Challenge 2: A ConstraintLayout Screen — Solution

<?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="match_parent">

    <TextView
        android:id="@+id/nameLabel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Your name:"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        android:layout_marginTop="24dp"
        android:layout_marginStart="16dp" />

    <EditText
        android:id="@+id/nameInput"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:hint="Enter your name"
        app:layout_constraintTop_toBottomOf="@id/nameLabel"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:layout_marginTop="8dp"
        android:layout_marginStart="16dp"
        android:layout_marginEnd="16dp" />

    <Button
        android:id="@+id/submitButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Submit"
        app:layout_constraintTop_toBottomOf="@id/nameInput"
        app:layout_constraintStart_toStartOf="parent"
        android:layout_marginTop="16dp"
        android:layout_marginStart="16dp" />

</androidx.constraintlayout.widget.ConstraintLayout>

Notes:
- Each View has both a vertical constraint (top_toTopOf / top_toBottomOf)
  and a horizontal constraint (start_toStartOf, and end_toEndOf for the
  EditText) — without both, a View's position on that axis is undefined.
- nameInput uses layout_width="0dp" with BOTH a start and end constraint
  to parent — this is ConstraintLayout's way of saying "stretch to fill
  the space between these two constraints," the constraint-based
  equivalent of LinearLayout's match_parent from Challenge 1.
- Chaining each View's top constraint to the previous View's bottom
  (toBottomOf="@id/...") is what produces the same visual top-to-bottom
  stacking LinearLayout gave for free, but as a flat hierarchy — none of
  these three Views is nested inside another.