Why ObservableCollection is not updated on items change?

问题

I noticed that ObservableCollection in WPF reflects changes in GUI only by adding or removing an item in the list, but not by editing it.

That means that I have to write my custom class MyObservableCollection instead. What is the reason for this behaviour?

Thanks

回答1:

The ObservableCollection has no way of knowing if you make changes to the objects it contains - if you want to be notified when those objects change then you have to make those objects observable as well (for example by having those objects implement INotifyPropertyChanged)

回答2:

another way of achieving this would be that you implement a new XXXViewModel class that derives from DependencyObject and you put this one in the ObservableCollection.

for this look at this very good MVVM introduction: http://blog.lab49.com/archives/2650

an example for such a class would be:

public class EntryViewModel : DependencyObject
{
    private Entry _entry;
    public EntryViewModel(Entry e)
    {
        _entry = e;
        SetProperties(e);
    }

    private void SetProperties(Entry value)
    {

        this.Id = value.Id;
        this.Title = value.Title;
        this.CreationTimestamp = value.CreationTimestamp;
        this.LastUpdateTimestamp = value.LastUpdateTimestamp;
        this.Flag = value.Flag;
        this.Body = value.Body;
    }


    public Entry Entry
    {
        get {
            SyncBackProperties();
            return this._entry;
        }
    }


    public Int64 Id
    {
        get { return (Int64)GetValue(IdProperty); }
        set { SetValue(IdProperty, value); }
    }

    // Using a DependencyProperty as the backing store for Id.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty IdProperty =
        DependencyProperty.Register("Id", typeof(Int64), typeof(EntryViewModel), new UIPropertyMetadata(new Int64()));

}}

important things here: - it derives from DependencyObject - it operates with DependencyProperties to support WPFs databinding

br sargola

回答3:

You can register a method in the view model class aginst the PropertyChanged event of data class objects and listen to them in View model when any change in the property of the data objects happen. This is very easy and straight way to have the control in View model when the items of an observable collection changes. Hope this helps...

回答4:

Probably because items have no way to alert the collection when they are edited - i.e. they might not be observable. Other classes would have similar behavior - no way to alert you to a every change in the graph of referenced classes.

回答5:

As a work-around, you could extract the object from the collection and then reinsert it after you are done processing. Depending on your requirements and concurrency model, this could just make the program ugly, though. This is a quick hack, and not suitable for anything that requires quality.

Instead, you could implement the collection with an update method that specifically triggers the ContentChanged (not sure about the name) event. It's not pretty, but it is at least quite easy to deal with.

Ideally, as kragen2uk says, it would be best to make the objects observable and keep your client code clean and simple.