activity_main

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    tools:context="com.robillo.customviewstutorial.MainActivity">

    <com.robillo.customviewstutorial.MyCustomView
        android:layout_width="300sp"
        android:layout_height="300sp"
        app:square_color="@color/colorPrimary"/>

</LinearLayout>