ObservableCollection

Question

Hi all,

I have a WPF dialog that is bound to a list of ObservableCollection<MyEntity> type. In the dialog, I want the "OK" button to be enabled only if changes are made to the ObservableCollection<MyEntity> list - that includes adding/removing items from the list and modifying the individual items in the list.

For adding/removing items from the list, it is easy - I implemented a handler for the CollectionChanged event.

What I don't know how to do is when an individual item is modified. Say, MyEntity.Name="New Value", what interface does MyEntity class need to implement to make it 'observable'?

Thanks,

Answer 1

MyEntity needs to implement INotifyPropertyChanged, then when a property change occurs you fire the PropertyChanged event. Like this:

public class MyEntity : INotifyPropertyChanged
{
    public bool MyFlag 
    {
        get { return _myFlag; }
        set 
        {
            _myFlag = value;
            OnPropertyChanged("MyFlag");
        }
    }

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

    public event PropertyChangedEventHandler PropertyChanged;
}

Two ways to approach this are:

• have an event listener internal to the object which then sets an IsDirty flag whenever a property changes. Then OK button is bound to a command (check out the usage of the ICommand interface), and in the CanExecute method of the command you check if any of the objects in the ObservableCollection have been set to dirty. This check can be done with a simple LINQ statement: myCollection.Any(x => x.IsDirty == true)

• this method is more clunky and smelly.... have an external object listening for changes (by subscribing to the PropertyChanged event on each object), and that external listener can then enable the OK button (via databinding or by setting it directly).

Answer 2

I like the answer provided by slugster, here is an alternative building on slugster's answer.

If you bind to your OK button using DelegateCommnd you can add event handlers for CollectionChanged and PropertyChanged to change a simple boolean flag to control the state of the OK button.

public class MainViewModel : ViewModelBase
{
  public DelegateCommand<object> RunCommand { get; set; }
  public DelegateCommand<object> OkCommand { get; set; }
  private bool enableOk = false;
  private bool setOK = false;
  private ObservableCollection<MyEntity> _entites = new ObservableCollection<MyEntity>();

  public MainViewModel()
  {
     _entites.CollectionChanged += (s, e) =>
     {
        if (e.Action == NotifyCollectionChangedAction.Add)
        {
           // handle property changing
           foreach (MyEntity item in e.NewItems)
           {
              ((INotifyPropertyChanged)item).PropertyChanged += (s1, e1) => { if (setOK) enableOk = true; };
           }
        }
        // handle collection changing
        if (setOK) enableOk = false;
     };

     MyEntity me1 = new MyEntity { Name = "Name", Information = "Information", Details = "Detials" };
     MyEntity me2 = new MyEntity { Name = "Name", Information = "Information", Details = "Detials" };
     MyEntity me3 = new MyEntity { Name = "Name", Information = "Information", Details = "Detials" };
     _entites.Add(me1);
     _entites.Add(me2);
     _entites.Add(me3);

     // allow collection changes now to start enabling the ok button...
     setOK = true;

     RunCommand = new DelegateCommand<object>(OnRunCommnad, CanRunCommand);
     OkCommand = new DelegateCommand<object>(OnOkCommnad, CanOkCommand);
  }

  private void OnRunCommnad(object obj)
  {
     MyEntity me = new MyEntity { Name = "Name", Information = "Information", Details = "Detials" };

     // causes ok to become enabled
     _entites.Add(me);

     MyEntity first = _entites[0];

     // causes ok to become enabled
     first.Name = "Zamboni";
  }

  private bool CanRunCommand(object obj)
  {
     return true;
  }

  private void OnOkCommnad(object obj)
  {
  }

  private bool CanOkCommand(object obj)
  {
     return enableOk;
  } 
}

Here is a version MyEntity (similar to the one provided by slugster):
Only the Name property fires an event in this example...

public class MyEntity : INotifyPropertyChanged
{
  private string _name = string.Empty;
  public string Name
  { 
     get
     {
        return _name;
     }
     set
     {
        _name = value;
        OnPropertyChanged("Name");
     }
  }
  public string Information { get; set; }
  public string Details { get; set; }

  public event PropertyChangedEventHandler PropertyChanged;

  protected void OnPropertyChanged(string propertyName)
  {
     PropertyChangedEventHandler handler = PropertyChanged;

     if (handler != null)
     {
        handler(this, new PropertyChangedEventArgs(propertyName));
     }
  }
}

Answer 3

You should implement INotifyPropertyChanged. You could do it by the following way (as you can see, this implementation is fully thread safe)

private readonly object _sync = new object();

public event PropertyChangedEventHandler PropertyChanged
{
   add { lock (_sync) _propertyChanged += value; }
   remove { lock (_sync) _propertyChanged -= value; }
} private PropertyChangedEventHandler _propertyChanged;

protected void OnPropertyChanged(Expression<Func<object>> propertyExpression)
{
   OnPropertyChanged(GetPropertyName(propertyExpression));
}

protected string GetPropertyName(Expression<Func<object>> propertyExpression)
{
    MemberExpression body;

    if (propertyExpression.Body is UnaryExpression)
        body = (MemberExpression) ((UnaryExpression) propertyExpression.Body).Operand;
    else
        body = (MemberExpression) propertyExpression.Body;

    return body.Member.Name;
}

protected virtual void OnPropertyChanged(string propertyName)
{
  var handler = _propertyChanged;
  if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}

Following the implementation I described above, you can notify about your changes by two ways 1) The first way

public int MyProperty
{
     get { return _myProperty; }
     set
        {
           if (value != __myProperty)
           {
               _subVersion = value;
               OnPropertyChanged(MyPropertyPropertyName);
            }
        }
} private int _myProperty; const string MyPropertyPropertyName = "MyProperty";

2) And the second way

public int MyProperty
{
     get { return _myProperty; }
     set
        {
           if (value != _myProperty)
           {
               _subVersion = value;
               OnPropertyChanged(() => MyProperty);
            }
        }
} private int _myProperty;