Question

I want to implement that when a textbox value is changed, my Add button become available.

I bind the textbox with viewModel:

<TextBox Name="nameTbx" Text="{Binding Path=NewNode.Name, Mode=TwoWay}" />

My button:

<Button Content="Add" Command="{Binding Path=AddNewNodeProperty}"/>

In the XAML code behind i set the DataContext to my ViewModel. ViewModel looks like:

/* code*/

private Node _newNode = new Node();
public Node NewNode
{
    get
    {
        return _newNode;
    }
    set
    {
        _newNode = value;
        OnPropertyChanged("NewNode");
    }
}

private AddNode _addNewNodeProperty;
    public AddNode AddNewNodeProperty
    {
        get
        {
            return _addNewNodeProperty;
        }
    }

In a constructor i initialize the _addNewNodeProperty

this._addNewNodeProperty = new AddNode(this);

Here is my AddNode class:

public class AddNode : ICommand
{
    private ServiceMapViewModel viewModel;

    public AddNode(ServiceMapViewModel viewModel)
    {
        this.viewModel = viewModel;
        this.viewModel.PropertyChanged += (s, e) =>
        {
            if (CanExecuteChanged != null)
            {
                CanExecuteChanged(this, new EventArgs());
            }
        };
    }

    public bool CanExecute(object parameter)
    {
        bool b = !string.IsNullOrWhiteSpace(this.viewModel.NewNode.Name);

        return b;
    }

    public event EventHandler CanExecuteChanged;

    public void Execute(object parameter)
    {
        this.viewModel.AddNewNode();
    }
}

And finally my Node class:

public class Node
{
    public string Name { get; set; }

    public bool? IsChecked { get; set; }

    public Group Group { get; set; }

    public Category Category { get; set; }

    public string Metadata { get; set; }

    public List<string> Children = new List<string>();

    public List<string> Parents = new List<string>();
}

The problem is that when i change my textbox text, NewNode is getting the value for me, but it supposed to set .

Tnx in advanced!

EDIT
Let me add something:

I also have a datagrid on the screen and when Selected Item is changed, my Add butom become available.

Selected Item:

<DataGrid Name="nodeDataGrid" ItemsSource="{Binding Path=MyServiceMap.Nodes}"
        Background="Silver" Margin="0,34,10,10" IsReadOnly="True" SelectedItem="{Binding Path=SelectedNode}"  >

And VM:

private Node _selectedNode = new Node();
    public Node SelectedNode
    {
        get
        {
            return _selectedNode;
        }
        set
        {
            _selectedNode = value;
            OnPropertyChanged("SelectedNode");
        }
    }

Answer 1

Your TextBox is bound to the Name property of your Node object. The Name property does not raise a PropertyChanged event when it changes. Only the entire NewNode property will raise the event when it is changed.

Edit to offer a suggestion

One approach would be to create a NodeViewModel class which, as mtaboy suggested, would implement the INotifyPropertyChanged interface. You would create whatever properties on the ViewModel that you would want to expose to your user. You can think of the ViewModel as sitting between your Model (i.e., Node class) and your View (the UI shown to the user).

One change I would recommend would be to encapsulate the ICommand logic into its own class. The one I often use is the RelayCommand class which can be found in Josh Smith’s MVVM article on MSDN. If you use that class, you can define either a property or a private method in your ViewModel class which would return whether the user could Add or not. For example,

private bool CanAddNewNode()
{
  return !String.IsNullOrWhitespace(Name);
}

When instantiating your RelayCommand, you can pass a lambda for the second parameter that refers to that method above.

var saveCommand = new RelayCommand(param => SaveMethod(), param => CanAddNewNode());

You would expose an ICommand property from your ViewModel which would return the RelayCommand object.

Hopefully, that gets you started.