Question

I have an issue trying to persist entities to the DB using code-first technique. For example of what I am doing you may look at the following MSDN Sample. The app generally works as intended except for one case.

If I have an existing entity and I bind it to a page that has a TextBox to hold the Title field and a AppBar icon to Save (similar to the ‘New Task’ screenshot in the above link, but with values pre-filled with existing entity with Two-Way binding), the following issue occurs. If I have the TextBox selected and I change the title and hit the save button, it updates the entity in-memory so that the full list now displays the new title. But the new title is not persisted to the DB (it does not auto-detect changes). This is weird, not just because the object in-memory has changed, but also because if I deselect the TextBox and then hit save, it will persist the changes to the DB.

I have seen other questions on SO with some change detection issues, they suggest adding this.Focus() or focusing some other element at the beginning of the save method. This does not help in my case. Unless I tap on screen to deselect the TextBox and hide the keyboard (or press Return key on the keyboard, which I bound to do this.Focus()), it won’t detect the object as changed.

How can I address this? What exactly is stopping EF from detecting the object change when the keyboard is still visible?

Answer 1

Not sure if I follow exactly what you described but I think the problem is that the property you have bound your textbox to does not get updated until the TextChanged is fired on the textbox, and this is done first when you leave the Textbox, basically it will lose Focus if you tap somewhere else.

There is a simple workaround for this and it behaviors. By making a small behavior you can force the textbox to update the binding on each keystroke – so everything is updated while you type and keyboard is still there.

Behavior:

/// <summary>
/// Update property on every keystroke in a textbox
/// </summary>
public class UpdateTextSourceTriggerBehavior : Behavior<TextBox>
{
    protected override void OnAttached()
    {
        this.AssociatedObject.TextChanged += OnTextBoxTextChanged;
    }

    void OnTextBoxTextChanged(object sender, TextChangedEventArgs e)
    {
        var bindingExpression = AssociatedObject.ReadLocalValue(TextBox.TextProperty) as BindingExpression;
        if (bindingExpression != null)
        {
            bindingExpression.UpdateSource();
        }
    }

    protected override void OnDetaching()
    {
        this.AssociatedObject.TextChanged -= OnTextBoxTextChanged;
    }
}

Now just attach this behavior to your textbox like this:

<TextBox Text="{Binding YourPropertyName, Mode=TwoWay}">
                <i:Interaction.Behaviors>
                    <UpdateTextSourceTriggerBehavior/>
                </i:Interaction.Behaviors>
            </TextBox>

This will keep the property on your viewmodel updated all the time, so that when you tap on save directly after typing in the textbox it will save the correct value. Hope it helps!

Cheers,
Anders