Android Basics

Constraints

• Limited resources (memory, processing, battery)
  • Android "stops" your app when not in use
• Primarily touch interaction
• Application model

App Components

• Each component is an entry point through which the system for user can enter your app

Activities

• Typically represents a single screen
• Main entry point (equivalent to main() method)
• For most purposes, this is your application class

Lifecycle

• Activities have an explicit lifecycle
  • One runs at a time, others are paused in the background
  • Users switch activities as they navigate through your application

• Every activity has a state: run, paused, or stopped

  • Changing a state fires a corresponding activity method

• Applications can stop at any time

  • Each activity needs to manage its own state
  • activities have methods for saving and restoring state

Intents

• A messaging object you can use to request an action from another app component
  • Starting an activity
  • Starting a service
  • Delivering a broadcast
• Used to pass data between acitivities
  • A data structure holding an abstract description of an action

• Use startActivity() to launch with intent

Fragments

• Can be thought as portions of a UI
  • With their own lifecycle and layout
  • Alternative to multiple activities
• An activity can contain and manage multiple fragments

Views

• Also referred to as a widget in Android
• Base widget class (drawing and event handling)
• Abstract container class
• Includes layout functionality directly

Layouts

• Defines the structure for a user interface
• View - draws something the user can see and interact with
• ViewGroup - an invisible container that defines the layout structure for the View and other ViewGroup objects

• Built using hierachy of View and ViewGroup

• Each subclasses of ViewGroup provides a unique way to display the views you nest within it

Linear Layout - a layout that organizes its children into single horizontal or vertical row

Relative Layout - enables us to specify the location of child objects relative to each other or to the parent

Grid View - displays item in two-dimensional scrollable grid

UI Definition and Layout

Define layout programmatically

• Write code to instantiate ViewGroups, Views and bind them together

Use XML to describe layout

• Describe screen elements in XML with properties

• Dynamically loaded

• XML is preferred

• See slides for sample code on XML

Pixel density

DPI (density per inch) - the number of pixels that fit into an inch

• High-density screens have more pixels per inch

$screen~{}density=\frac{width(px)}{width(inches)}$

• Can also use height

dp Layout units

• Stands for Density-independent pixels
• A dp is equal to one physical pixel on screen with 160 dpi

dp = (width in pixels * 160) / screen density

Events

• Android uses the Java event model with additional mobile events