UWP绑定到用户控件的可观察集合不起作用

Question

我有这个页面：NewsPage

<Page
x:Class="TouchTypeRacing.Views.NewsPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:TouchTypeRacing.Views"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:controls="using:TouchTypeRacing.Controls"
xmlns:models="using:TouchTypeRacing.Models"
DataContext="{Binding}"
mc:Ignorable="d">

<Grid Background="White">
    ....
    <ScrollViewer Grid.Row="1"
                  VerticalScrollBarVisibility="Auto"
                  Margin="5,10,5,0">
        <ItemsControl ItemsSource="{Binding Posts}"
                      x:Name="itemsControl">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Vertical"/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.ItemTemplate>
                <DataTemplate x:DataType="models:Post">
                    <controls:Post/>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
    </ScrollViewer>
</Grid>
</Page>

页面的数据上下文绑定到视图模型。 PageViewmModel
ItemsControl 数据模板是 Post 控件。
页面上ItemsControl 的ItemsSource 绑定到viewmodel 的Posts 属性。

public NewsPage()
{
    this.InitializeComponent();

    _viewModel = new NewsPageViewModel();
    DataContext = _viewModel;           
}

然后，视图模型：

public class NewsPageViewModel
{
    private ObservableCollection<Post> _posts = new ObservableCollection<Post>();
    public ObservableCollection<Post> Posts { get { return _posts; } }
    public NewsPageViewModel()
    {
        GetPosts(_posts);
    }
    public static void GetPosts(ObservableCollection<Post> posts)
    {
        posts.Clear();
        posts = new ObservableCollection<Post>
        {
            new Post
            {
                Id = "1",
                DateTime = DateTime.Today,
                User = Application.CurrentUser,
                Likes = 10,
                ImagePath = Application.CurrentUser.ImagePath,
                Message = "Test message",
                Comments = new ObservableCollection<Comment>
                {
                    new Comment {Id= "1", Content="Comment1", User = Application.CurrentUser },
                    new Comment {Id= "2", Content="Comment2", User = Application.CurrentUser },
                    new Comment {Id= "3", Content="Comment3", User = Application.CurrentUser },
                    new Comment {Id= "4", Content="Comment4", User = Application.CurrentUser },
                },
                Last2Comments = new List<Comment>
                {
                    new Comment {Id= "3", Content="Comment3", User = Application.CurrentUser },
                    new Comment {Id= "4", Content="Comment4", User = Application.CurrentUser },
                }
            }
        };
    }

ItemsControl 显示为空。 我做错了什么？

Answer 1

如果您想继续切换集合，NewsPageViewModel 必须实现 INotifyPropertyChanged 并在您更改集合实例时引发 PropertyChanged。这是怎么回事。我还在下面提供了一个更简单的“快速修复”解决方案。

public class VIewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public void OnPropertyChanged([CallerMemberName] String propName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
    }
}

public class NewsPageViewModel : ViewModelBase
{
    public NewsPageViewModel()
    {
        Posts = GetPosts();
    }

    private ObservableCollection<Post> _posts;
    public ObservableCollection<Post> Posts
    {
        get { return _posts; }
        set {
            _posts = value;
            OnPropertyChanged();
        }
    }

    protected ObservableCollection<Post> GetPosts()
    {
        //  do stuff to get posts, return new ObservableCollection<Post>
    }
}

快速修复

更简单的方法是从集合实例开始，这样 UI 在第一次（在这种情况下，仅）获取 Posts 的值时获得一些东西，保持相同的集合。但是，如果没有INotifyPropertyChanged，您将遇到无数这样的问题。尝试在没有它的情况下使用 WPF 确实是一种自我惩罚的练习。

ObservableCollection<Post> _posts = new ObservableCollection<Post>();

public ObservableCollection<Post> Posts { get { return _posts; } }
public NewsPageViewModel()
{
    GetPosts(_posts);
}
public static void GetPosts(ObservableCollection<Post> posts)
{
    posts.Clear();
    //  ...get posts
}

ObservableCollection 的特别之处在于它实现了INotifyCollectionChanged，当您从集合中添加或删除项目时，它将通知 UI ——但 UI 必须知道要侦听哪个集合以进行更改。

Answer 2

我还不能添加 cmets...所以：

在 NewsPageViewModel 的构造函数中，您初始化一个新的（但为空的）ObservableCollection。

如果您没有在 GetPost 方法中添加元素，_posts 将保持为空。