Maintaining the collection in a List/ObservableCollection

问题

i have been successfully adding an item to list in a MVVM, and now my problem is maintaining the list in the view model. Every time i navigate to a page or go back to a page and return to that listview, the list resets. how will i able to achieve that? i am currently using the prism to build the MVVM.

The ViewModel:

public ObservableCollection<CartData> _cartData;

public ObservableCollection<CartData> CartData
        {

         get {
             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 });
                   }

View:

 .....

 <Page.DataContext>
        <vm:CartingDataSource/>
    </Page.DataContext>
   ....
<ListView
            x:Name="itemListView"
            AutomationProperties.AutomationId="ItemsListView"
            AutomationProperties.Name="Items"
            TabIndex="1"
            Margin="-10,130,0,264"
            Padding="120,0,0,60"

            ItemsSource="{Binding cartData}"
            IsSwipeEnabled="False" Grid.RowSpan="2" ItemClick="itemListView_ItemClick" SelectionChanged="itemListView_SelectionChanged_1" IsItemClickEnabled="True">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <Grid Margin="6">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>
                        <Border Background="{ThemeResource ListViewItemPlaceholderBackgroundThemeBrush}" Width="60" Height="60">
                            <Image Stretch="UniformToFill" AutomationProperties.Name="{Binding Title}" Source="Assets/wewewew.jpg"/>
                        </Border>
                        <StackPanel Grid.Column="1" Margin="10,0,0,0">
                            <TextBlock Text="{Binding Cakename}" Style="{StaticResource TitleTextBlockStyle}" TextWrapping="NoWrap" MaxHeight="40"/>
                            <TextBlock Text="{Binding Cakeprice}" Style="{StaticResource CaptionTextBlockStyle}" TextWrapping="NoWrap"/>
                        </StackPanel>
                    </Grid>
                </DataTemplate>
            </ListView.ItemTemplate>
            <ListView.ItemContainerStyle>
                <Style TargetType="FrameworkElement">
                    <Setter Property="Margin" Value="0,0,0,10"/>
                </Style>
            </ListView.ItemContainerStyle>
        </ListView>

回答1:

Here if your ViewModel is CartingDataSource in that case it is being instatiated on every page load. Now if that is the case, then you are creating a new instance of your collection in your constructor as below:

public CartingDataSource() {


        CartData = new ObservableCollection<CartData>();

      }

As a result of which it re-initializes your collection.

You need to remove the initialization from your constructor and do something like this:

public ObservableCollection<CartData> _cartData;

public ObservableCollection<CartData> cartData
    {

     get {
            if(_cartData == null)
            {
                 _cartData = new ObservableCollection<CartData>();
            } 
         return _cartData;
         }
    set {

            _cartData = value;
       }
    }

来源：https://stackoverflow.com/questions/22163110/maintaining-the-collection-in-a-list-observablecollection

标签

xaml

mvvm

prism