Layouts and Adapter-Based Containers

Roughly speaking, there are two major types of ViewGroup implementations:

• There are those that organize a set of children known at the time that you are writing the app
• There are those that organize a set of children that cannot be known until the app is run

For example, if we were to write a to-do list app like the one shown above, we do not know how many to-do items there are in the list. That depends on how many the user enters that we save in a database, a Web service, or some other place. However, for each individual to-do list item, we know that the visual representation of that item is a checkbox and a label showing the task to be performed.

So, in this case, we have both types of ViewGroup. The list of to-do items is managed by a ViewGroup that is optimized for organizing data at runtime, while each row in that list is managed by a ViewGroup that is optimized for organizing data when we write the app.

The most popular ViewGroup for organizing data at runtime is RecyclerView, though there are other options. We will explore RecyclerView more later in the book. The recommended ViewGroup for organizing widgets in layout resources is a ConstraintLayout, though there are other options.