Why app namespace doesn't have autocompletion in just one of my Android project layouts?

Issue

When I put "app:" in main_activity.xml, doesn't work the autocompletion, it only shows 1 option:

   appNs

But when I do in another xml like fragment_owl.xml, it gives me all the attributes of app namespace. I have xmlns:app="http://schemas.android.com/apk/res-auto in both of them

this are my xmls

activity_main.xml: (that has autocompletion of app namespace attributes)

    <?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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/holamundo"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="69dp"
        android:text="Hola Mundo"
        android:textAllCaps="false"
        android:textSize="36sp"
        android:textStyle="bold"

        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/roll"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="134dp"
        android:text="GIRAR"
        android:textSize="36sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <ImageView
        android:id="@+id/dice"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:srcCompat="@drawable/empty_dice"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:srcCompat = "@drawable/dice_1"
        />

<!--        android:src="@drawable/dice_1"-->


</androidx.constraintlayout.widget.ConstraintLayout>

but in this another one:

<?xml version="1.0" encoding="utf-8"?>

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    >

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical"
            >

            <fragment

                android:id="@+id/navHostFragment"
                android:name="androidx.navigation.fragment.NavHostFragment"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                app:navGraph="@navigation/navigation"
                app:defaultNavHost="true"
                app <- NO AUTOCOMPLETION :(


                />
        </LinearLayout>

</layout>

I've already try invalite cache and delete or override some folders but nothings seems to work for me.

Solution

After doing some digging i found out the following

Changing your LinearLayout to a ConstraintLayout solves your problem, but why?

ConstraintLayout is from a library (you import it via your build.gradle androidx.constraintlayout:constraintlayout:*.*.*)

CommonsWare explains it here android.support.constraint is from a library, not the platform.

Taking this into account Android Studio maybe doesnt understand the app namespace for layouts that belong to the Android platform like LinearLayout as they are part of the android namespace.

This is my interpretation, hope it answers your question

Answered By - Jaime