Why is TextChanged event for TextBox called BEFORE the ViewModel is updated (via TwoWay binding)? Now the EventHandler concludes nothing is changed!

Why is TextChanged event for TextBox called BEFORE the ViewModel is updated (via TwoWay binding)? Now the EventHandler concludes nothing is changed!

06-22-2009 5:07 PM |

I have a DataGrid that shows items and a details form that allows editing of individual items. (Editing in the grid itself makes accidental changes more likely.) I have a Save button that updates the data. This Save button is only enabled if the data has been changed.

I tried to accomplish this via TwoWay binding. Unfortunately the events are not handled in the order that suits my needs. Am I doing something wrong?

The core code (DataContext = ViewModel):

        void PeopleGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            Person selectedPerson = PeopleGrid.SelectedItem as Person;
            ViewModel.CurrentPersonIndex = PeopleGrid.SelectedIndex;
            ViewModel.CurrentPerson = selectedPerson.Copy();
        }
       
...

    public class Person : IComparable, INotifyPropertyChanged
    {
        private string _name;
        public string Name
        {
            get { return _name; }
            set
            {
                if (_name != value)
                {
                    _name = value;
                    if (PropertyChanged != null)
                    {
                        PropertyChanged(this, new PropertyChangedEventArgs("Name"));
                    }
                }
            }
        }
       
...

        NameTextBox.TextChanged += new TextChangedEventHandler(NameTextBox_TextChanged);

...

        void NameTextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            AdjustSaveButton();
        }

        private void AdjustSaveButton()
        {
            Person originalPerson = ViewModel.People[ViewModel.CurrentPersonIndex];
            SaveButton.IsEnabled = (originalPerson.CompareTo(ViewModel.CurrentPerson) > 0);
        }
       
       
       
Some related XAML:

    <data:DataGrid x:Name="PeopleGrid" ItemsSource="{Binding People}" />

    <TextBox x:Name="NameTextBox" Text="{Binding CurrentPerson.Name, Mode=TwoWay}" />
   
   
The problem I face is that AdjustSaveButton() is called BEFORE the Name setter and hence SaveButton.IsEnabled remains false upon the first change in the NameTextBox. Upon the second change the setter has been called once, and hence SaveButton.IsEnabled is set to true. That's obviously one key stroke too late ...

 Thanks!

Re: Why is TextChanged event for TextBox called BEFORE the ViewModel is updated (via TwoWay binding)? Now the EventHandler concludes nothing is changed!

06-24-2009 10:18 PM |

Thanks  you! Are you sure this applies to Silverlight? On the Internet I see GetBindingExpression in the context of WPF but not Silverlight.

You would do me a favor if you elaborated on why it makes sense that the binding happens only after the TextChanged event handler is executed. In my scenario that does not seem to be helpful. Or maybe I should take a different approach?

Re: Why is TextChanged event for TextBox called BEFORE the ViewModel is updated (via TwoWay binding)? Now the EventHandler concludes nothing is changed!

06-24-2009 11:53 PM |

Yes, it works on Silverlight.

Regarding the TextChanged event.

The way I see TextChanged event is independent of the binding itself.

TextChanged means that the control property Text has changed.

It may ot may not have a binding expression attached to it.

And, because you know that the text has been changed we can all sort of

things just before the binding is actually executed.

Maybe we are missing an event here. BindingExecuted?

This behavior does not goes well with MVVM I have to agree.

Klinger

Re: Why is TextChanged event for TextBox called BEFORE the ViewModel is updated (via TwoWay binding)? Now the EventHandler concludes nothing is changed!

06-25-2009 10:29 AM |

Must be Silverlight 3 then. Will check solution at home tonight. Thanks!

Re: Why is TextChanged event for TextBox called BEFORE the ViewModel is updated (via TwoWay binding)? Now the EventHandler concludes nothing is changed!

09-29-2009 8:48 AM |

Well.. encapsulated in a user control the workaround...

public class TextBoxChangeExt : System.Windows.Controls.TextBox

// based on

// http://betaforums.silverlight.net/forums/t/103695.aspx

var be = txCtl.GetBindingBLOCKED EXPRESSION

{