TextBox TextChanged event doesn't cause data binding to update

TextBox TextChanged event doesn't cause data binding to update

11-19-2008 11:11 AM |

I have a text box that is bound (two way) to the following property:

 private String m_description;

        public String Description
        {
            get { return m_description; }
            set
            {
                m_description = value;
                this.OnPropertyChanged("Description"); // This goes on to call the INotifyPropertyChanged.PropertyChanged event
            }
        }

But the data binding is only ever updated when the text loses focus. The TextChanged event does fire, but it doesn't  update the binding.

 I've had a look at the BindingExpression code via Reflector:

  if (this._targetObject is TextBox)
        {
            this._targetIsTextBox = true;
            TextBox box = this._targetObject as TextBox;
            box.LostFocus += new RoutedEventHandler(this.TargetTextBoxLostFocus);
            if (this._targetProperty == TextBox.TextProperty)
            {
                box.TextChanged += new TextChangedEventHandler(this.TargetTextBoxTextChanged);
            }
        }

It looks like it *should* work, but it isnt? The TargetTextBoxLostFocus function is definitely being called, I can see that in the call stack.

Any ideas?

Mark Ingram

Re: TextBox TextChanged event doesn't cause data binding to update

11-19-2008 5:04 PM |

 But looking at the code in the BindingExpression class, it looks like it *should* work?

I've got around it by manually updating my data in the data context in the TextChanged method. It's not nice though.

Re: TextBox TextChanged event doesn't cause data binding to update

08-03-2009 5:33 AM |

I ran over the same isssue. Not sure what's best to do now...

I wonder if there is a reason for why binding was implemented like this or if it's just bad design/a slip.

Re: TextBox TextChanged event doesn't cause data binding to update

08-26-2009 5:02 PM |

bogdan_vre:

I wonder if there is a reason for why binding was implemented like this or if it's just bad design/a slip.

I'm guessing it has to do with validation. If it changed the underlying binding object on every keystroke, and there was something like a minimum length requirement, it would cause pre-mature validation errors.

Re: TextBox TextChanged event doesn't cause data binding to update

09-25-2009 10:17 AM |

Mmm...

 I thought there was a way to choose whether you want in textbox to propagate the value to the binded property on focus or new character typed, seems not :-(.

 I use the same workaround TextboxChanged change the property... ugly stuff.

 Is not good that a technology forces you to follow a way for the functional flow of the app (e.g. enabling button when the textbox change not supported or just by a workaround.... you have to enable the button always and show error information in the case the textbox didn't contain what you expected :-( ).

Cheers

   Braulio

Re: TextBox TextChanged event doesn't cause data binding to update

09-25-2009 11:26 PM |

andulvar:

The only way to have the TextBox update a two-way binding is to cause it to lose focus

You can manually trigger the update of the source by calling UpdateSource() on the BindingExpression, which can be retrieved by calling GetBindingExpression on the TextBox.

This is not a bug or an oversight. Lots of automatic features have been cut to keep the Silverlight plugin as small as possible. In this case, you can see in WPF a "PropertyChanged" member on the UpdateSourceTrigger enum. This member does not exist in Silverlight.

Re: Re: TextBox TextChanged event doesn't cause data binding to update

09-26-2009 4:32 PM |

Thanks nelligan for your response...

Well... is not a bug... it's lack of functionallity. But I find it something serious beacuse it affects the functional side of an application, What if my client has an standard on validations (e.g. enable save button just when the textbox has a valida value) ? I have to say him... well... we have to use a lot of manual hacks to make this work :-(.

I thought that this was fixed /enhanced on SL 3.0, or maybe some control on the toolkit... don't know, but it's a pity having to go with workarounds to acomplish a quite standard and well know functionallity.

Re: Re: TextBox TextChanged event doesn't cause data binding to update

09-29-2009 8:49 AM |

Well.. I have encapsulated the workaround in an user control... to make it a bit less painful

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

{