Question

I have a WPF MVVM app that contains a data entry form with several text boxes. I noticed that when the user is in a textbox and makes a change that the Context object does not know a change was made until the user tabs out of that text box. Once the user tabs out of the textbox, everything works fine. But I would like to know a change was made without the user having to tab off the textbox.

Is this possible?

The way my form works is that the Save and Cancel buttons bind to ICommands. These commands have a “CanSave” and “CanCancel” method that checks to see if the EntityState changed in anyway but allowing the buttons to enable. This works great but the user has to tab off the textbox to make things work.

How can I make this work without the user tabbing off a changed textbox?

Answer 1

Set the binding direction (Mode) of the TextBox to be TwoWay instead of the default and set the UpdateSourceTrigger to be PropertyChanged instead of default… like so:

<TextBox x:Name="txtPersonLastname" Text="{Binding Person.LastName, Mode=TwoWay, NotifyOnValidationError=True, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True, ValidatesOnExceptions=True}" />

(I have some additional attributes for validation in this excerpt.)

The key difference is the PropertyChanged which will update your backing property in the ViewModel. When the user types anything into the TextBox, that PropertyChanged event will fire, and in turn should trigger your CanSave, Save routines.

In Blend, it should look like this: