Question

这是我的第一个windows8应用程序，它非常基于Visual Studio提供的默认模板。问题是当我尝试通过在LoadData（）中创建Observable集合的新实例来分配Items属性值时数据没有绑定，但是当我使用Items.Add方法将项添加到列表时，我可以在UI中看到数据。我希望有人可以解释我的行为，如果我错过任何非常明显的事情。

namespace Sample.ViewModels
{
    public class MainViewModel : INotifyPropertyChanged
    {
        public MainViewModel()
        {
           this.Items = new ObservableCollection<ItemViewModel>();                    
        }

        /// <summary>
        /// A collection for ItemViewModel objects.
        /// </summary>
        public ObservableCollection<ItemViewModel> Items { get; private set; }

        private string _sampleProperty = "Sample Runtime Property Value";

        /// <summary>
        /// Sample ViewModel property; this property is used in the view 
        /// to display its value using a Binding
        /// </summary>
        /// <returns></returns>
        public string SampleProperty
        {
            get
            {
                return _sampleProperty;
            }
            set
            {
                if (value != _sampleProperty)
                {
                    _sampleProperty = value;
                    NotifyPropertyChanged("SampleProperty");
                }
            }
        }

        /// <summary>
        /// Sample property that returns a localized string
        /// </summary>
        public string LocalizedSampleProperty
        {
            get
            {
                return AppResources.SampleProperty;
            }
        }

        public bool IsDataLoaded
        {
            get;
            private set;
        }

        /// <summary>
        /// Creates and adds a few ItemViewModel objects into the Items collection.
        /// </summary>
        public void LoadData()
        {

            try
            {
                using (IQContext context = new IQContext("isostore:/Test.sdf"))
                {
                    var query = (from c in context.Categories

                                 select new ItemViewModel
                                 {
                                     CategoryId = c.CategoryId,
                                     CategoryName = c.CategoryName

                                 });

                    // Items present in the list.
                    this.Items = 
                        new ObservableCollection<ItemViewModel>(query);

                    // this.Items.Add(new ItemViewModel() 
                    //     { CategoryId = 1, CategoryName = "Rishi"}); // This Works

                    this.IsDataLoaded = true;
                }
            }

            catch (Exception ex)
            {
                throw ex;
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged(String propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (null != handler)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

Answer 1

您的Items属性应该实现INPC，因为您要更改LoadData中的引用，并且需要通知UI：

public ObservableCollection<ItemViewModel> Items { get; private set; }

到

private ObservableCollection<ItemViewModel> items;
public ObservableCollection<ItemViewModel> Items 
{ 
    get
    {
        return this.items;
    }

    private set
    {
        this.items = value;
        NotifyPropertyChanged("Items");
    }
}

基于MVVM的Windows Phone 8应用程序中的绑定问题

1 个答案: