21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Introduction to Android

• Overview of Android System• Android Components• Component lifecycles

• Slides rely heavily on http://developers.android.com

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Android Introduction

Android OS is based the Linux Kernel Aimed at small-scale devices

– Phones, tablets, TVs, games consoles…– Touch screen or mouse/pointer– Small internal memory (256Mbyte to 3GB)– Usually low power devices

Access to multiple sensors– Accelerometers (up down etc)– Proximity, Light, Magnetic– Geo location (GPS etc)

Resources defined in XML documents rather than inside code

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Android OS Features

Application framework enabling reuse and replacement of components

Dalvik virtual machine optimized for mobile devices Integrated browser based on the open source WebKit engine Optimized graphics powered by a custom 2D graphics library;

3D graphics based on (at least) OpenGL ES 1.0 specification, hardware acceleration optional but common

SQLite for structured data storage Media support for common audio, video, and still image formats Telephony, Bluetooth and WiFi (hardware dependent) Camera, GPS, compass, and accelerometer (hardware

dependent) Rich development environment including a device emulator,

tools for debugging, memory and performance profiling, and a plugin for the Eclipse IDE

Android OS Structure

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Android Application Components

Components– Activities – most important for us– Services, “Broadcast Receivers”, “Content

Providers” “Intents” are used to activate components

– Can use existing apps within your app, very easy to do

Intent Filters define what a component can do

The Manifest file is where most app capabilities are declared

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Activity

A visual user interface for one action (Activity base class)– Eg a Text messaging app might have activities to

• show list of contacts• write message• review old messages• change settings

– They work together but each is independent One of the activities in identified as the first to be

launched. Moving from one activity to another is accomplished

by the current activity starting the next one. Each activity has a default window to draw in.

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Views

Content of a window is a hierarchy of views (View Class).

Each view controls a rectangular space within the window.

Parent view contain children views. Ready made views to use include:-

– Buttons– Text Fields– Scroll bars– Menu items– Check boxes etc

Build in the GUI designer!

A contains

B Contains C Contains

D detail

E detail

Fdetail

G detail

H detail

K detail

I detail

J detail

L M

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Services

A service does not have a visual user interface Runs in the background for an indefinite period

– Eg service might play background music as user does something else.

– Might fetch data over the network– Calculate something – Provide a result to an activity

Each service extends the Service base class Services run in the main thread of the application

process.– Don’t block other components or user interface– Often spawn another thread for time consuming tasks

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Broadcast Receivers

A component that does nothing but receive and react to broadcast announcements.

Many broadcasts originate in system code– Eg timezone change announcement– Battery low announcement– Picture has been taken announcement

Applications can initiate broadcasts– Data has been downloaded and ready to use

An application can have any number of broadcast receivers

Receivers extend the BroadcastReceiver base class.

Notifications are often used by them.

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Intents

Intents are asynchronous messages that activate– activities, services and broadcast receivers.

For activities and services it – Names the action being requested– Specifies the URI of the data to act on

• Allow user to edit some specific text

Each type of component is activated by sending an intent object to – Activity - Context.startActivity() or Activity.startActivityForResult()

• Android calls the activity’s onNewIntent() method and passes it the intent object

– Service – Context.startService()• Android calls the services OnStart() method and passes it the intent object

– Broadcast Receiver – Context.sendBroadcast() • Android delivers the intent to all interested broadcast receivers by calling their

Onreceive() method.

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

The Manifest File

Applications declare their components in a manifest file bundled in the Android package .apk

The manifest is an XML file. It also

– Names any libraries needed to run app– Identify any permissions the app needs– Declares intent filters (what can the app do)

Example Manifest document <?xml version="1.0" encoding="utf-8"?>

<manifest . . . >    <application . . . >        <activity android:name="com.example.project.FreneticActivity"                  android:icon="@drawable/small_pic.png"                  android:label="@string/freneticLabel"                   . . .  >            <intent-filter . . . >                <action android:name="android.intent.action.MAIN" />                <category android:name="android.intent.category.LAUNCHER" />            </intent-filter>            <intent-filter . . . >                <action android:name="com.example.project.BOUNCE" />                <data android:mimeType="image/jpeg" />                <category android:name="android.intent.category.DEFAULT" />            </intent-filter>        </activity>        . . .    </application></manifest>

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Activity Lifecycle

An activity has three states– Active or running when in the foreground ie has the focus for

the user’s actions– Paused if it has lost focus but is still visible

• A paused activity is completely alive• Can be killed by the system in extreme low memory situations

– Stopped if completely obscured by another activity.• It still retains all state and member information• Often killed by the system when memory needed elsewhere

As activity state changes various methods called:-– onCreate(), onStart(),– onRestart(), onResume(),– onPause(), onStop(),– onDestroy()

Activity Starts

Process is killed

Activity is running

Activity is no longer visible

Activity is shut down

OnCreate()

OnResume()

OnDestroy()

OnStop()

OnStart()

OnPause()

Another Activity in front

Activity comes to the

Foreground

Activity comes to the Foreground

User navigates back to the

activity

Other Applications need

memory

OnRestart()

Activity Lifecycle

21/04/23E.R.Edwards 21/04/23 Staffordshire UniversitySchool of Computing

Summary

Android is open source – anyone can join in! Fairly radical change in perspective making

programming interesting! Apps can use other apps as content providers Very powerful emulator to develop on, integrated

with Eclipse IDE or Android Studio. MIT has the AppInventor2 site – interesting way to

get started Battle for dominance between iPhone and

Android?– Anybody else in the running?