liftOnScroll not working with ViewPager?

Overview

When I set app:liftOnScroll="true" to my AppBarLayout, and then launch the app, I don’t see any shadow, and scrolling down or up doesn’t work.

My Layout:

<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  android:id="@+id/coordinatorLayoutRoot"
  android:layout_width="match_parent"
  android:layout_height="match_parent">

    <com.google.android.material.appbar.AppBarLayout
      android:id="@+id/appbar"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      app:liftOnScroll="true">

        <androidx.appcompat.widget.Toolbar
          android:id="@+id/toolbar"
          android:layout_width="match_parent"
          android:layout_height="?android:attr/actionBarSize"
          android:theme="?android:toolbarStyle"
          app:layout_scrollFlags="scroll|enterAlways"
          app:title="@string/app_name"
          app:titleTextAppearance="@style/ToolbarTitleAppearance"/>

        <com.google.android.material.tabs.TabLayout
          android:id="@+id/tabs"
          style="@style/TabLayout.Style"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"/>

    </com.google.android.material.appbar.AppBarLayout>

    <androidx.viewpager.widget.ViewPager
      android:id="@+id/viewPager"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:overScrollMode="always"
      app:layout_behavior="@string/appbar_scrolling_view_behavior"/>
    
</androidx.coordinatorlayout.widget.CoordinatorLayout>

Version number

com.google.android.material:material:1.0.0-beta01

Operating system and device

• OnePlus 6 with Android 8.1
• Emulator (Pixel 2 XL) Android P (latest DP)

Related issues

Someone already posted a similar issue (#140 ) but his issue was fixed by removing the bottom margin of his NestedScrollView. However I’m using a ViewPager, and I don’t have any bottom margin in my layout.

[seth-gravy]

You can now set liftOnScrollTargetViewId on your AppBarLayout to point to your ViewPager directly. Or in my case, a RecyclerView inside of the ViewPager (but make sure each RecyclerView has a unique id and you update liftOnScrollTargetViewId in the ViewPager’s onTabSelected).

[khaykov]

If by any chance you have multiple fragments with the same layout in ViewPager, having unique IDs for scroll containers and communicating them to AppBar is a whole new can of headache.

While changing ID of view after getting an initial reference to them (using findViewById or with KTX view binding) works, I can’t help but feel like it’s going to come and bite me later, or not me, but another unsuspecting developer who will try to get the view using its original ID for whatever reason.

The safest way to do it, that I’m aware of, is by having an ID for each fragment in ViewPager stored in ids.xml. This adds a lot of manual labor, especially if you have a good number of ViewPager (also, it will not work with dynamically added fragments, if that’s your things).

Anything else I’m missing?