ListView 项目源在初始化时为空

Question

我试图在初始化时和初始化时对 ListView 使用过滤器，但该项目正在为 ListView 抛出一个空指针，即使我在构造函数和绑定中设置它。这是我的代码：

 public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            var viewModel = new MainViewModel();
            this.ProjectListView.ItemsSource = viewModel.Projects;

这里是引用它的 XAML。

<ListView x:Name="ProjectListView" Margin="0,0,10,0" ItemsSource="{Binding Projects}" FontSize="16" Foreground="Black">

如果在构造函数中初始化，我不确定为什么 ItemSource 为空。我需要应用 PropertyChanged 绑定吗？

这是我的大部分 XAML

  <TextBox x:Name="FolderNameBox" Grid.Column="1" Background="White" Grid.Row="1" Grid.ColumnSpan="5" 
               Margin="0,0,287,654.333" VerticalContentAlignment="Center"
               Padding="6" FontSize="16"
               IsReadOnly="True"
               Text="{Binding ElementName=Hierarchy, Path=SelectedItem.Path, UpdateSourceTrigger=PropertyChanged, Mode=OneWay}">
    </TextBox>
    <TextBox x:Name="SearchProjectsBox" Grid.Column="5" Background="White" Grid.Row="1" Text="Search Projects"
        Margin="47.333,0,0,654.333" VerticalContentAlignment="Center" Foreground="LightGray" Padding="6" FontSize="16" HorizontalAlignment="Left" Width="268" GotFocus="TextBox_GotFocus" LostFocus="TextBox_LostFocus" TextChanged="FilterListView"/>
    <TreeView x:Name="Hierarchy" Grid.Column="4" HorizontalAlignment="Left" Height="631" Margin="0,58,0,0" Grid.Row="1" VerticalAlignment="Top" Width="226" 
              ItemsSource="{Binding Path=Projects}">
        <TreeView.ItemTemplate>
            <HierarchicalDataTemplate ItemsSource="{Binding ChildFolders}">
                <StackPanel Orientation="Horizontal" >
                    <Image Source="{Binding Icon}" Margin="5, 5, 5, 5"></Image>
                    <TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" BorderThickness="0" FontSize="16" Margin="5" IsReadOnly="True" PreviewMouseDoubleClick="SelectAll" LostFocus="TextBoxLostFocus"/>
                </StackPanel>
            </HierarchicalDataTemplate>
        </TreeView.ItemTemplate>
    </TreeView>
    <Grid Grid.ColumnSpan="2" Grid.Column="4" HorizontalAlignment="Left" Height="631" Margin="245,58,0,0" Grid.Row="1" VerticalAlignment="Top" Width="540">
        <ScrollViewer HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden">
            <ListView x:Name="ProjectListView" Margin="0,0,10,0" ItemsSource="{Binding Projects}" FontSize="16" Foreground="Black">
                <ListView.ItemContainerStyle>
                    <Style TargetType="{x:Type ListViewItem}">
                        <Style.Triggers>
                            <Trigger Property="IsMouseOver" Value="true">
                                <Setter Property="Background" Value="Transparent"/>
                                <Setter Property="BorderBrush" Value="Transparent"/>
                            </Trigger>
                            <Trigger Property="IsMouseOver" Value="false">
                                <Setter Property="Background" Value="Transparent"/>
                                <Setter Property="BorderBrush" Value="Transparent"/>
                            </Trigger>
                        </Style.Triggers>
                    </Style>
                </ListView.ItemContainerStyle>

这是我的基本视图模型

        private ObservableCollection<Project> projects;
    public ObservableCollection<Project> Projects
    {
        get { return projects; }
        set
        {
            if (value != projects)
            {
                projects = value;
                OnPropertyChanged("Projects");
            }
        }
    }

这是背后的代码

        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainViewModel();
            MainViewModel model = new MainViewModel();
            model.BuildData();
            this.ProjectListView.ItemsSource = model.Projects;

        }

        private void FilterListView(object sender, TextChangedEventArgs e)
        {
            CollectionViewSource.GetDefaultView(this.ProjectListView.ItemsSource).Refresh();
        }

FilterListView 是 TextChange 方法。它在第一行中断。 ProjectListVIew 注册为 null，但它甚至不应该启动，因为它依赖于 TextBox 中的更改。

编辑。这是后面的代码

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainViewModel();
            MainViewModel model = new MainViewModel();
            model.BuildData();
            this.ProjectListView.ItemsSource = model.Projects;

        }

        private void FilterListView(object sender, TextChangedEventArgs e)
        {
            CollectionViewSource.GetDefaultView(this.ProjectListView.ItemsSource).Refresh();
        }


        private void SelectAll(object sender, MouseButtonEventArgs e)
        {
            TextBox box = sender as TextBox;
            box.IsReadOnly = false;
            box.SelectAll();
        }

        private void TextBoxLostFocus(object sender, RoutedEventArgs e)
        {
            TextBox box = sender as TextBox;
            box.IsReadOnly = true;
        }

        private void TextBox_GotFocus(object sender, RoutedEventArgs e)
        {
            TextBox tb = (TextBox)sender;
            tb.Text = string.Empty;
            tb.Foreground = Brushes.Black;
            tb.GotFocus -= TextBox_GotFocus;
        }

        private void TextBox_LostFocus(object sender, RoutedEventArgs e)
        {
            TextBox box = (TextBox)sender;
             box.Text = "Search Projects";
             box.Foreground = Brushes.LightGray;
             box.GotFocus += TextBox_GotFocus;  
        }
    }
}

基础视图模型

public class ViewModelBase : INotifyPropertyChanged
{
    #region INotifyPropertyChanged
    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string projectName)
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(projectName));
        }
    }
    #endregion INotifyPropertyChanged
}

以及我正在使用的视图模型

class MainViewModel : ViewModelBase
{
    private ObservableCollection<Project> projects;
    public ObservableCollection<Project> Projects
    {
        get { return projects; }
        set
        {
            if (value != projects)
            {
                projects = value;
                OnPropertyChanged("Projects");
            }
        }
    }

编辑：

我的 ListView 不断变化 - 例如，显示的数据将基于用户在 TreeView 中的选择。现在，我的过滤器没有考虑屏幕上的实际内容。它仅基于绑定的集合进行过滤。

因此，如果我的集合是水果、蔬菜、肉类、乳制品，而我的 ListView 是 Apple、Carrot 和 Milk，如果我搜索 Apple，ListView 将为空白，但如果我搜索 Fruit，我将有一个填充的行。显然不正确。

听起来我很接近。

这是我的视图模型：

public ICollectionView SourceCollection
    {
        get
        {
            return this.projectCollection.View;
        }
    }

    public string FilterText
    {
        get
        {
            return filterText;
        }
        set
        {
            filterText = value;
            this.projectCollection.View.Refresh();
            RaisePropertyChanged("SearchProjectsText");
        }
    }

    private void projectCollection_Filter(object sender, FilterEventArgs e)
    {
        if (string.IsNullOrEmpty(FilterText))
        {
            e.Accepted = true;
            return;
        }

        Project project = e.Item as Project;
        if (project.Name.ToUpper().Contains(FilterText.ToUpper()) || project.Path.ToUpper().Contains(FilterText.ToUpper()))
        {
            e.Accepted = true;
        }
        else
        {
            e.Accepted = false;
        }
    }

    public void RaisePropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

其中 SearchProjectText 是用于过滤的实际文本框。我的相关 XAML：

<TextBox x:Name="SearchProjectsBox" Grid.Column="5" Background="White" Grid.Row="1" Text="{Binding FilterText, UpdateSourceTrigger=PropertyChanged}"
        Margin="47.333,0,0,654.333" VerticalContentAlignment="Center" Foreground="Black" Padding="6" FontSize="16" HorizontalAlignment="Left" Width="268"/>
    <TreeView x:Name="Hierarchy" Grid.Column="4" HorizontalAlignment="Left" Height="631" Margin="0,58,0,0" Grid.Row="1" VerticalAlignment="Top" Width="226" 
              ItemsSource="{Binding Path=Projects}">

<ListView x:Name="ProjectListView" Margin="0,0,10,0" ItemsSource="{Binding SourceCollection}" FontSize="16" Foreground="Black">

Answer 1

您从不设置数据上下文。不要同时绑定和分配；你不需要两者。只需设置父级的 DataContext，并在 XAML 中绑定适当的父级属性（您已经这样做了——您的 XAML 看起来不错）：

    public MainWindow()
    {
        InitializeComponent();
        DataContext = new MainViewModel();
    }

并确保当您将新集合实例分配给其 Projects 属性时，您的视图模型正确引发 INotifyPropertyChanged.PropertyChanged。

并确保您确实为 Projects 提供一个集合，并且其中有一些东西。

我希望你使用ObservableCollection<T>，而不是List<T>，来收集？

更新

我不知道为什么（已经很晚了，我很累，抱歉！），但是 FilterListView 在 InitializeComponent() 之前被调用。当你描述症状时我应该想到这一点。这为我解决了这个问题：

private void FilterListView(object sender, TextChangedEventArgs e)
{
    if (this.ProjectListView != null)
    {
        CollectionViewSource.GetDefaultView(this.ProjectListView.ItemsSource).Refresh();
    }
}

Answer 2

设置将创建其绑定的 ViewModel 的新实例的 DataContext。

public MainWindow()
{
   InitializeComponent();
   DataContext = new MainViewModel();
 }