Android Studio Design Time Data Binding Fallback/Default value

You should read the Data Binding Guide posted on the Android developers website. The last section of the document, Android Studio Support for Data Binding explain how you can use a placeholder that can help you during the design phase. It's very simple:

<TextView android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   android:text="@{user.firstName, default=Placeholder}"/>

If you want to have text with spaces as placeholder you can use single quotes ('), back quotes (`) or "

android:text='@{user.firstName, default="Placeholder text"}'
android:text="@{user.firstName, default=`Placeholder text`}"
android:text="@{user.firstName, default="Placeholder text"}"
android:text="@{user.firstName, default=@string/placeholder_text}"

The Preview pane displays default values for data binding expressions.

android:text="@{user.firstName, default=PLACEHOLDER}"

This can set default value .

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center_horizontal"
    android:text="@{defaultString ?? @string/hello_world}"/>