Xamarin Forms SearchBar + ListView slow to update - forms

Question

I would like some help into speeding up the process of filtering a long list of list items and viewing them on a ListView.
My app has a search bar, a ListView and a very long list of strings to choose from.
When the user enter a search term, the ListView is updated with every key stroke and filter out the irrelevant items.
Sorting itself takes a few milliseconds, but updating the ListView afterwards with the new filter-event takes a long time (20 seconds easy, if only a single character has been entered as search criteria)
I believe the time is spent on inflating a large number of ViewCells every time the filtered list updates.
Do any of you know how to speed up the process? I thought the way it could work was to have a very limited number of ViewCells (like 10 or 20) and then have them update and just show a selection of the filtered list. Scrolling would to be reusing the top/bottom one, update the content and put it back on the bottom/top - but I have not been able to wrap my head around how to do this.
Maybe it is the wrong approach and you know a better way?

Answer 1

I just had a similar problem that my list with just 20 elements would search extremely slow. Maybe your problem is similar. Sadly you didn't post any code. I had something like this:
List l = originalItems.Where((i) => i.Name.Contains(filterText));
listView.ItemsSource = l;
And I could not understand why this would be so slow. I found a different approach with more overhead that for some reason is faster and more responsive and overall feels better for the user. My ListView always has an ObservableCollection as ItemsSource. When I filter I calculate the difference to this ObservableCollection (the extra items and the removed items) and then remove or add accordingly. This avoids replacing the ItemsSource property which seems to be too harsh on the ListView.
//property of the class
ObservableCollection<FlightListItem> listViewItems;
// ....
//somewhere at initialization
listView.ItemsSoure = listViewItems;
// ....
//in the filter method:
List l = originalItems.Where((i) => i.Name.Contains(filterText));
IEnumerable itemsToAdd = l.Except(listViewItems).ToList();
IEnumerable itemsToRemove = listViewItems.Except(l).ToList();
listView.BeginRefresh();
foreach (FlightListItem item in removes)
listViewItems.Remove(item);
foreach (FlightListItem item in added)
listViewItems.Add(item);
listView.EndRefresh();
Notes:
removing the listView.BeginRefresh() and EndRefresh() did not seem to impact performance, but it seems the right thing to call them here.
We need to call ToList() on the itemsToAdd and itemsToRemove even though we only need IEnumerables! This is because Except is a lazy operation and will otherwise only be evaluated during the for loop. However during the for loop one of the parameters to Except changes which leads to an IllegalArgumentException due to modifying an IEnumerable while going over it.
If anyone knows a good filterable observable collection that would probably be a nicer solution.