21.1 Manually Creating an XML Layout
The structure of an XML layout file is actually quite straightforward and follows the hierarchical approach of the view tree. The first line of an XML resource file should ideally include the following standard declaration:
<?xml version="1.0" encoding="utf-8"?>
This declaration should be followed by the root element of the layout, typically a container view such as a layout manager. This is represented by both opening and closing tags and any properties that need to be set on the view. The following XML, for example, declares a ConstraintLayout view as the root element, assigns the ID activity_main and sets match_parent attributes such that it fills all the available space of the device display:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"...