multithreading - Transfer data from background to UI thread

Question

I have a quite long list of items (about 100) , which I am getting from server and that I show to an user in a LongListSelector. Every item is a picture, name and some description.

The main problem is that when I am sending the list to ObservableCollection, that it noticeably slows down the ui.

Everything is even worse, when I am trying to scroll up and down: sometimes I can see delay during the rendering of new items.

I tried it 2 ways,

using TaskEx.Run():

var answer = await shoppingCartListDataService.GetShoppingListAsync();

var groupedObjects = await TaskEx.Run(() => from item in answer.collection
                                                       group item by
                                                            item.name[0].ToString(CultureInfo.InvariantCulture)
                                                        into it
                                                        orderby it.Key
                                                        select
                                                            new ProductSearchCategoryCollection<ProductItem>(
                                                            it.Key, it.OrderBy(i => i.SumPrice)));
FoundProductItems = new ObservableCollection<ProductSearchCategoryCollection<ProductItem>>(groupedObjects);

and

Dispatcher.BeginInvoke() (called from ThreadWorker)

var answer = await shoppingCartListDataService.GetShoppingListAsync();
var groupedObjects = from item in answer.collection
                                     group item by item.aisle
                                     into it
                                     orderby it.Key
                                     select
                                     new ProductSearchCategoryCollection<ProductItem>(it.Key, it.OrderBy(i => i.SumPrice));
Deployment.Current.Dispatcher.BeginInvoke(() =>
                    FoundProductItems = new ObservableCollection<ProductSearchCategoryCollection<ProductItem>>(groupedObjects));

In the both cases the ui slowdown is noteable.

Answer 1

里奇梅尔顿的最终版本：

_items = groupedObjects.ToList();
Deployment.Current.Dispatcher.BeginInvoke(() => 
                        RaisePropertyChanged("FoundProductItems", _items, _items, true)); // using shortcut from MVVMlight