Binding to Source Updates not happening with TextBox and AppBarButton

Seeing that the TextBox in focus doesn’t update the bound object? The reason might be that the focus is not lost. The issue can be related to an AppBarButton. This button doesn’t take the focus on click – and binding to the source happens on losing the focus.

The issues and how this can be solved is described here – with a Universal Windows Platform (UWP) application.

Lost Focus

XAML Compiled Binding

The sample code is available at More Samples.

Let’s get into a sample. The sample application defines simple properties One, Two, and Three which will be entered in the UI, and shown with the UWP MessageDialog in the method ShowValues.

In the XAML file, the first TextBox is bound to the property One with a simple two-way binding. The second TextBox configures the UpdateSourceTrigger to PropertyChanged. This way, binding from the UI to the source code happens with every property change. The default value for the UpdateSourceTrigger is LostFocus. The third TextBox control has the same binding as the first one.

The sample code makes use of compiled binding. Similar, you can use the traditional binding based on reflection. The issues are the same.

Showing the updated values

The actual values from the code are shown on invoking the ShowValues method. This method is invoked via the Click event of AppBarButton and Button controls. The first AppBarButton just resets the properties. The second AppBarButton defines a handler for the Click event to show the values, and a Button control that follows defines the Click event similar.

Running the application and enter values in the three TextBox controls, and clicking on the AppBarButton, the first two values are shown, but the last one not. The first TextBox control has lost the focus while selecting the second one. With the second TextBox, the source is updated with every property change – because of the change with UpdateSourceTrigger. If you keep the focus in this textbox, the current values are shown. Clicking on the AppBarButton while the third TextBox has the focus, source updates do not occur.

LostFocus not happening

This behavior is different clicking on the Button control. The Button control gets the focus, thus updates on losing the focus do occur.

Taking the Focus with the AppBarButton

What’s the reason for the AppBarButton not taking the focus? With some scenarios you don’t want the focus to be taken away from the TextBox. For example, the user wants to mark some text bold or italics by clicking on a button. Here, the focus should stay in the TextBox control.

In the scenario where the user enters multiple values, the focus should be lost. To solve the issue, one way is to use the Button control instead of the AppBarButton. You can use SymbolIcon or FontIcon as content for the Button to get a similar look. Another way is to use the PropertyChanged value with the UpdateSourceTrigger property. However, this creates too much runtime activity with every property change – and you might get in issues with a custom converter receiving interim values.

The best way is to just take the focus clicking the AppBarButton. You just need to set the property AllowFocusOnInteraction with this control. Setting the value to True, the AppBarButton control now behaves like the Button control in this scenario and takes the focus.

The AllowFocusOnInteraction property has been introduced in the Windows 10 Anniversary Edition (build 14393, API contract v3).

Be aware, that using the AppBarButton in a CommandBar, setting the AllowFocusOnInteraction property on the AppBarButton is not enough. You need to set this property on the CommandBar as well.

Summary

There are cases where you see unexpected results like no source updates happening. There are reasons behind it – such as the AppBarButton not taking the focus. This issue can be solved easily by setting the AllowFocusOnInteraction property.

If this information helped you fix an issue, consider buying me a coffee which helps me staying up longer and writing more articles.

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More information on XAML and writing UWP applications is in my book Professional C# 7 and .NET Core 2.0, and in my Programming Windows Apps workshops.

Enjoy learning and programming!

Christian

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