UWP: The Master-Detail-Problem with the events SelectionChanged & LostFocus and How to Fix it

While creating apps with the Universal Windows Platform (UWP) I noticed a weird behavior, but I’m not sure if it’s a bug. Let’s just call it the “Master-Detail-Problem”.

To explain it, I stripped  down my code to the XAML-snippet below. No code-behind needed. Continue reading “UWP: The Master-Detail-Problem with the events SelectionChanged & LostFocus and How to Fix it”

LostFocus (TextBox) vs. Buttons IsDefault-Property

If you bind the Text-Property of a TextBox to something, the "something" is updated when the TextBox loses focus. This is the Default-UpdateSourceTrigger defined in the Metadata for the TextBox.TextProperty. In a Data Binding you can specify another UpdateSourceTrigger, like e.g. PropertyChanged.

If a TextBox has LostFocus as UpdateSourceTrigger, which is the default, you can get problems if the TextBox is inside of a Dialog that contains a OK-Button with the IsDefault-Property set to true. When the TextBox has the Focus, the user can press Enter and the Button is clicked, but the TextBox still keeps the Focus. The LostFocus-Event doesn’t occur, so the Source of a Data Binding isn’t updated with the value of the TextBoxs Text-Property. You need to update the source explicit. Let’s take an example and use the Person-class of my previous post:

The Window below just contains a TextBox and a Button. The Text-Property of the TextBox is bound to the Name of the data in the DataContext. This would be a Person-instance, as you’ll see below in the codebehind-file. The Button has it’s IsDefault-Property set to true and defines an eventhandler for the Click-Event.

In the Constructor in the Codebehind-file a Person-instance is created. It’s assigned to the Windows DataContext-Property, so the TextBox above will display the Persons name, which is set to "Thomas". The Click-Eventhandler for the Button simply displays the name of the Person in a MessageBox.

If I start the application and type "Urs" into the TextBox and press Enter, the Button is clicked but the TextBox keeps the focus. So the Source, which is the Person-Instance, isn’t updated and the MessageBox still displays the old value (“Thomas”), as you can see in the Image below:

LostFocus vs. IsDefault

For such a scenario, you have to update the source explicit. You could do this in the Click-Eventhandler. To update the source, you have to get the BindingExpression for the TextProperty. You get the BindingExpression by calling the GetBindingExpression-Method on the TextBox. Pass in the Text-Property as parameter. On the BindingExpression call the UpdateSource-Method to update the Person-instance explicitly with the value defined in the Text-Property of the TextBox. With the Eventhandler below the Person-Instance would always be actualized when the Button is clicked. The MessageBox would display the right value:

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