c# - 无法让 ListView 显示初始选择的项目

Question

似乎没有办法让初始选择显示在 Xamarin Forms ListView 中。我看到这个问题已被问过几次，但在每种情况下都没有相关答案，或者未回答的问题太复杂而无法正确分析，或者问题已回答但答案不起作用。所以我想我会提出这个问题的简化版本，希望有人能告诉我一个解决方法。解决方法是所有可以做的事情，因为很明显 SelectedItem 无法绑定(bind)并且无法从代码中设置。这是我检查过的 SO 问题......

:Why can't I set the SelectedItem property of Xamarin.Forms.ListView?

Xamarin Forms ListView SelectedItem Binding Issue

Xamarin.Forms ListView set SelectedItem by Code

Xamarin XAML ListView - How to select programatically

这是 XAML...

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:local="clr-namespace:XamarinFormsBench"
         x:Class="XamarinFormsBench.ListViewPage1">
    <ContentPage.BindingContext>
        <local:ViewModel1/>
    </ContentPage.BindingContext>
    <ListView x:Name="MyListView" Margin="0,20,0,0"
        ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem, Mode=TwoWay}" 
          BindingContextChanged="MyListView_BindingContextChanged">
    </ListView>
</ContentPage>

这是代码...

public class Item
{
    public Item(string name, string description)
    {
        Name = name;
        Description = description;
    }
    string Name { get; set; }
    string Description { get; set; }
    public override string ToString()
    {
        return Name + " = " + Description;
    }
}

public class ViewModel1 : INotifyPropertyChanged
{
    public ObservableCollection<Item> Items { get; set; }
    public ViewModel1()
    {

        Items = new ObservableCollection<Item>
        {
            new Item("Item 1","First item"),
            new Item("Item 2","Second item"),
            new Item("Item 3","Third item"),
            new Item("Item 4","Fourth item"),
            new Item("Item 5","Fifth item")
        };
        _selectedItem = Items[2];
    }
    Item _selectedItem;
    public event PropertyChangedEventHandler PropertyChanged;
    public Item SelectedItem
    {
        get
        {
            return _selectedItem;
        }
        set
        {
            _selectedItem = value;
            NotifyPropertyChanged(nameof(SelectedItem));
        }
    }
    public void NotifyPropertyChanged(string name)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }
}
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class ListViewPage1 : ContentPage
{
    public ListViewPage1()
    {
        InitializeComponent();
    }
    public ViewModel1 ViewModel
    {
        get
        {
            return BindingContext as ViewModel1;
        }
    }

    private void MyListView_BindingContextChanged(object sender, EventArgs e)
    {
    //         MyListView.SelectedItem = ViewModel.SelectedItem;
    }
}

我的期望是出现一个包含 5 个项目的列表，并且第 3 个项目将被突出显示。发生的情况是出现了 5 个项目，但没有突出显示任何项目。绑定(bind)显然有效，因为每次单击项目时所选项目都会更新。

现在，如果我删除绑定(bind)并尝试通过取消注释接近末尾的行来设置代码中的选定项，那也不起作用。

所以我没有想法。我一直在考虑模拟鼠标点击，但这真的超出了我愿意去的地方。

我已经在 UWP 和 Android 上运行了这段代码。还没有尝试过 iOS。

Answer 1

我刚刚创建了一个快速测试解决方案，这对我来说在 iOS 和 Android 上都适用。

区别在于我使用 FreshMvvm 绑定(bind) View 和 ViewModel 以及 PropertyChanged.Fody 来自动通知。

这表明是您的绑定(bind)或属性更改导致了问题。

这是 View 模型

[AddINotifyPropertyChangedInterface]
public class MainPageModel : FreshBasePageModel
{
    public MainPageModel()
    {
        Items = new ObservableCollection<Item>
        {
            new Item("Item 1","First item"),
            new Item("Item 2","Second item"),
            new Item("Item 3","Third item"),
            new Item("Item 4","Fourth item"),
            new Item("Item 5","Fifth item")
        };

        SelectedItem = Items[2];
    }

    public ObservableCollection<Item> Items { get; set; }

    public Item SelectedItem { get; set; }
}

和 View

<?xml version="1.0" encoding="utf-8"?>

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ScratchPad.Pages.MainPage"
             Title="List Test">

    <ListView ItemsSource="{Binding Items}"
              SelectedItem="{Binding SelectedItem, Mode=TwoWay}">
    </ListView>

</ContentPage>

App.xaml.cs 只是为了很好的衡量标准

public partial class App : Application
{
    public App()
    {
        InitializeComponent();

        var page = FreshPageModelResolver.ResolvePageModel<MainPageModel>();
        var nav = new FreshNavigationContainer(page);
        MainPage = nav;

    }

    ...
}

编辑“纯”Xamarin Forms 代码

这是 View

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:selectedItemTest="clr-namespace:SelectedItemTest;assembly=SelectedItemTest"
             x:Class="SelectedItemTest.MainPage"
             Title="List Test">

    <ContentPage.BindingContext>
        <selectedItemTest:MainPageModel/>
    </ContentPage.BindingContext>

    <ListView ItemsSource="{Binding Items}"
              SelectedItem="{Binding SelectedItem, Mode=TwoWay}">
    </ListView>

</ContentPage>

和 View 模型

public class MainPageModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public ObservableCollection<Item> Items { get; set; }

    public MainPageModel()
    {
        Items = new ObservableCollection<Item>
        {
            new Item("Item 1","First item"),
            new Item("Item 2","Second item"),
            new Item("Item 3","Third item"),
            new Item("Item 4","Fourth item"),
            new Item("Item 5","Fifth item")
        };

        SelectedItem = Items[2];
    }

    private Item _selectedItem;
    public Item SelectedItem
    {
        get
        {
            return _selectedItem;
        }
        set
        {
            _selectedItem = value;
            OnPropertyChanged(nameof(SelectedItem));
        }
    }
}