ObservableCollection 和 Item PropertyChanged

Question

我已经看到很多关于这个问题的讨论，但也许我只是一个新手，无法理解。如果我有一个可观察的集合，它是 msdn 示例 (http: //msdn.microsoft.com/en-us/library/ms748365.aspx) 中的“PersonNames”集合，如果添加或删除了 PersonName 等，我会更新我的视图。我想获得更新当我更改PersonName 中的属性时，我的视图也是如此。就像我改变名字一样。我可以为每个属性实现OnPropertyChanged，并让这个类派生自INotifyPropertyChanged，这似乎按预期调用。

我的问题是，视图如何从ObservableCollection 获取更新的数据，因为属性更改不会导致ObservableCollection 发生任何事件？

这可能是一件非常简单的事情，但为什么我似乎找不到一个例子让我感到惊讶。任何人都可以为我阐明这一点或有任何指向示例的指针，我将不胜感激。我们当前的 WPF 应用程序中的多个地方都有这种情况，并且正在努力解决这个问题。

---

“通常，负责显示数据的代码会为当前显示在屏幕上的每个对象添加一个PropertyChanged 事件处理程序。”

谁能给我一个例子来说明这意味着什么？我的视图绑定到我的ViewModel，它有一个ObservableCollection。此集合由 RowViewModel 组成，它具有支持 PropertiesChanged 事件的属性。但我不知道如何让集合自行更新，以便更新我的视图。

Answer 1

正如您所发现的，没有集合级别的事件表明集合中某个项目的属性已更改。通常，负责显示数据的代码会为当前显示在屏幕上的每个对象添加一个 PropertyChanged 事件处理程序。

Answer 2

这是您将如何附加/分离到每个项目的 PropertyChanged 事件。

ObservableCollection<INotifyPropertyChanged> items = new ObservableCollection<INotifyPropertyChanged>();
items.CollectionChanged += items_CollectionChanged;

static void items_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    if (e.OldItems != null)
    {
        foreach (INotifyPropertyChanged item in e.OldItems)
            item.PropertyChanged -= item_PropertyChanged;
    }
    if (e.NewItems != null)
    {
        foreach (INotifyPropertyChanged item in e.NewItems)
            item.PropertyChanged += item_PropertyChanged;
    }
}

static void item_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    throw new NotImplementedException();
}

Answer 3

比尔，

我确信您现在已经找到了解决您的问题的变通方法或解决方案，但我为遇到此常见问题的任何人发布了此信息。您可以用此类替换 ObservableCollections，这些 ObservableCollections 是实现 INotifyPropertyChanged 的​​对象的集合。这有点苛刻，因为它表示列表需要重置而不是找到已更改的一个属性/项目，但对于小型列表，性能影响应该是不明显的。

马克

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.ComponentModel;

namespace WCIOPublishing.Helpers
{
    public class ObservableCollectionWithItemNotify<T> : ObservableCollection<T> where T: INotifyPropertyChanged 
    {

        public ObservableCollectionWithItemNotify()
        {
            this.CollectionChanged += items_CollectionChanged;
        }


        public ObservableCollectionWithItemNotify(IEnumerable<T> collection) :base( collection)
        {
            this.CollectionChanged += items_CollectionChanged;
            foreach (INotifyPropertyChanged item in collection)
                item.PropertyChanged += item_PropertyChanged;

        }

        private void items_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if(e != null)
            {
                if(e.OldItems!=null)
                    foreach (INotifyPropertyChanged item in e.OldItems)
                        item.PropertyChanged -= item_PropertyChanged;

                if(e.NewItems!=null)
                    foreach (INotifyPropertyChanged item in e.NewItems)
                        item.PropertyChanged += item_PropertyChanged;
            }
        }

        private void item_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            var reset = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset);
            this.OnCollectionChanged(reset);

        }

    }
}

Answer 4

我们在 WPF 聊天中写道：

public class OcPropertyChangedListener<T> : INotifyPropertyChanged where T : INotifyPropertyChanged
{
    private readonly ObservableCollection<T> _collection;
    private readonly string _propertyName;
    private readonly Dictionary<T, int> _items = new Dictionary<T, int>(new ObjectIdentityComparer());
    public OcPropertyChangedListener(ObservableCollection<T> collection, string propertyName = "")
    {
        _collection = collection;
        _propertyName = propertyName ?? "";
        AddRange(collection);
        CollectionChangedEventManager.AddHandler(collection, CollectionChanged);
    }

    private void CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Add:
                AddRange(e.NewItems.Cast<T>());
                break;
            case NotifyCollectionChangedAction.Remove:
                RemoveRange(e.OldItems.Cast<T>());
                break;
            case NotifyCollectionChangedAction.Replace:
                AddRange(e.NewItems.Cast<T>());
                RemoveRange(e.OldItems.Cast<T>());
                break;
            case NotifyCollectionChangedAction.Move:
                break;
            case NotifyCollectionChangedAction.Reset:
                Reset();
                break;
            default:
                throw new ArgumentOutOfRangeException();
        }

    }

    private void AddRange(IEnumerable<T> newItems)
    {
        foreach (T item in newItems)
        {
            if (_items.ContainsKey(item))
            {
                _items[item]++;
            }
            else
            {
                _items.Add(item, 1);
                PropertyChangedEventManager.AddHandler(item, ChildPropertyChanged, _propertyName);
            }
        }
    }

    private void RemoveRange(IEnumerable<T> oldItems)
    {
        foreach (T item in oldItems)
        {
            _items[item]--;
            if (_items[item] == 0)
            {
                _items.Remove(item);
                PropertyChangedEventManager.RemoveHandler(item, ChildPropertyChanged, _propertyName);
            }
        }
    }

    private void Reset()
    {
        foreach (T item in _items.Keys.ToList())
        {
            PropertyChangedEventManager.RemoveHandler(item, ChildPropertyChanged, _propertyName);
            _items.Remove(item);
        }
        AddRange(_collection);
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void ChildPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
            handler(sender, e);
    }

    private class ObjectIdentityComparer : IEqualityComparer<T>
    {
        public bool Equals(T x, T y)
        {
            return object.ReferenceEquals(x, y);
        }
        public int GetHashCode(T obj)
        {
            return System.Runtime.CompilerServices.RuntimeHelpers.GetHashCode(obj);
        }
    }
}

public static class OcPropertyChangedListener
{
    public static OcPropertyChangedListener<T> Create<T>(ObservableCollection<T> collection, string propertyName = "") where T : INotifyPropertyChanged
    {
        return new OcPropertyChangedListener<T>(collection, propertyName);
    }
}

• 弱事件
• 跟踪多次添加到集合中的同一项目
• 它~冒泡~了孩子们的财产改变事件。
• 静态类只是为了方便。

像这样使用它：

var listener = OcPropertyChangedListener.Create(yourCollection);
listener.PropertyChanged += (sender, args) => { //do you stuff}

Answer 5

只需使用 BindingList<T> 代替 ObservableCollection。
以下代码显示了与 List 和项目属性的 DataGrid 绑定。

<Window x:Class="WpfApplication1.MainWindow"
                    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    Title="MainWindow" Height="350" Width="525">
    <DataGrid ItemsSource="{Binding}" AutoGenerateColumns="False" >
        <DataGrid.Columns>
            <DataGridTextColumn Header="Values" Binding="{Binding Value}" />
        </DataGrid.Columns>
    </DataGrid>
</Window>

---

using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Threading;

namespace WpfApplication1 {
    public partial class MainWindow : Window {
        public MainWindow() {
            var c = new BindingList<Data>();
            this.DataContext = c;
            // add new item to list on each timer tick
            var t = new DispatcherTimer() { Interval = TimeSpan.FromSeconds(1) };
            t.Tick += (s, e) => {
                if (c.Count >= 10) t.Stop();
                c.Add(new Data());
            };
            t.Start();
        }
    }

    public class Data : INotifyPropertyChanged {
        public event PropertyChangedEventHandler PropertyChanged = delegate { };
        System.Timers.Timer t;
        static Random r = new Random();
        public Data() {
            // update value on each timer tick
            t = new System.Timers.Timer() { Interval = r.Next(500, 1000) };
            t.Elapsed += (s, e) => {
                Value = DateTime.Now.Ticks;
                this.PropertyChanged(this, new PropertyChangedEventArgs("Value"));
            };
            t.Start();
        }
        public long Value { get; private set; }
    }
}

Answer 6

下面的代码给出了@Stack 的答案的简单解释，并显示了BindingList 如何观察它是否有项目更改，并显示ObservableCollection 不会观察项目内部的变化。

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;

namespace BindingListExample
{
    class Program
    {
        public ObservableCollection<MyStruct> oc = new ObservableCollection<MyStruct>();
        public System.ComponentModel.BindingList<MyStruct> bl = new BindingList<MyStruct>();

        public Program()
        {
            oc.Add(new MyStruct());
            oc.CollectionChanged += CollectionChanged;

            bl.Add(new MyStruct());
            bl.ListChanged += ListChanged;
        }

        void ListChanged(object sender, ListChangedEventArgs e)
        {
            //Observe when the IsActive value is changed this event is triggered.
            Console.WriteLine(e.ListChangedType.ToString());
        }

        void CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            //Observe when the IsActive value is changed this event is not triggered.
            Console.WriteLine(e.Action.ToString());
        }

        static void Main(string[] args)
        {
            Program pm = new Program();
            pm.bl[0].IsActive = false;
        }
    }

    public class MyStruct : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        private bool isactive;
        public bool IsActive
        {
            get { return isactive; }
            set
            {
                isactive = value;
                NotifyPropertyChanged("IsActive");
            }
        }

        private void NotifyPropertyChanged(String PropertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(PropertyName));
            }
        }
    }
}