Xamarin.Forms ListView项目删除包含旧值

Question

我有一个ListView并通过DataBinding将其填充到我的ViewModel中的Lists属性。 另外，我有一个ListView菜单，带有一个删除命令，也绑定到我的ViewModel。 我的问题是，如果我已经初始化ListView，我可以删除其中的列表。 如果我添加新列表，我可以删除所有列表。 但是，如果我添加新项目，我无法删除它们，因为我从DeleteCommand获取的列表是旧的，已经删除的列表。

因此，删除列表后，它们似乎在某种程度上，某处仍然存在，我只能删除新列表，如果当前列表的总量高于以前任何已删除列表的数量。

我希望这是对我的问题的某种可理解的解释。

Binding工作正常，我的ViewModel中的Lists Property保存正确的值，但DeleteListCommand中的“sender”ItemList是旧的ItemList。

这是我的ListView的XAML：

<ListView x:Name="listView" ItemsSource="{Binding Lists}" >
    <ListView.ItemTemplate>
        <DataTemplate>
            <ViewCell x:Name="viewCell">
                <ViewCell.ContextActions>
                    <MenuItem Command="{Binding BindingContext.RenameListCommand, Source={x:Reference listView}}" CommandParameter="{Binding .}" Text="Rename" />
                    <MenuItem Command="{Binding BindingContext.DeleteListCommand, Source={x:Reference listView}}" CommandParameter="{Binding .}" IsDestructive="True" Text="Delete" />
                </ViewCell.ContextActions>
                <ContentView Margin="0,2,0,2"
                                    HeightRequest="50"
                                    BackgroundColor="{Binding Color}">

                    <ContentView.GestureRecognizers>
                        <TapGestureRecognizer BindingContext="{Binding Source={x:Reference listView}, Path=BindingContext}"
                                                        Command="{Binding ListTappedCommand}" 
                                                        CommandParameter="{Binding Source={x:Reference viewCell}, Path=BindingContext}" />
                    </ContentView.GestureRecognizers>
                    <ContentView.Content>

                        <Label Text="{Binding Name}"
                                    HorizontalTextAlignment="Center"
                                    VerticalTextAlignment="Center"
                                    TextColor="White" 
                                    IsEnabled="True"/>
                    </ContentView.Content>

                </ContentView>
            </ViewCell>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

这是我的ViewModel：

...
public ObservableCollection<ItemList> lists = new ObservableCollection<ItemList>();
        public ObservableCollection<ItemList> Lists
        {
            get { return lists; }
            set
            {
                lists = value;
                OnPropertyChanged("Lists");
            }
        }
public event PropertyChangedEventHandler PropertyChanged;

...

this.DeleteListCommand = new Command<ItemList>((sender) =>
            {
                    OnDeleteList(sender);
            });

...

public ICommand DeleteListCommand { get; set; }
private void OnDeleteList(ItemList itemList)
        {
            Lists.Remove(itemList);
        }

...

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

Answer 1

根据示例项目，您需要执行以下操作：将单元命名为“viewCell”，因为您需要将单元格传递给模型才能重置ContextAction。 更改菜单项上的绑定以传递单元格而不是ItemList。 然后在模型重置上下文操作中，从单元格的绑定上下文中获取项目

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:TobyList_XamarinForms"
                 xmlns:localVM="clr-namespace:TobyList_XamarinForms.ViewModels"
                 Title="Toby" 
             x:Class="TobyList_XamarinForms.Views.MasterPage">

    <StackLayout Padding="5" VerticalOptions="FillAndExpand" BackgroundColor="#F9F9F9">

        <StackLayout.BindingContext>
            <localVM:MasterPageViewModel />
        </StackLayout.BindingContext>


        <ListView x:Name="listView" ItemsSource="{Binding Lists}" CachingStrategy="RecycleElement">
        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell x:Name="viewCell">
                    <ViewCell.ContextActions>
                            <MenuItem Command="{Binding Path=BindingContext.DeleteListCommand, Source={x:Reference Name=listView}}" CommandParameter="{Binding Source={x:Reference viewCell}}" Text="Delete" />
                        </ViewCell.ContextActions>
                    <ContentView Margin="0,2,0,2"
                                        HeightRequest="50"
                                        BackgroundColor="{Binding Color}">

                        <ContentView.GestureRecognizers>
                            <TapGestureRecognizer BindingContext="{Binding Source={x:Reference listView}, Path=BindingContext}"
                                                            Command="{Binding ListTappedCommand}" 
                                                            CommandParameter="{Binding Source={x:Reference viewCell}, Path=BindingContext}" />
                        </ContentView.GestureRecognizers>
                        <ContentView.Content>

                            <Label Text="{Binding Name}"
                                        HorizontalTextAlignment="Center"
                                        VerticalTextAlignment="Center"
                                        TextColor="White"/>
                        </ContentView.Content>

                    </ContentView>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>

        <StackLayout Orientation="Horizontal" HeightRequest="30" Margin="7">
            <Label Text="Add">
                <Label.GestureRecognizers>
                    <TapGestureRecognizer Command="{Binding AddListCommand}" />
                </Label.GestureRecognizers>
            </Label>

        </StackLayout>

    </StackLayout>

</ContentPage>

模型：

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Windows.Input;
using TobyList_XamarinForms.Models;
using Xamarin.Forms;
using System.Linq;

namespace TobyList_XamarinForms.ViewModels
{
    public class MasterPageViewModel : INotifyPropertyChanged
    {
        public ObservableCollection<ItemList> lists = new ObservableCollection<ItemList>();
        public ObservableCollection<ItemList> Lists
        {
            get { return lists; }
            set
            {
                lists = value;
                OnPropertyChanged("Lists");
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public MasterPageViewModel()
        {
            this.AddListCommand = new Command(() =>
            {
                OnAddList();
            });
            //this.DeleteListCommand = new Command<ItemList>((sender) =>
            //{
            //  OnDeleteList(sender);
            //});

            this.DeleteListCommand = new Command<ViewCell>((sender) =>
            {
                OnDeleteList(sender);
            });

        }

        public ICommand AddListCommand { get; protected set; }
        private void OnAddList()
        {
            ItemList itemList = new ItemList() { Id = Guid.NewGuid().ToString().ToUpper(), Name = "Lorem Ipsum", Color = "#000000" };
            Lists.Add(itemList);
        }

        public ICommand DeleteListCommand { get; set; }
        //public void OnDeleteList(ItemList itemList)
        //      {
        //          Lists.Remove(itemList);
        //      }
        public void OnDeleteList(ViewCell viewCell)
        {
            viewCell.ContextActions.Clear();
            Lists.Remove((ItemList)viewCell.BindingContext);
        }

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