Remove shadow from toolbar when fragment with viewpager is launched

Question

My MainActivity implements NavigationView, so mostly the whole contents of the screen are inside FrameLayout. In one of the Fragments, I have a ViewPager, I want that when that Fragment is launched, the elevation from Toolbar gets removed, and otherwise gets re-added.

My target API is 29, and I don't care for the support of older versions.

activity_main.xml

<androidx.drawerlayout.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">
    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <com.google.android.material.appbar.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">
            <include
                layout="@layout/toolbar"/>
        </com.google.android.material.appbar.AppBarLayout>
        <FrameLayout
            android:id="@+id/fragment_main"
            android:layout_width="match_parent"
            android:layout_height="match_parent"/>
    </LinearLayout>
    <include layout="@layout/navigation_drawer"/>
</androidx.drawerlayout.widget.DrawerLayout>

fragment_view_pager.xml

<androidx.viewpager.widget.ViewPager
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/tabs_vp"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <com.google.android.material.tabs.TabLayout
        android:id="@+id/tabs_tl"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:elevation="4dp"
        android:background="?android:attr/colorPrimary">
    </com.google.android.material.tabs.TabLayout>
</androidx.viewpager.widget.ViewPager>

Answer 1

I think this is the simplest answer, in the ViewPager fragment:

private var myElevation: Float? = null

override fun onCreateView(/* params */) {
    // other code
    myElevation = activity!!.app_bar_id.elevation
    activity!!.app_bar_id.elevation = 0f
}    

override fun onDestroyView() {
    super.onDestroyView()
    activity!!.app_bar_id.elevation = myElevation!!
}

In this way, the elevation is 0 only when the ViewPager Fragment is active.