[Tech notes] Android UI basics

In an Android application, the user interface is built using View and ViewGroup objects. There are many types of views and view groups, each of which is a descendant of the View class.

View objects are the basic units of user interface expression on the Android platform. The View class serves as the base for subclasses called "widgets," which offer fully implemented UI objects, like text fields and buttons.

The ViewGroup class serves as the base for subclasses called "layouts," which offer different kinds of layout architecture, like linear, tabular and relative.

On the Android platform, you define an Activity's UI using a hierarchy of View and ViewGroup nodes (as shown here). In order to attach the view hierarchy tree to the screen for rendering, your Activity must call the setContentView() method and pass a reference to the root node object. The Android system receives this reference and uses it to invalidate, measure, and draw the tree. The root node of the hierarchy requests that its child nodes draw themselves — in turn, each view group node is responsible for calling upon each of its own child views to draw themselves.

Android parses the elements of your layout in-order (from the top of the hierarchy tree), instantiating the Views and adding them to their parent(s). Because these are drawn in-order, if there are elements that overlap positions, the last one to be drawn will lie on top of others previously drawn to that space.