Change view transparrency in CoordinatorLayout

Question

I have the following screen

What I want to achieve is when I scroll up the view, and the AppBarLayout is collapsed the "TEST TEXT" becomes transparrent. What is the best way to do this? May I achieve this with xml only or do I need to write code.

Here is my xml

<android.support.design.widget.CoordinatorLayout
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@android:color/background_light"
android:fitsSystemWindows="true">

<android.support.design.widget.AppBarLayout
    android:id="@+id/main.appbar"
    android:layout_width="match_parent"
    android:layout_height="300dp"
    android:background="@android:color/transparent"
    android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
    android:fitsSystemWindows="true">

    <android.support.design.widget.CollapsingToolbarLayout
        android:id="@+id/main.collapsing"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_scrollFlags="scroll|exitUntilCollapsed"
        android:fitsSystemWindows="true"
        app:contentScrim="@android:color/transparent"
        app:expandedTitleMarginStart="48dp"
        app:expandedTitleMarginEnd="64dp">

        <ImageView
            android:id="@+id/main.backdrop"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:scaleType="centerCrop"
            android:fitsSystemWindows="true"
            android:src="@drawable/material_flat"
            app:layout_collapseMode="parallax"/>

        <TextView
            android:id="@+id/textView"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:scaleType="centerCrop"
            android:gravity="center"
            android:textSize="26sp"
            android:text="TEST TEXT"
            app:layout_collapseMode="parallax"/>

        <android.support.v7.widget.Toolbar
            android:id="@+id/main.toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
            app:layout_collapseMode="pin"
            />
    </android.support.design.widget.CollapsingToolbarLayout>
</android.support.design.widget.AppBarLayout>

<android.support.v4.widget.NestedScrollView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    >

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textSize="20sp"
        android:lineSpacingExtra="8dp"
        android:text="@string/lorem"
        android:padding="@dimen/activity_horizontal_margin"
        />
</android.support.v4.widget.NestedScrollView>

<android.support.design.widget.FloatingActionButton
    android:layout_height="wrap_content"
    android:layout_width="wrap_content"
    android:layout_margin="@dimen/activity_horizontal_margin"
    android:src="@drawable/ic_comment_24dp"
    app:layout_anchor="@id/main.appbar"
    app:layout_anchorGravity="bottom|right|end"
    />

Answer 1

mAppBarLayout.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
    @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
        if (verticalOffset == -mCollapsingToolbarLayout.getHeight() + mToolbar.getHeight()) {

           //change color of textview.
           textview.setBackgroundColor(Color.TRANSPARENT);
        }
    }
});