommand;
public ItemsViewModel()
{
GetItems();
tapCommand = new Command(ExecuteTap);
}
public ICommand TapCommand
{
get { return tapCommand; }
}
// Find the item selected and change the property
private async void ExecuteTap(object obj)
{
var item = ItemsPassed.FirstOrDefault(i => i.ItemId.ToString() == obj.ToString());
if (item != null)
{
if (item.Favourited == true)
{
item.Favourited = false;
}
else
{
item.Favourited = true;
}
await App.Database.SaveItemAsync(item);
Console.WriteLine("update the database");
}
}
}
You then want to make sure the changes occur in the view - this is done through extending INotifyPropertyChange in the Model and calling it when the property changes.
Model
When the item is changed in the viewmodel the OnPropertyChanged is fired which cause the view to update.
public class Item : INotifyPropertyChanged
{
[PrimaryKey, AutoIncrement]
public int ItemId { get; set; }
public string ItemText { get; set; }
public string Author { get; set; }
private bool _favourited;
public bool Favourited
{
get { return _favourited; }
set
{
_favourited = value;
OnPropertyChanged("Favourited");
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
Hope this helps. Sorry I had to put things in snippets the code section wasn't working properly probably because some of the code is wrong, but its just for example.