ObservableCollection通知更新（MVVM WPF GridView）

Question

I have an issue where I have a GridView, which is populated by "People"

ObservableCollection<Person> People
{

    get { return directory.People }

    set { directory.People = value; } 
}

This is perfectly updating the GridView, but I want to instead only show people with the name "Andy", so I've changed it to be the following:

ObservableCollection<Person> People
{
    get {

        var andys = new ObservableCollection<Person>();

        foreach (var person in directory.People)
        {
            if (person.Name == "Andy")
            {
                  andys.Add(person);
            }
        }

        return andys;
     }

     set { directory.People = value; } 

}

The above code will perfectly populate the GridView, but it will not update it if I do the following

Private Void AddAndy()
{
    directory.People.Add(new Person{ name = Andy2 });
    //People.Add(new Person { name = Andy2 }); -- Doesn't work either
}

What this means is that although the GridView will populate initially, any other changes will have no effect on the Grid. I've switched back to looking directly at Directory.People, and it goes back to updating when people are added.

I've checked that when I add the person to "dirctory.People" it does say there is a new item in the list.

Is there something I can do to force the "People" variable to recognise that there are changes, an in turn for the GridView to update?

Thanks a lot in advance,

Andy

Answer 1

Put simply there is no connection between the People property on the directory object and the ObservableCollection you are creating in the property accessor.

So how does the XAML framework know when the directory.People collection has been updated?

I would suggest using a ListCollectionView and expose this as the property on the ViewModel,

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class PeoplesViewModel
{
    private ObservableCollection<Person> _people;
    private ListCollectionView _filteredPeople;

    public PeoplesViewModel()
    {
        _people = new ObservableCollection<Person>();
        _filteredPeople = new ListCollectionView(_people);

        _people.Add(new Person { FirstName = "John", LastName = "Smith" });
        _people.Add(new Person { FirstName = "David", LastName = "Smith" });
        _people.Add(new Person { FirstName = "John", LastName = "Jones" });

        _filteredPeople.Filter = x => ((Person) x).FirstName == "John";
    }

    public IEnumerable People { get { return _filteredPeople; } } 
}

Now when a new Person is added to View will automatically update.

Now I realize this is slightly different to the above, but if the directory.People is an ObservableCollection<T> then this should work.

Answer 2

I think the best solution is to create a property for your filtered results.

public ICollection<People> FilteredPeople
    {
        get
        {
            return this._FilteredPeople;
        }

        set
        {
            if (value != null && !value.Equals(this._FilteredPeople))
            {
                this._FilteredPeople = value;
                this.OnPropertyChanged("FilteredPeople");
            }
        }
    }

Implement INotifyPropertyChanged

    /// <summary>
    /// Triggered when a class property value is changed
    /// </summary>
    public event PropertyChangedEventHandler PropertyChanged;

    /// <summary>
    /// Helper for PropertyChanged event
    /// </summary>
    /// <param name="propertyName">Name of the property</param>
    protected void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

Then add an event handler to your observable collections CollectionChanged event. In that handler, set the collection property to your filtered results. Bind to the FilteredPeople property and away you go. You can easily extend this to parametise the name used.

This will cause a complete redraw of your control rather than just adding the elements so be aware of that if your collection runs in to high counts.