使用MVVM在ObservableCollection上更新UI中的项

Question

因此，我在视图模型上有一个ObservableCollection<Term>，Term项具有一些数据属性。

Term类：

using SQLite;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Neomilano.Model
{
    public class Term
    {
        public int ID { get; set; }

        public string Name { get; set; }

        public DateTimeOffset TermStart { get; set; }

        public DateTimeOffset TermEnd { get; set; }

        public string DurationText 
        {
            get 
            {
                return "from " + TermStart.Date.ToString("MMMM d, yyyy") + " to " + TermEnd.Date.ToString("MMMM d, yyyy");
            }        
        }

        public Term()
        {
        }
    }
}

这是一个有问题的ViewModel，它有ObservableCollection (ProjectBaseViewModel实现了MVVM的ViewModelBase，并且有一些我认为这里不需要的属性)：

using GalaSoft.MvvmLight.Command;
using GalaSoft.MvvmLight.Threading;
using Microsoft.Practices.ServiceLocation;
using Neomilano.Model;
using Neomilano.Services;
using SQLite;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;
using Windows.Foundation;


namespace Neomilano.ViewModel
{
    public partial class TermsViewModel : ProjectBaseViewModel
    {

        public IDialogService DialogService
        {
            get { return ServiceLocator.Current.GetInstance<IDialogService>(); }
        }

#region properties
        private ObservableCollection<Term> _terms;
        public ObservableCollection<Term> Terms {
            get { return this._terms; }
            private set { Set(() => Terms, ref _terms, value); }
        }
#endregion

#region commands

        private RelayCommand<int> _deleteTermCommand;
        public RelayCommand<int> DeleteTermCommand
        {
            get 
            {
                return _deleteTermCommand ??
                    (_deleteTermCommand = new RelayCommand<int>(async id => {
                        var t = "Are you sure you want to delete this term?";
                        var c = "This cannot be undone.";
                        var ct = "delete";
                        bool deleteConfirmed = await DialogService.ShowConfirmationDialogAsync(t, c, ct);

                        if (deleteConfirmed == true)
                            await ExecuteDeleteTermAsync(id);
                    }));
            }
        }

#endregion

        public TermsViewModel()
        {
            if (IsInDesignMode)
            {
                List<Term> t = new List<Term>();

                t.Add(new Term() { ID=1, Name = "Sample Term 1", TermStart = DateTimeOffset.Parse("October 1, 2013"), TermEnd = DateTimeOffset.Parse("December 17, 2013") });
                t.Add(new Term() { ID=2, Name="Sample Term 2", TermStart=DateTimeOffset.Parse("January 1, 2014"), TermEnd=DateTimeOffset.Parse("April 30, 2014") });

                Terms = new ObservableCollection<Term>(t);
            }

        }

        /// <summary>
        /// Gets the list of Terms from the database and adds it to the Terms property of the ViewModel.
        /// </summary>
        /// <returns></returns>
        public async Task GetTermsListAsync()
        {
            IsProgressEnabled = true;

            List<Term> list = new List<Term>();
            await Task.Run(() =>
            {
                using (var db = new SQLiteConnection(app.DBFileName))
                {
                    list = db.Table<Term>().ToList<Term>();
                    db.Close();
                }
            });
            Terms = new ObservableCollection<Term>(list);

            IsProgressEnabled = false;
        }

        /// <summary>
        /// Returns the term ID of the selected term from the view.
        /// </summary>
        /// <param name="clickedItem"></param>
        /// <returns></returns>
        public Term ReturnSelectedTerm(object clickedItem)
        {
            return (Term)clickedItem;
        }

        public async Task ExecuteDeleteTermAsync(int termId)
        {
            IsProgressEnabled = true;

            Term t = new Term();
            await Task.Run(() =>
            {
                using (var db = new SQLiteConnection(app.DBFileName))
                {
                    var q = db.Table<Term>().Where(tr => tr.ID.Equals(termId));
                    t = q.First<Term>();

                    db.Delete<Term>(termId);
                    db.Close();
                }
            });

            var target = Terms.Single<Term>(tr => tr.ID.Equals(termId));
            Terms.Remove(target);

            IsProgressEnabled = false;
        }
    }
}

问题:当然，从另一个页面中添加和删除术语效果很好。但是，当Term项被更新时，只有标题才会被更新。

这里的风景是：

            <ListView 
                x:Name="TermsListView"
                ItemsSource="{Binding Terms}" 
                IsItemClickEnabled="True"
                ItemClick="OnTermItemClick">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Holding="OnTermItemHold">
                            <FlyoutBase.AttachedFlyout>
                                <MenuFlyout x:Name="TermItemContextMenu">
                                    <MenuFlyoutItem Text="edit" Command="{Binding TermsViewModel.NavigateToFormCommand, Mode=OneWay, Source={StaticResource Locator}}" CommandParameter="{Binding}" />
                                    <MenuFlyoutItem Text="delete" Command="{Binding TermsViewModel.DeleteTermCommand, Mode=OneWay, Source={StaticResource Locator}}" CommandParameter="{Binding ID}" />
                                </MenuFlyout>
                            </FlyoutBase.AttachedFlyout>
                            <TextBlock Text="{Binding Name}"  Style="{ThemeResource ListViewItemTextBlockStyle}"/>
                            <TextBlock Text="{Binding DurationText}" Style="{ThemeResource ListViewItemSubheaderTextBlockStyle}"/>
                        </StackPanel>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>

当两个日期值发生更改时，Term.DurationText不会更新。但是，如果要查看编辑页面，则更改将反映在DatePickers中。

是否有任何方法可以通过绑定使DurationText相应更新，使其与使用MVVM 的实现一起工作？

我试过这个，但是错误说：

在泛型类型或方法'Neomilano.Common.ItemsChangeObservableCollection'.中，类型'Neomilano.Model.Term‘不能用作类型参数'T’没有从“'System.ComponentModel.INotifyPropertyChanged'.”到“Neomilano.Model.Term”的隐式引用转换

项目说明：*这是一个WinRT应用程序*它是一个通用的Windows，但我现在正在开发Windows。我认为这不重要，因为趋同押注。WinRT和WinPRT，但是谁知道这可能是问题所在，正如我所提到的，我正在使用might (仅使用库)。

因此，一般来说，如何使用MVVM更新ObservableCollection中WinRT中的项的内容？

谢谢你的帮助!

Answer 1

您不需要链接文章中的任何内容，只需使用Term实现INotifyPropertyChanged。ObservableCollection<T>只通知绑定到它的ListView集合的内容已更改，即通知添加、删除、移动、替换、重置等。它不监视集合中的项是否发生更改。

在本例中，您可以使Term从ViewModelBase派生，因为这包含INotifyPropertyChanged的一个实现。

下面是一个例子。请注意，TermStart的setter也是如何为DurationText引发更改事件的，因为该属性是依赖的。

public class Term : ViewModelBase
{
    private int _id;
    private string _name;
    private DateTimeOffset _termStart;
    private DateTimeOffset _termEnd;

    public int Id
    {
        get { return _id; }
        set
        {
            if (value == _id) return;
            _id = value;
            RaisePropertyChanged("Id");
        }
    }

    public string Name
    {
        get { return _name; }
        set
        {
            if (value == _name) return;
            _name = value;
            RaisePropertyChanged("Name");
        }
    }

    public DateTimeOffset TermStart
    {
        get { return _termStart; }
        set
        {
            if (value.Equals(_termStart)) return;
            _termStart = value;
            RaisePropertyChanged("TermStart");
            RaisePropertyChanged("DurationText");
        }
    }

    public DateTimeOffset TermEnd
    {
        get { return _termEnd; }
        set
        {
            if (value.Equals(_termEnd)) return;
            _termEnd = value;
            RaisePropertyChanged("TermEnd");
            RaisePropertyChanged("DurationText");
        }
    }

    public string DurationText
    {
        get { return "from " + TermStart.Date.ToString("MMMM d, yyyy") + " to " + TermEnd.Date.ToString("MMMM d, yyyy"); }
    }
}