Xamarin Forms Collection 视图 ItemsSource Binding 未更新 UI

Question

Xamarin.Forms 如果我在后面的代码中设置绑定，4.0 集合视图 ItemsSource 绑定无法按预期工作。这些项目基于源集合的初始值显示，但是 UI 在更新源集合时不会更新。如果我在 xaml.

中设置绑定，同样有效

后面的代码：

    public MainPage()
    {
        this.InitializeComponent();
        this.BindingContext = this.mainViewModel = new MainViewModel();


        CollectionView courseCollectionView = new CollectionView
        {
            ItemSizingStrategy = ItemSizingStrategy.MeasureFirstItem,
            ItemTemplate = new DataTemplate(typeof(ItemView))
        };


     courseCollectionView.SetBinding(CollectionView.ItemsSourceProperty, 
     new Binding() { Source = this.mainViewModel.CountryNames, Mode = 
     BindingMode.TwoWay });

     courseCollectionView.ItemsLayout = new GridItemsLayout(4, 
      ItemsLayoutOrientation.Vertical);

        this.CoursesStack.Children.Clear();
        this.CoursesStack.Children.Add(courseCollectionView);

    }

用于 ItemsSource 绑定的视图模型属性：

   ObservableCollection<Country> countryNames;

    public ObservableCollection<Country> CountryNames
    {
        get => this.countryNames;
        set
        {
            this.countryNames = value;
            RaisePropertyChanged("CountryNames");
        }
    }

预期：应根据对绑定到 ItemsSource 属性.

的 ObsevableCollection（add/delete 个集合中的项目）所做的更改更新视图

实际：视图未随 ObservableCollection 的更改而更新。

Answer 1

在这种情况下，尝试从 INotifyPropertyChanged 扩展 MainPage 而不是 RaisePropertyChanged("CountryNames") in set 属性，使用 OnPropertyChanged()

Answer 2

我认为您的绑定有误。尝试： courseCollectionView.SetBinding(CollectionView.ItemsSourceProperty, nameof(mainViewModel.CountryNames));

您需要指定 Path (mainViewModel.CountryNames) 而不是 Source

Answer 3

关于在使用 CollectionView 时更新 UI，我做了一个示例，您可以看一下：

public partial class MainPage : ContentPage
{
    public mainvidemodel viewmodel { get; set; }
    public MainPage()
    {
        InitializeComponent();
        viewmodel = new mainvidemodel();
        this.BindingContext = viewmodel;

        CollectionView collectionView = new CollectionView();

        collectionView.SetBinding(ItemsView.ItemsSourceProperty, "countries");

        collectionView.ItemTemplate = new DataTemplate(() =>
        {
            StackLayout stacklayout = new StackLayout();

            Label label1 = new Label();
            label1.SetBinding(Label.TextProperty,"Id");

            Label label2 = new Label();
            label2.SetBinding(Label.TextProperty, "Name");

            Label label3 = new Label();
            label3.SetBinding(Label.TextProperty, "caption");


            stacklayout.Children.Add(label1);
            stacklayout.Children.Add(label2);

            stacklayout.Children.Add(label3);
            return stacklayout;

        });
        Button btn = new Button() { Text = "btn", WidthRequest = 200, HeightRequest = 50 };
        btn.Clicked += Btn_Clicked;
        stacklayout1.Children.Add(collectionView);
        stacklayout1.Children.Add(btn);

    }

    private void Btn_Clicked(object sender, EventArgs e)
    {
        viewmodel.countries.Add(new Country()  { Id = 8, Name = "country8", caption = "caption 8" });
    }
}
public class mainvidemodel
{
    public ObservableCollection<Country> countries { get; set; }
    public mainvidemodel()
    {
        countries = new ObservableCollection<Country>()
        {
            new Country(){Id=1,Name="country1",caption="caption 1"},
            new Country(){Id=2,Name="country2",caption="caption 2"},
            new Country(){Id=3,Name="country3",caption="caption 3"},
            new Country(){Id=4,Name="country4",caption="caption 4"},
            new Country(){Id=5,Name="country5",caption="caption 5"},
            new Country(){Id=6,Name="country6",caption="caption 6"},
            new Country(){Id=7,Name="country7",caption="caption 7"},

        };

    }
}
public class Country
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string caption { get; set; }
}

Xamarin Forms Collection 视图 ItemsSource Binding 未更新 UI

Xamarin Forms Collection view ItemsSource Binding not updating the UI

itemssource

collectionview

xamarin

xamarin.forms

后面的代码：