Xamarin.Forms - 在OnAppearing方法中没有更新ListView

Question

我正在使用Xamarin.Forms开发一个应用程序,我有一个用户可以发布Feed更新的屏幕,此Feed更新包含文本和一些附加文件.我的XAML视图基本上,一个StackLayout包含ListView,并Editor和一对夫妇的Buttons.问题是我正试图在方法中设置ItemSource我ListView的onAppearing.该方法被称为正常.我正在设置的集合按预期更改.但由于某种原因,ListView没有更新.ListView在设置Source调用Editor焦点方法后,使工作正常的解决方法.我之所以这样做,是因为我发现当我在设置集合后点击编辑器时,ListView正确渲染了.但这是一个非常讨厌的解决方法.

这是我的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"
             x:Class="HalliganTL.View.UpdateFeedPage">
  <StackLayout Orientation="Vertical" >
    <ListView x:Name="AttachedFilesListView" MinimumHeightRequest="50" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" >
      <ListView.ItemTemplate>
        <DataTemplate>
          <ViewCell>
            <StackLayout HorizontalOptions="StartAndExpand" Orientation="Horizontal">
              <Button Text="[X]" />
              <StackLayout Padding="5,0,0,0" VerticalOptions="StartAndExpand" Orientation="Vertical">
                <Label Text="{Binding Name}" VerticalTextAlignment="Center" FontSize="Medium" />
              </StackLayout>
            </StackLayout>
          </ViewCell>
        </DataTemplate>
      </ListView.ItemTemplate>
    </ListView>
    <Editor x:Name="UpdateFeedEditor" FontSize="15" Text="{Binding PostText}"  VerticalOptions="FillAndExpand" />
    <Button Text="Attach file" TextColor="White" BackgroundColor="#77D065" Clicked="onAttachFileClicked"/>
    <Button Text="Post" TextColor="White" BackgroundColor="#77D065" Clicked="onPostClicked"/>
  </StackLayout>
</ContentPage>

这是我的页面代码:

public partial class UpdateFeedPage : ContentPage
    {
        public static List<FileObject> CurrentSelectedFile = new List<FileObject>();
        public static string PostText = "";

        public UpdateFeedPage()
        {
            InitializeComponent();
            Title = "New Update";
            UpdateFeedEditor.Text = PostText;
        }

        protected override void OnAppearing()
        {
            AttachedFilesListView.ItemsSource = CurrentSelectedFile;
            UpdateFeedEditor.Focus();
            base.OnAppearing();
        }

        async void onPostClicked(object sender, EventArgs e)
        {
            await Navigation.PopAsync();
        }

        async void onAttachFileClicked(object sender, EventArgs e)
        {
            await Navigation.PushAsync(new FilePickerPage());
        }
    }

我玩了很多东西,比如:

• 从ListView和包含ListView的StackLayout更改Horizo​​ntalOptions和VerticalOptions.

• 删除除StackLayout和子ListView之外的所有其他控件.

但到目前为止没有任何工作,除了以编程方式调用编辑器,Focus方法

注意:

正如您所看到的,我设置的ListView集合是静态的,我直接从另一个页面修改该集合,但我认为这没关系,因为当我在OnAppearing中设置断点时,正在正确地修改集合.

Answer 1

解决该刷新问题的两种方法.

第一路)

List<FileObject>通过重置以下命令强制ListView从您的集合中刷新ItemsSource:

当前:

protected override void OnAppearing()
{
    AttachedFilesListView.ItemsSource = CurrentSelectedFile;
    UpdateFeedEditor.Focus();
    base.OnAppearing();
}

改成:

protected override void OnAppearing()
{
    base.OnAppearing();
    AttachedFilesListView.ItemsSource = null;
    AttachedFilesListView.ItemsSource = CurrentSelectedFile;
}

第二路)

将您的List<FileObject>收藏替换为,ObservableCollection<FileObject>以便通过将要收听的NotifyCollectionChangedAction.Add|Remove|...基于事件发布更改ListView.

当前:

public static List<FileObject> CurrentSelectedFile = new List<FileObject>();

改成:

public static readonly ObservableCollection<FileObject> CurrentSelectedFile = new ObservableCollection<FileObject>();

注意:还要添加一个 using System.Collections.ObjectModel;

你OnAppearing成了:

protected override void OnAppearing()
{
    base.OnAppearing();
    AttachedFilesListView.ItemsSource = CurrentSelectedFile;
}