A View is the building block of a basic UI element. Views are bounded by a rectangle and are in charge of event handling and drawing. A Widget is a type of View. A ViewGroup is also a type of View.
A ViewGroup is a container class that contains other Views, including ViewGroups. Layouts are a type of ViewGroup.
Layouts define the structure of a user interface, typically in the XML, although you can dynamically create the UI in Java.
When loading a layout, call setContentView. You have probably seen this in your onCreate, but take note that
you can change the layout at any time. Activities are not implicitly associated with layouts.
A Layout is a rectangle - the boundaries of the a Layout are defined by getLeft(), getRight(), getTop(), and
getBottom(). You can also use getHeight() and getWidth() to get the actual size of the Layout box.
In a Linear Layout, all the children are aligned either horizontally or vertically, depending on the
android:orientation. That is, all the children are placed one after another linearly.
- Orientation: Vertical for rows, horizontal for columns.
- Gravity: Gravity and layout_gravity
- Weight: Weight describes the relative importance of the Views, which allows these views to expand to fit the parent proportionally. For example, two views with a weight of 1 will fill the layout at a 1:1 ratio (or, the same amount), and two views with weights of 1 and 2 will fill the layout with a 1:2 ratio (so, 1/3 and 2/3 of the screen).
In a Relative Layout, the children can define their position relative to the parent or the other children available.
- Create a form using a Linear Layout that asks a user for first and last name, username, DOB, etc.
- Create a calculator layout using a Linear Layout. Use this opportunity to figure out how to nest layouts.