Keep Collections to the List in MVVM Prism

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C#

XAML

Metro-design

PRISM

currently im having troubles in keeping the collections in the list in the mvvm, every time that i navigate away and return to a page, the list keeps getting empty. how am i going to deal with this? can someone help me with this? The Model: I have not implemented the others yet because i cant add items to the list

The ViewModel:

class CartingDataSource : BindableBase
{
public ObservableCollection<CartData> _cartData;
 
public ObservableCollection<CartData> CartData
        {
 
         get {
 
               if (_cartData == null)
                {
                    _cartData = new ObservableCollection<CartData>();
                }
             return _cartData;
             }
        set {
 
                SetProperty(ref _cartData, value);
           }
        }
 

 

        private DelegateCommand _addItemCommand;
        public ICommand AddItemCommand
        {
            get
            {
                if (_addItemCommand == null)
                {
                    _addItemCommand = new DelegateCommand(AddToCart);
                }
                return _addItemCommand;
            }
        }
       public void AddToCart() {
 
       CartData.Add(new CartData 
         { 
            Cakename = "Black Forest", Cakeprice = 104 });
}
 
                   }

i have already asked in the stackoverflow but no proper answer yet. can you please help me? this is my second post in here thank you!