res/layout/knob.xml

<?xml version="1.0" encoding="utf-8"?>
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<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content">

    <ImageView
        android:id="@+id/knob_foreground"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <ImageView
        android:id="@+id/knob_toggle_on"
        android:layout_gravity="center_horizontal|center_vertical"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/knob_toggle_on"
        android:visibility="gone" />
    <ImageView
        android:id="@+id/knob_toggle_off"
        android:layout_gravity="center_horizontal|center_vertical"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/knob_toggle_off"
        android:visibility="gone" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_gravity="center_horizontal"
        android:orientation="vertical"
        android:gravity="center_horizontal">

        <TextView
            android:id="@+id/knob_value"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:visibility="gone" />
        <TextView
            android:id="@+id/knob_label"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:ellipsize="marquee"
            android:visibility="gone" />
    </LinearLayout>

</FrameLayout>