Element Button is not allowed here

Issue

I created a fragment_b.xml file in my resources/layout folder. On my two buttons I get the following warning:

Element Button is not allowed here

Why is this warning popping up? My code is working fine.

fragment_b.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="match_parent">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent">

        <Button
            android:id="@+id/btn_fragment_b"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/go_to_fragment_c"
            app:layout_constraintBottom_toTopOf="@id/btn_nagivate_to_fragment_f"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            />

        <Button
            android:id="@+id/btn_nagivate_to_fragment_f"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/go_to_fragment_f"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@id/btn_fragment_b"
            android:layout_marginTop="@dimen/a_lot_of_margin"/>

    </androidx.constraintlayout.widget.ConstraintLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

Solution

This will happen if you don't have the androidx ConstraintLayout library added as a dependency in your gradle files.

In your app/build.gradle file, in the dependencies block, add this:

dependencies {
    // ...
    implementation "androidx.constraintlayout:constraintlayout:2.0.0-beta3"
}

Of course you can use a different library version, but the androidx.constraintlayout:constraintlayout: part is important.

Once that's done, do a gradle sync and rebuild your project and the warning should go away.

Answered By - Ben P.