About
The graphical user interface for an Android app is built using a hierarchy of UI elements where:
- ViewGroup (Layout) objects are invisible container element (and therefore one is at the top). Layouts are invisible containers that control how its child views are positioned on the screen.
- View objects (Widget: button, text, …). Views are rectangle on the screen.
You can declare then a user interface by building a viewgroup
The UI elements are instantiated from an activity or a fragment (sub-activity). They instantiated what is called as the main thread.
From a performance point of view, you are better off with more siblings and less children in your UI hierarchy (no more than 18 views in total or 10 nested views).
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Declaration
The GUI can be declared:
- or at load time through layout XML file
You can declare a user interface (that always start with a viewGroup then) in two ways:
- Declare UI elements in XML. In general, element names correspond to class names and attribute names correspond to methods.
- Instantiate layout elements at runtime. Your application can create View and ViewGroup objects (and manipulate their properties) programmatically.
Design
Debugging
