ObservableCollection and Item PropertyChanged

Question

I've seen lots of talk about this question but maybe I'm just too much of a newbie to get it. If I have an observable collection that is a collection of "PersonNames" as in the msdn example (http: //msdn.microsoft.com/en-us/library/ms748365.aspx), I get updates to my View if a PersonName is added or removed, etc. I want to get an update to my View when I change a property in the PersonName as well. Like if I change the first name. I can implement OnPropertyChanged for each property and have this class derive from INotifyPropertyChanged and that seems to get called as expected.

My question is, how does the View get the updated data from the ObservableCollection as the property changed does not cause any event for the ObservableCollection?

This is probably something really simple but why I can't seem to find an example surprises me. Can anyone shed any light on this for me or have any pointers to examples I would greatly appreciate it. We have this scenario in multiple places in our current WPF app and are struggling with figuring it out.

---

"Generally, the code responsible for displaying the data adds a PropertyChanged event handler to each object currently displayed onscreen."

Could someone please give me an example of what this means? My View binds to my ViewModel which has a ObservableCollection. This collection is made up of a RowViewModel which has properties that support the PropertiesChanged event. But I can't figure out how to make the collection update itself so my view will be updated.

Answer 1

Here is how you would attach/detach to each item's PropertyChanged event.

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 2

Following is the code giving a simple explanation of answer by @Stack and showing how BindingList is observing if it has a item changed and shows ObservableCollection will not observe the change inside an item.

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));
            }
        }
    }
}

Answer 3

Instead of ObservableCollection simply use the BindingList<T>.
The following code shows a DataGrid binding to a List and to item's properties.

<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 4

We wrote this in the WPF-chat:

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);
    }
}

• Weak events
• Keeps track of the same item being added multiple times to the collection
• It ~bubbles~ up the property changed events of the children.
• The static class is just for convenience.

Use it like this:

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

Answer 5

Bill,

I'm sure that you have found a workaround or solution to your issue by now, but I posted this for anyone with this common issue. You can substitute this class for ObservableCollections that are collections of objects that implement INotifyPropertyChanged. It is kind of draconian, because it says that the list needs to Reset rather than find the one property/item that has changed, but for small lists the performance hit should be unoticable.

Marc

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 6

As you found out, there is no collection-level event that indicates that a property of an item in the collection has changed. Generally, the code responsible for displaying the data adds a PropertyChanged event handler to each object currently displayed onscreen.