维护收藏列表/的ObservableCollection [英] Maintaining the collection in a List/ObservableCollection

问题描述

我已经成功地将一个项目在MVVM上市，现在我的问题是如何保持在视图模型的列表。每次我浏览一个页面或返回到一个页面，并返回到列表视图，列表重置。我将如何能够做到这一点？ 。我目前使用的棱镜来构建MVVM

视图模型：

 公开的ObservableCollection< CartData> _cartData; 
 
公众的ObservableCollection< CartData> CartData 
 {
 
获得{
返回_cartData; 
} 
集合{
 
的SetProperty（REF _cartData，价值）; 
} 
} 
 
 
 
 
私人DelegateCommand _addItemCommand; 
公众的ICommand AddItemCommand 
 {
得到
 {
如果（_addItemCommand == NULL）
 {
 _addItemCommand =新DelegateCommand（AddToCart）; 
} 
返回_addItemCommand; 
} 
} 
 
公共无效AddToCart（）{
 
 CartData.Add（新CartData {Cakename =黑森林，Cakeprice = 104} ）; 
} 
  

查看：

  ..... 
 
< Page.DataContext> 
< VM：CartingDataSource /> 
< /Page.DataContext> 
 .... 
< ListView的
X：NAME =itemListView
 AutomationProperties.AutomationId =ItemsListView
 AutomationProperties.Name =项目
的TabIndex =1
保证金= -  10,130,0,264
填充=120,0,0,60
 
的ItemsSource ={结合cartData}
 IsSwipeEnabled =FALSEGrid.RowSpan =2项目单击=itemListView_ItemClick的SelectionChanged =itemListView_SelectionChanged_1IsItemClickEnabled =真> 
< ListView.ItemTemplate> 
<&DataTemplate的GT; 
<电网保证金=6> 
< Grid.ColumnDefinitions> 
< ColumnDefinition WIDTH =自动/> 
< ColumnDefinition WIDTH =*/> 
< /Grid.ColumnDefinitions> 
<边框背景={ThemeResource ListViewItemPlaceholderBackgroundThemeBrush}WIDTH =60HEIGHT =60> 
<图像拉伸=UniformToFillAutomationProperties.Name ={结合标题}来源=资产/ wewewew.jpg/> 
< /边框> 
< StackPanel的Grid.Column =1保证金=10,0,0,0> 
< TextBlock的文本={结合Cakename}风格={StaticResource的TitleTextBlockStyle}TextWrapping =NoWrap的了maxHeight =40/> 
< TextBlock的文本={结合Cakeprice}风格={StaticResource的CaptionTextBlockStyle}TextWrapping =NoWrap的/> 
< / StackPanel的> 
< /网格和GT; 
< / DataTemplate中> 
< /ListView.ItemTemplate> 
< ListView.ItemContainerStyle> 
<风格的TargetType =FrameworkElement的> 
< setter属性=保证金VALUE =0,0,0,10/> 
< /样式和GT; 
< /ListView.ItemContainerStyle> 
< /&的ListView GT; 
  

解决方案

下面，如果您的视图模型是 CartingDataSource 在这种情况下，它是被instatiated在每一页上的负荷。现在，如果是这样的话，那么你就是在构造函数创建集合的一个新的实例如下：

 公共CartingDataSource（ ）{
 
 
 CartData =新的ObservableCollection< CartData>（）; 
 
} 
  

其结果是它重新初始化您的收藏。

您需要从构造函数中删除初始化和做这样的事情：

 公开的ObservableCollection< CartData> _cartData; 
 
公众的ObservableCollection< CartData> cartData 
 {
 
获得{
如果（_cartData == NULL）
 {
 _cartData =新的ObservableCollection< CartData>（）; 
} 
返回_cartData; 
} 
集合{
 
 _cartData =价值; 
} 
} 
  

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>

解决方案

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;
       }
    }

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