CardView inside CardView Doesn't Have Shadow

Question

I have a CardView inside another CardView, but the child CardView doesnt have shadow around it. Any idea why?

        <android.support.v7.widget.CardView
                android:layout_width="match_parent"
                android:layout_height="match_parent"
            card_view:cardBackgroundColor="@color/WHITE_COLOR"
            card_view:cardCornerRadius="20dp"
            card_view:cardElevation="3dp"
            card_view:cardPreventCornerOverlap="false"
            card_view:contentPadding="0dp">

            <android.support.v7.widget.CardView
                android:layout_width="200dp"
                android:layout_height="200dp"
                android:layout_centerInParent="true"
                card_view:cardBackgroundColor="@color/RED_COLOR"
                card_view:cardCornerRadius="20dp"
                card_view:cardElevation="6dp"
                card_view:cardPreventCornerOverlap="false"
                card_view:contentPadding="0dp">


            </android.support.v7.widget.CardView>

        </android.support.v7.widget.CardView>

Answer 1

Use this code temporarily..

<android.support.v7.widget.CardView
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                card_view:cardBackgroundColor="@color/WHITE_COLOR"
                card_view:cardCornerRadius="20dp"
                card_view:cardElevation="3dp"
                card_view:cardPreventCornerOverlap="false"
                card_view:contentPadding="0dp">

                <android.support.v7.widget.CardView
                    android:layout_width="200dp"
                    android:layout_height="200dp"
                    android:layout_centerInParent="true"
                    card_view:cardBackgroundColor="@color/RED_COLOR"
                    card_view:cardCornerRadius="20dp"
                    card_view:cardElevation="6dp"
                    card_view:cardPreventCornerOverlap="false"
                    card_view:contentPadding="0dp">


                </android.support.v7.widget.CardView>
                 <View
                android:layout_width="match_parent"
                android:layout_height="4dp"
                android:background="@drawable/shadow" />

            </android.support.v7.widget.CardView>

Include this xml code in drawable folder under the name shadow.xml

<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <gradient
        android:startColor="@android:color/transparent"
        android:endColor="#40000000"
        android:angle="90" />
</shape