使用MVVM Light在ObservableCollection的UI上更新项

本文关键字：UI 更新 ObservableCollection MVVM Light 使用 | 更新日期: 2023-09-27 18:02:38

所以我有一个ObservableCollection<Term>上的视图模型，与Term项目有一些数据属性。

Term class:

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()
        {
        }
    }
}

这是有问题的ViewModel有ObservableCollection (ProjectBaseViewModel实现MVVM光的ViewModelBase，并有几个属性，我不认为是必要的在这里):

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，从另一个页面添加和删除术语很好。然而，当Term项被更新时，只有标题被更新。

下面是视图中的内容:

            <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>
当两个日期值发生变化时，

Term.DurationText不更新。但是，如果要看到编辑页面，则更改将反映在日期pickers中。

是否有任何方法可以使DurationText通过绑定相应地更新，使与使用MVVM Light的实现一起工作?

我已经试过了，但是错误提示:

类型"Neomilano.Model."术语'不能用作类型参数'T'在泛型类型或方法中"Neomilano.Common.ItemsChangeObservableCollection"。没有从Neomilano.Model的隐式引用转换。术语"System.ComponentModel.INotifyPropertyChanged"。

项目规格:*这是一个WinRT应用程序*这是一个通用的Windows应用程序，但我现在首先在Windows Phone上工作。我认为这无关紧要，因为我赌的是收敛性。WinRT和WinPRT，但谁知道这可能是问题所在*正如我所提到的，我正在使用MVVM Light(仅限库)。

所以，在一般情况下，我怎么能更新一个项目的ObservableCollection在WinRT使用MVVM光的内容?

谢谢你的帮助!

使用MVVM Light在ObservableCollection的UI上更新项

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

在这种情况下，您可以使Term派生自ViewModelBase，因为它包含INotifyPropertyChanged的实现。

下面是一个例子。请注意，TermStart的setter也会为DurationText引发一个更改事件，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"); }
    }
}
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