an android button with transparent background and round ripple

button_borderless_round.xml

<?xml version="1.0" encoding="utf-8"?>
<ripple xmlns:android="http://schemas.android.com/apk/res/android"
    android:color="@color/colorAccent_alpha26">
    <item>
        <selector>

            <item android:state_pressed="false">
                <shape
                    android:shape="oval">
                    <size android:height="56dp"
                        android:width="56dp"/>
                    <solid android:color="@android:color/transparent" />
                </shape>
            </item>

            <item android:state_pressed="true">
                <shape
                    android:shape="oval">
                    <size android:height="56dp"
                        android:width="56dp"/>
                    <solid android:color="@color/colorPrimary"/>
                </shape>
            </item>

        </selector>
    </item>
</ripple>