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#1: Layouts in Android

Layouts

A layout defines the structure for a user interface in your app, such as in an activity. All elements in the layout are built using a hierarchy of View and ViewGroup objects. A View usually draws something the user can see and interact with. Whereas a ViewGroup is an invisible container that defines the layout structure for View and otherViewGroup objects, as shown in figure 1.



Figure 1. Illustration of a view hierarchy, which defines a UI layout
The View objects are usually called "widgets" and can be one of many subclasses, such as Button or TextView. The ViewGroup objects are usually called "layouts" can be one of many types that provide a different layout structure, such as LinearLayout or ConstraintLayout .
You can declare a layout in two ways:
  • Declare UI elements in XML. Android provides a straightforward XML vocabulary that corresponds to the View classes and subclasses, such as those for widgets and layouts.
    You can also use Android Studio's Layout Editor to build your XML layout using a drag-and-drop interface.
  • Instantiate layout elements at runtime. Your app can create View and ViewGroup objects (and manipulate their properties) programmatically.

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