Hide TextView when CollapsingToolbarLayout is Collapsed, and show it when expanded

Issue

I am working on an app using a CollapsingToolbarLayout, with an ImageView inside it. I wanted to add a gradient on top of it to look nicer and to be able to read the CollapsingToolBar title better, so I made a little hack and added a Relative Layout with a textview inside it, then I added a background to that same TextView (which is the gradient I was talking about). The problem with this is that when the ToolBar is collapsed, the gradient still shows over it and I dont want it to happen, how can I make it invisible when the ToolBar is collapsed?

Design:

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout 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=".anime_page">

    <com.google.android.material.appbar.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="450dp"
        android:fitsSystemWindows="true">

        <com.google.android.material.appbar.CollapsingToolbarLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:fitsSystemWindows="true"
            app:contentScrim="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|snap|exitUntilCollapsed"
            app:collapsedTitleTextAppearance="@style/collapsedToolbarLayoutTitleColor"
            app:expandedTitleTextAppearance="@style/expandedToolbarLayoutTitleColor"
            android:theme="@style/Theme.AnimeWatcher"
            android:id="@+id/anime_page_collapsing_toolbar">

            <ImageView
                android:id="@+id/anime_page_cover"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:scaleType="centerCrop"
                app:layout_collapseMode="parallax" />
            
            <androidx.appcompat.widget.Toolbar
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                app:layout_collapseMode="pin" >

                <ImageView
                    android:id="@+id/anime_page_back"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:src="@drawable/ic_baseline_arrow_back_24"
                    android:paddingEnd="10dp" />

            </androidx.appcompat.widget.Toolbar>

            <RelativeLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content">

                <TextView
                    android:layout_width="match_parent"
                    android:layout_height="100dp"
                    android:layout_alignParentBottom="true"
                    android:background="@drawable/black_gradient" />

            </RelativeLayout>


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

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

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/anime_page_rcv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior">


    </androidx.recyclerview.widget.RecyclerView>

</androidx.coordinatorlayout.widget.CoordinatorLayout>

Images of the design:

ToolBar Expanded

ToolBar Collapsed

Solution

Add an OnOffsetChangedListener to the AppBar and listen to the changes when it's collapsed or expanded and hide/show your TextView based on that.

Add an ID to the appbar

<com.google.android.material.appbar.AppBarLayout
    android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="450dp"
    android:fitsSystemWindows="true">

Then, access the AppBar and add an OnOffsetChangedListener as

appbar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {

    @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
        float percentage = (float) Math.abs(verticalOffset) / appBarLayout.getTotalScrollRange();
        if (Math.abs(verticalOffset) == appBarLayout.getTotalScrollRange()) {
            //  Collapsed
            //Hide your TextView here
            tv.setVisibility(View.GONE);
        } else if(verticalOffset == 0) {
            //Expanded
            //Show your TextView here
            tv.setVisibility(View.VISIBLE);
        } else {
            //In Between
            tv.setVisibility(View.VISIBLE);
            tv.animate().alpha(percentage);
    }
});

You can choose to play with the alpha property only instead of visibility, that's upto you.

Answered By - Lalit Fauzdar