Why reference of a view using a binding class is marked with @Nullable?

Question

I have a layout with some TextView and a CardView. Only the reference binding.mycardview returns an object CardView?, but according to docs:

Null safety: Since view binding creates direct references to views, there's no risk of a null pointer exception due to an invalid view ID. Additionally, when a view is only present in some configurations of a layout, the field containing its reference in the binding class is marked with @Nullable.

My layout row.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <androidx.cardview.widget.CardView
        android:id="@+id/cVComune"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="@dimen/space"
        android:layout_marginTop="@dimen/space"
        android:layout_marginEnd="@dimen/space"
        android:layout_marginBottom="@dimen/space"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <TextView
                android:id="@+id/tvComune"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="@dimen/space"
                android:layout_marginTop="@dimen/space"
                android:textSize="@dimen/text_label_medium"
                android:textStyle="bold"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

            <TextView
                android:id="@+id/tvCap"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="@dimen/space"
                android:layout_marginTop="@dimen/space"
                android:textSize="@dimen/text_label_medium"
                app:layout_constraintStart_toEndOf="@+id/ivMail"
                app:layout_constraintTop_toBottomOf="@+id/tvComune" />

            <TextView
                android:id="@+id/tvPrefisso"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="@dimen/space"
                android:layout_marginTop="@dimen/space"
                android:textSize="@dimen/text_label_medium"
                app:layout_constraintStart_toEndOf="@+id/ivPhone"
                app:layout_constraintTop_toBottomOf="@+id/tvComune" />

            <TextView
                android:id="@+id/tvAbitanti"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="@dimen/space"
                android:layout_marginTop="@dimen/space"
                android:textSize="@dimen/text_label_medium"
                app:layout_constraintStart_toEndOf="@+id/ivAbitanti"
                app:layout_constraintTop_toBottomOf="@+id/tvComune" />

            <TextView
                android:id="@+id/tvLink"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="@dimen/space"
                android:layout_marginTop="@dimen/space"
                android:textColor="@color/hyperlink"
                android:textSize="@dimen/text_label_medium"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/tvCap" />

            <TextView
                android:id="@+id/tvProvincia"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginTop="@dimen/space"
                android:layout_marginEnd="@dimen/space"
                android:textSize="@dimen/text_label_medium"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

            <TextView
                android:id="@+id/tvRegione"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginTop="@dimen/space"
                android:layout_marginEnd="@dimen/space"
                android:textSize="@dimen/text_label_medium"
                app:layout_constraintEnd_toStartOf="@+id/tvProvincia"
                app:layout_constraintTop_toTopOf="parent" />

            <ImageView
                android:id="@+id/ivMail"
                android:layout_width="@dimen/widthimg"
                android:layout_height="0dp"
                android:layout_marginStart="@dimen/space"
                android:layout_marginTop="@dimen/space"
                android:contentDescription="@string/codicepostale"
                app:layout_constraintBottom_toBottomOf="@+id/tvCap"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/tvComune"
                app:layout_constraintVertical_bias="0.3"
                app:srcCompat="@drawable/mail" />

            <ImageView
                android:id="@+id/ivPhone"
                android:layout_width="@dimen/widthimg"
                android:layout_height="0dp"
                android:layout_marginStart="@dimen/spaceimg"
                android:layout_marginTop="@dimen/space"
                android:contentDescription="@string/telefono"
                app:layout_constraintBottom_toBottomOf="@+id/tvCap"
                app:layout_constraintStart_toEndOf="@+id/ivMail"
                app:layout_constraintTop_toBottomOf="@+id/tvComune"
                app:layout_constraintVertical_bias="0.0"
                app:srcCompat="@drawable/phone" />

            <ImageView
                android:id="@+id/ivAbitanti"
                android:layout_width="@dimen/widthimg"
                android:layout_height="0dp"
                android:layout_marginStart="@dimen/spaceimg"
                android:layout_marginTop="@dimen/space"
                android:contentDescription="@string/abitanti"
                app:layout_constraintBottom_toBottomOf="@+id/tvPrefisso"
                app:layout_constraintStart_toEndOf="@+id/ivPhone"
                app:layout_constraintTop_toBottomOf="@+id/tvComune"
                app:layout_constraintVertical_bias="0.0"
                app:srcCompat="@drawable/stick" />

        </androidx.constraintlayout.widget.ConstraintLayout>

    </androidx.cardview.widget.CardView>

</androidx.constraintlayout.widget.ConstraintLayout>

So, why RowBinding.tvLink is an object TextView marked with @NonNull but RowBinding.cVComune is a CardView? marked @Nullable? Is this a bug?

Answer 1

Do you have multiple versions of row.xml? For example, res/layout/row.xml and res/layout-land/row.xml? If the CardView isn't present in one of the two layout files, or if the android:id attribute isn't on the CardView, its type will be nullable.

You ought to be able to view the generated binding file in Android Studio by looking inside your module's build/generated/data_binding_base_class_source_out directory. That file should have a comment explaining why the view is nullable.

For example, here I deleted the android:id attribute from my TextView in the layout-land version of my layout file:

  /**
   * This binding is not available in all configurations.
   * <p>
   * Present:
   * <ul>
   *   <li>layout/</li>
   * </ul>
   *
   * Absent:
   * <ul>
   *   <li>layout-land/</li>
   * </ul>
   */
  @Nullable
  public final TextView text;