Notify ObservableCollection when Item changes

问题:

I found on this link

ObservableCollection not noticing when Item in it changes (even with INotifyPropertyChanged)

some techniques to notify a Observablecollection that an item has changed. the TrulyObservableCollection in this link seems to be what i'm looking for.

public class TrulyObservableCollection<T> : ObservableCollection<T>
where T : INotifyPropertyChanged
{
    public TrulyObservableCollection()
    : base()
    {
        CollectionChanged += new NotifyCollectionChangedEventHandler(TrulyObservableCollection_CollectionChanged);
    }

    void TrulyObservableCollection_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.NewItems != null)
        {
            foreach (Object item in e.NewItems)
            {
                (item as INotifyPropertyChanged).PropertyChanged += new PropertyChangedEventHandler(item_PropertyChanged);
            }
        }
        if (e.OldItems != null)
        {
            foreach (Object item in e.OldItems)
            {
                (item as INotifyPropertyChanged).PropertyChanged -= new PropertyChangedEventHandler(item_PropertyChanged);
            }
        }
    }

    void item_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        NotifyCollectionChangedEventArgs a = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset);
        OnCollectionChanged(a);
    }
}

But when I try to use it, I don't get notifications on the collection. I'm not sure how to correctly implement this in my C# Code:

XAML :

    <DataGrid AutoGenerateColumns="False" ItemsSource="{Binding MyItemsSource, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
        <DataGrid.Columns>
            <DataGridCheckBoxColumn Binding="{Binding MyProperty, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        </DataGrid.Columns>
    </DataGrid>

ViewModel :

public class MyViewModel : ViewModelBase
{
    private TrulyObservableCollection<MyType> myItemsSource;
    public TrulyObservableCollection<MyType> MyItemsSource
    {
        get { return myItemsSource; }
        set 
        { 
            myItemsSource = value; 
            // Code to trig on item change...
            RaisePropertyChangedEvent("MyItemsSource");
        }
    }

    public MyViewModel()
    {
        MyItemsSource = new TrulyObservableCollection<MyType>()
        { 
            new MyType() { MyProperty = false },
            new MyType() { MyProperty = true },
            new MyType() { MyProperty = false }
        };
    }
}

public class MyType : ViewModelBase
{
    private bool myProperty;
    public bool MyProperty
    {
        get { return myProperty; }
        set 
        {
            myProperty = value;
            RaisePropertyChangedEvent("MyProperty");
        }
    }
}

public class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChangedEvent(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChangedEventArgs e = new PropertyChangedEventArgs(propertyName);
            PropertyChanged(this, e);
        }
    }
}

When i run the program, i have the 3 checkbox to false, true, false as in the property initialisation. but when i change the state of one of the ckeckbox, the program go through item_PropertyChanged but never in MyItemsSource Property code.

回答1:

The spot you have commented as // Code to trig on item change... will only trigger when the collection object gets changed, such as when it gets set to a new object, or set to null.

With your current implementation of TrulyObservableCollection, to handle the property changed events of your collection, register something to the CollectionChanged event of MyItemsSource

public MyViewModel()
{
    MyItemsSource = new TrulyObservableCollection<MyType>();
    MyItemsSource.CollectionChanged += MyItemsSource_CollectionChanged;

    MyItemsSource.Add(new MyType() { MyProperty = false });
    MyItemsSource.Add(new MyType() { MyProperty = true});
    MyItemsSource.Add(new MyType() { MyProperty = false });
}


void MyItemsSource_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    // Handle here
}

Personally I really don't like this implementation. You are raising a CollectionChanged event that says the entire collection has been reset, anytime a property changes. Sure it'll make the UI update anytime an item in the collection changes, but I see that being bad on performance, and it doesn't seem to have a way to identify what property changed, which is one of the key pieces of information I usually need when doing something on PropertyChanged.

I prefer using a regular ObservableCollection and just hooking up the PropertyChanged events to it's items on CollectionChanged. Providing your UI is bound correctly to the items in the ObservableCollection, you shouldn't need to tell the UI to update when a property on an item in the collection changes.

public MyViewModel()
{
    MyItemsSource = new ObservableCollection<MyType>();
    MyItemsSource.CollectionChanged += MyItemsSource_CollectionChanged;

    MyItemsSource.Add(new MyType() { MyProperty = false });
    MyItemsSource.Add(new MyType() { MyProperty = true});
    MyItemsSource.Add(new MyType() { MyProperty = false });
}

void MyItemsSource_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    if (e.NewItems != null)
        foreach(MyType item in e.NewItems)
            item.PropertyChanged += MyType_PropertyChanged;

    if (e.OldItems != null)
        foreach(MyType item in e.OldItems)
            item.PropertyChanged -= MyType_PropertyChanged;
}

void MyType_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "MyProperty")
        DoWork();
}

回答2:

I solved this case by using static Action

---

public class CatalogoModel 
{
    private String _Id;
    private String _Descripcion;
    private Boolean _IsChecked;

    public String Id
    {
        get { return _Id; }
        set { _Id = value; }
    }
    public String Descripcion
    {
        get { return _Descripcion; }
        set { _Descripcion = value; }
    }
    public Boolean IsChecked
    {
        get { return _IsChecked; }
        set
        {
           _IsChecked = value;
            NotifyPropertyChanged("IsChecked");
            OnItemChecked.Invoke();
        }
    }

    public static Action OnItemChecked;
} 

public class ReglaViewModel : ViewModelBase
{
    private ObservableCollection<CatalogoModel> _origenes;

    CatalogoModel.OnItemChecked = () =>
            {
                var x = Origenes.Count;  //Entra cada vez que cambia algo en _origenes
            };
}

回答3:

The ObservableCollection and its derivatives raises its property changes internally. The code in your setter should only be triggered if you assign a new TrulyObservableCollection<MyType> to the MyItemsSource property. That is, it should only happen once, from the constructor.

From that point forward, you'll get property change notifications from the collection, not from the setter in your viewmodel.

回答4:

I know it's late, but maybe this helps others. I have created a class NotifyObservableCollection, that solves the problem of missing notification to item itself, when a property of the item changes. The usage is as simple as ObservableCollection.

public class NotifyObservableCollection<T> : ObservableCollection<T> where T : INotifyPropertyChanged
{
    private void Handle(object sender, PropertyChangedEventArgs args)
    {
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset, null));
    }

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        if (e.NewItems != null) {
            foreach (object t in e.NewItems) {
                ((T) t).PropertyChanged += Handle;
            }
        }
        if (e.OldItems != null) {
            foreach (object t in e.OldItems) {
                ((T) t).PropertyChanged -= Handle;
            }
        }
        base.OnCollectionChanged(e);
    }

While Items are added or removed the class forwards the items PropertyChanged event to the collections PropertyChanged event.

usage:

public abstract class ParameterBase : INotifyPropertyChanged
{
    protected readonly CultureInfo Ci = new CultureInfo("en-US");
    private string _value;

    public string Value {
        get { return _value; }
        set {
            if (value == _value) return;
            _value = value;
            OnPropertyChanged();
        }
    }
}

public class AItem {
    public NotifyObservableCollection<ParameterBase> Parameters {
        get { return _parameters; }
        set {
            NotifyCollectionChangedEventHandler cceh = (sender, args) => OnPropertyChanged();
            if (_parameters != null) _parameters.CollectionChanged -= cceh;
            _parameters = value;
            //needed for Binding to AItem at xaml directly
            _parameters.CollectionChanged += cceh; 
        }
    }

    public NotifyObservableCollection<ParameterBase> DefaultParameters {
        get { return _defaultParameters; }
        set {
            NotifyCollectionChangedEventHandler cceh = (sender, args) => OnPropertyChanged();
            if (_defaultParameters != null) _defaultParameters.CollectionChanged -= cceh;
            _defaultParameters = value;
            //needed for Binding to AItem at xaml directly
            _defaultParameters.CollectionChanged += cceh;
        }
    }


public class MyViewModel {
    public NotifyObservableCollection<AItem> DataItems { get; set; }
}

If now a property of an item in DataItems changes, the following xaml will get a notification, though it binds to Parameters[0] or to the item itself except to the changing property Value of the item (Converters at Triggers are called reliable on every change).

<DataGrid CanUserAddRows="False" AutoGenerateColumns="False" ItemsSource="{Binding DataItems}">
    <DataGrid.Columns>
        <DataGridTextColumn Binding="{Binding Parameters[0].Value}" Header="P1">
            <DataGridTextColumn.CellStyle>
                <Style TargetType="DataGridCell">
                    <Setter Property="Background" Value="Aqua" />
                    <Style.Triggers>
                        <DataTrigger Value="False">
                            <!-- Bind to Items with changing properties -->
                            <DataTrigger.Binding>
                                <MultiBinding Converter="{StaticResource ParameterCompareConverter}">
                                    <Binding Path="DefaultParameters[0]" />
                                    <Binding Path="Parameters[0]" />
                                </MultiBinding>
                            </DataTrigger.Binding>
                            <Setter Property="Background" Value="DeepPink" />
                        </DataTrigger>
                        <!-- Binds to AItem directly -->
                        <DataTrigger Value="True" Binding="{Binding Converter={StaticResource CheckParametersConverter}}">
                            <Setter Property="FontWeight" Value="ExtraBold" />
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </DataGridTextColumn.CellStyle>
        </DataGridTextColumn>

回答5:

You could use an extension method to get notified about changed property of an item in a collection in a generic way.

public static class ObservableCollectionExtension
{
    public static void NotifyPropertyChanged<T>(this ObservableCollection<T> observableCollection, Action<T, PropertyChangedEventArgs> callBackAction)
        where T : INotifyPropertyChanged
    {
        observableCollection.CollectionChanged += (sender, args) =>
        {
            //Does not prevent garbage collection says: http://stackoverflow.com/questions/298261/do-event-handlers-stop-garbage-collection-from-occuring
            //publisher.SomeEvent += target.SomeHandler;
            //then "publisher" will keep "target" alive, but "target" will not keep "publisher" alive.
            if (args.NewItems == null) return;
            foreach (T item in args.NewItems)
            {
                item.PropertyChanged += (obj, eventArgs) =>
                {
                    callBackAction((T)obj, eventArgs);
                };
            }
        };
    }
}

public void ExampleUsage()
{
    var myObservableCollection = new ObservableCollection<MyTypeWithNotifyPropertyChanged>();
    myObservableCollection.NotifyPropertyChanged((obj, notifyPropertyChangedEventArgs) =>
    {
        //DO here what you want when a property of an item in the collection has changed.
    });
}

回答6:

All the solutions here are correct,but they are missing an important scenario in which the method Clear() is used, which doesn't provide OldItems in the NotifyCollectionChangedEventArgs object.

this is the perfect ObservableCollection .

public class ObservableCollectionEX<T> : ObservableCollection<T>
{
    #region Constructors
    public ObservableCollectionEX() : base()
    {
        CollectionChanged += ObservableCollection_CollectionChanged;
    }
    public ObservableCollectionEX(IEnumerable<T> c) : base(c)
    {
        CollectionChanged += ObservableCollection_CollectionChanged;
    }
    public ObservableCollectionEX(List<T> l) : base(l)
    {
        CollectionChanged += ObservableCollection_CollectionChanged;
    }

    #endregion



    public new void Clear()
    {
        foreach (var item in this)
        {
            if (item is INotifyPropertyChanged i)
            {
                if (i != null)
                    i.PropertyChanged -= Element_PropertyChanged;
            }
        }
        base.Clear();
    }
    private void ObservableCollection_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.OldItems != null)
            foreach (var item in e.OldItems)
            {
                if (item != null && item is INotifyPropertyChanged i)
                {
                    i.PropertyChanged -= Element_PropertyChanged;
                }
            }
        if (e.NewItems != null)
            foreach (var item in e.NewItems)
            {
                if (item != null && item is INotifyPropertyChanged i)
                {
                    i.PropertyChanged -= Element_PropertyChanged;
                    i.PropertyChanged += Element_PropertyChanged;
                }
            }
    }
    private void Element_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        //raise the event
        ItemPropertyChanged?.Invoke(sender, e);
    }


    /// <summary>
    /// the sender is the Item
    /// </summary>
    public PropertyChangedEventHandler ItemPropertyChanged;

}

You can even go the extra mile and change the ItemPropertyChanged to provide the owner list like this

Outside the class in some namespace :
public delegate void ListedItemPropertyChangedEventHandler(IList SourceList, object Item, PropertyChangedEventArgs e);

in the class change these :

private void Element_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    //raise the event
    ItemPropertyChanged?.Invoke(this,sender, e);
}
public ListedItemPropertyChangedEventHandler ItemPropertyChanged;  

回答7:

A simple solution is to use BindingList<T> instead of ObservableCollection<T> . Indeed the BindingList relay item change notifications. So with a binding list, if the item implements the interface INotifyPropertyChanged then you can simply get notifications using the ListChanged event.

See also this SO answer.