android-calculatorpp/app/src/main/res/drawable/metro_widget_button.xml

<?xml version="1.0" encoding="utf-8"?>

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<selector xmlns:a="http://schemas.android.com/apk/res/android">

    <item a:drawable="@drawable/metro_button_pressed" a:state_pressed="true" />

    <item a:drawable="@drawable/metro_blue_button_shape" />

</selector>