TextBox Binding TwoWay Doesn't Update Until Focus Lost WP7

I assume your Save button is an ApplicationBarButton (not a normal button). For normal buttons it will just work because they take focus and hence the data-binding will kick in.

For ApplicationBarButtons on the phone it's a little different because they don't take focus away from the client app. To ensure the data-binding kicks in when your Save button is clicked, you can add the following code in your handler:

object focusObj = FocusManager.GetFocusedElement();
if (focusObj != null && focusObj is TextBox)
{
    var binding = (focusObj as TextBox).GetBindingExpression(TextBox.TextProperty);
    binding.UpdateSource();
}

Download Charles Petzold's free book Programming Windows Phone 7. On page 387 he talks about how to do this.

Basically, set the UpdateSourceTrigger property of the Binding to Explicit. Then, in the TextBox's TextChanged callback, update the Binding source.

try to set UpdateSourceTrigger property to 'PropertyChanged'

like this

Property="{Binding PropertyBinding, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"

You can use the UpdateTextBindingOnPropertyChanged behavior from the Prism Library for WP7 to update the bound value when the text changes instead of on lost focus.