Android’Overview - HKUSTmuppala/aad/notes/Android_Overview.pdf · AAD Hands-On (Muppala) Android Overview 5 Name* Version* APILevel ReleaseDate LinuxKernel Base’ 1.0 1 Sep.2008’

Android Overview Most of the material in this sec5on comes from

h6p://developer.android.com/guide/

Android Overview

•  A so=ware stack for mobile devices

•  Developed and managed by Open Handset Alliance

•  Open-‐sourced under Apache License

Key Applica5ons

Middleware Opera5ng System (Linux Kernel 2.6)

AAD Hands-On (Muppala) Android Overview 2

Android Features •  Applica'on framework enabling reuse and replacement of components •  Dalvik virtual machine op5mized for mobile devices •  Integrated browser based on the open source WebKit engine •  Op'mized graphics powered by a custom 2D graphics library; 3D graphics

based on the OpenGL ES 1.0 specifica5on (hardware accelera5on op5onal) •  SQLite for structured data storage •  Media support for common audio, video, and s5ll image formats (MPEG4,

H.264, MP3, AAC, AMR, JPG, PNG, GIF) •  GSM Telephony (hardware dependent) •  Bluetooth, EDGE, 3G, 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

Source: h6p://developer.android.com


Open Handset Alliance (OHA)

•  Open Handset Alliance™, a group of 80 technology and mobile companies


Open Handset Alliance

Mobile Operators

Handset Manufacturers

Semiconductor Companies

So=ware Companies

Commercializa5on Companies

Android Plaòrm Versions

•  Moving target, new plaòrms released regularly


Name Version API Level Release Date Linux Kernel Base 1.0 1 Sep. 2008 Base_1_1 1.1 2 Feb. 2009 Cupcake 1.5 3 Apr. 2009 2.6.27 Donut 1.6 4 Sep.2009 2.6.29 Éclair 2.0 Oct. 2009 2.6.29

2.1-‐update 1 7 Jan. 2010 2.6.29 Froyo 2.2 8 May 2010 2.6.32

Gingerbread 2.3/2.3.1/ 2.3.2 9 Dec. 2010 2.6.35

2.3.3/2.3.4 10 Jul. 2011 2.6.35

Honeycomb 3.0.x 11 Feb. 2011 3.1.x 12 May 2011 3.2.x 13 Jul. 2011

Ice Cream Sandwich

4.0/4.0.1/4.0.2 14 Dec. 2011

4.0.3 15 Jan. 2012

Android Plaòrm Versions as of Feb. 1, 2012


Android Architecture


Android Architecture -‐ Applica5ons

•  Android ships with a set of core applica5ons: –  Email Client –  SMS Program –  Calendar –  Maps –  Browser –  Contacts

•  All applica5ons are wri6en using the Java language.


Android Architecture – Applica5on Framework

•  Enabling and simplifying the reuse of components –  Developers have full access to the same framework APIs used by the

core applica5ons. –  Applica5on architecture designed to simplify reuse of components –  Users are allowed to replace components.


Android Architecture – Applica5on Framework •  Set of services and features


Feature Role View System

Used to build an applica5on, including lists, grids, text boxes, bu6ons, and embedded web browser

Content Provider

Enabling applica5ons to access data from other applica5ons or to share their own data

Resource Manager

Providing access to non-‐code resources such as localized strings, graphics, and layout files

No5fica5on Manager

Enabling all applica5ons to display custom alerts in the status bar

Ac5vity Manager

Manages the lifecycle of applica5ons and provides a common naviga5on backstack

Android Architecture -‐ Libraries

•  Includes a set of C/C++ libraries used by various components of the Android system

•  Exposed to developers through the Android applica5on framework

•  Features –  System C Library (Bionic) a

version of libc –  Media Libraries suppor5ng

playback and recording of popular audio/video formats

–  Surface Manager (Surface Flinger)

–  Audio Manager (Audio Flinger)

–  LibWebCore (WebKit) –  SGL –  3D Libraries: Open GL ES –  WebKit browser support –  SQLite


Android Architecture -‐ Run5me

•  Core Libraries –  Provides most of the func5onality available in the core libraries of the

Java programming language –  APIs

•  Data Structures •  U5li5es •  File Access •  Network Access •  Graphics •  Etc


Android Architecture – Run5me (Contd.)

•  Every Android applica5on runs in its own process, with its own instance of the Dalvik Virtual Machine

•  Dalvik Virtual Machine –  Dalvik VM has been wri6en so that a device can run mul5ple VMs

efficiently –  Register-‐based virtual machine


Android Architecture – Run5me (Contd.)

•  Dalvik Virtual Machine (Cont) –  Executes files in the Dalvik Executable (.dex) format, which is op5mized for minimal memory footprint

–  Runs classes compiled by a Java language compiler transformed into the .dex format

–  Relies on the Linux kernel for underlying func5onality such as threading and low-‐level memory management

a.java Java compiler a.class Dx tool a.dex


Android Architecture – Linux Kernel

•  Android relies on Linux Kernel 2.6 for core system services –  Memory Management –  Process Management –  Network Stack –  Driver Model –  Security

•  The Kernel acts as an abstrac5on layer between the hardware and the rest of the so=ware stack


Android Applica5on Fundamentals

Android Applica5ons

•  Android applica5ons are wri6en in Java •  The compiled Java code (along with any data and resource

files required by the applica5on) is bundled by the aapt tool into an Android package (.apk)


.apk package

Java classes

Data files

Resources

Android Applica5ons

•  Each Android applica5on lives in its own world: –  Runs in its own Linux process

•  Started when any of the applica5on code needs to be executed •  Shuts down when no longer needed or system resources are required by other applica5ons

–  Each process has its own virtual machine (sandboxing) •  Applica5on code runs in isola5on from all other applica5ons

–  Each applica5on assigned a unique Linux user ID


Applica5on Components

•  One applica5on can make use of elements of other applica5ons –  Applica5ons are composed of parts (components) that can be started

up when the need arises, and instan5ate the Java objects for that part –  Android applica5ons do not have a single entry point (e.g. no main()

func5on). –  Essen5al components that the system can instan5ate and run as

needed



•  Four basic components: Ac5vi5es, Services, Broadcast Receivers, Content Providers

Ac5vi5es Services

Broadcast Receivers Content Providers



Ac5vity

•  An ac5vity is visual UI for one endeavour the user can undertake: –  Extends the Activity base class –  Displays user interface controls (views) –  Provides for user interac5on


Ac5vity (Contd.)

•  An applica5on typically consists of one or more ac5vi5es: –  Each ac5vity has a default window to draw in (the UI) –  One ac5vity usually marked as the main ac5vity (started when the

applica5on begins) –  Moving from one ac5vity to another is accomplished by having the

current ac5vity start the next one –  An ac5vity may return a result to the previous ac5vity.

•  Details of ac5vi5es specified in the Applica5on’s Android Manifest file


Example Android Manifest File <?xml version="1.0" encoding="u/-‐8"?> <manifest xmlns:android="h5p://schemas.android.com/apk/res/android" package="hkust.comp355.courseinfo” android:versionCode="1” android:versionName="1.0"> <applica5on android:icon="@drawable/icon" android:label="@string/app_name" android:debuggable="true"> <ac5vity android:name=".welcome" android:label="@string/app_name"> <intent-‐filter> <ac5on android:name="android.intent.acHon.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-‐filter> </ac5vity> <ac5vity android:name=".menu" android:label="@string/app_name"> </ac5vity> </applica5on> <uses-‐sdk android:minSdkVersion="8" /> </manifest>


Service

•  A service runs in the background for an indefinite period 5me. –  Example: play music, network download, etc –  Does not have a user interface –  extends the Service base class.

•  We can bind to a running service (or start the service if it's not already running)


Broadcast Receivers

•  Receives and reacts to broadcast announcements (Intents) –  Many broadcasts originate in system code. –  E.g. announcements that a picture is taken, that the 5me zone has

changed, that the ba6ery is low, etc. –  Applica5ons can also ini5ate broadcasts.

•  E.g. to let other applica5ons know that some data has been downloaded to the device and is available for them to use

–  Do not have a user interface –  May start an ac5vity in response to the received broadcast –  May also use no5fica5ons to alert the user


Content Providers

•  Makes a specific set of the applica5on's data available to other applica5ons –  Data can be stored in the file system, in an SQLite, or in any other

manner that makes sense –  extends the ContentProvider base class to implement a standard

set of methods that enable other applica5ons to retrieve and store data of the type it controls

•  Applica5ons do not call these methods directly. –  They use a ContentResolver object and call its methods instead –  A ContentResolver can talk to any content provider


Exercise: Crea5ng a Simple Applica5on

A Simple Applica5on

•  Create a new Android Project. In Eclipse, click File-‐> New -‐> Android project. Meaning of different fields: –  Project Name

•  Use MyInfo.

–  Build Target •  Android 4.0.3.

–  ApplicaHon Name •  Use My Personal Info.

–  Package Name •  Here we use the "hkust.cse.myinfo" namespace.


A Simple Applica5on

–  Create AcHvity •  This is the name for the class stub that will be generated by the plugin. This

will be a subclass of Android's Ac5vity class. An Ac5vity is simply a class that can run and do work. It can create a UI if it chooses, but it doesn't need to. As the checkbox suggests, this is op5onal, but an Ac5vity is almost always used as the basis for an applica5on. Here we use MyInfoAc'vity.

•  Now that your Android project is ready, we will define colors for the applica5on –  Find the folder /res/Values, right click, select New-‐> Android XML File, set

the file name as colors.xml, and set the content to the following code: <?xml version="1.0" encoding="u/-‐8"?> <resources> <color name=”myblue">#003366</color> <color name=”myyellow">#996600</color> </resources>

–  This is how we define new colors that can be used in our applica5ons.


A Simple Applica5on

•  Open strings.xml, and change its content with following code: <?xml version="1.0" encoding="u/-‐8"?> <resources> <string name="app_name”>My Personal Info</string> <string name="Welcome">Welcome to</string> <string name=”MyName”>Your Name\’s\nApp</string> </resources>

–  We are basically defining three new strings that we can use for the text in the user interface that we design next. It is very easy to replace these string values with other values to reconfigure the applica5on user interface without needing to change any code.


A Simple Applica5on

•  Import some pictures to use in designing the user interface: There are icons for three different resolu5ons (high, medium and low): hdpi, mdpi and ldpi –  On the course page you will find four Zip files: ldpi.zip, mdpi.zip,

hdpi.zip and xhdpi.zip. Download the four zip files to the respec5ve folders in the project home directory (must be D:\workspace\MyInfo\res\drawable-‐<type>) and unzip them

–  Then, right click on the project name in Eclipse, and select refresh. You can see the icons appear in the project folder.

–  The Android device will use the appropriate icons that are suitable for the resolu5on of the device.


A Simple Applica5on

•  Configure the user interface layout: In the /res/layout folder, rename main.xml to myinfo.xml (just press F2 to rename), and change its content to the following code:

<?xml version="1.0" encoding="u`-‐8"?> <LinearLayout xmlns:android="h6p://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:background="@color/myblue" android:orienta5on="ver5cal" >


A Simple Applica5on

<TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_margin="20dp" android:gravity="center_ver5cal|center_horizontal" android:text="@string/Welcome" android:textColor="@android:color/white" android:textSize="14pt" />


A Simple Applica5on <ImageView android:id="@+id/imageView1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_gravity="center" android:layout_margin="40dp" android:src="@drawable/boy" /> <TextView android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center_ver5cal|center_horizontal" android:text="@string/MyName" android:textColor="@color/myyellow" android:textSize="14pt" /> </LinearLayout>


A Simple Applica5on

•  What we have done by modifying the myinfo.xml file is to define the new user interface

•  We use the TextView layout and set the text of the textview layout to the appropriate string value –  Note how the string values are referenced

•  We use ImageView to include the pictures.


A Simple Applica5on

•  Then change the content of cseinfo.java file and replace the onCreate() method with following code:

public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.myinfo); }

•  Now run the applica5on and no5ce the screen layout


Intents

Ac5va5ng Components


Ac5vity

Service

Broadcast Receiver

Intent

Content Provider

Content Resolver

Intents

•  Intents are asynchronous message objects to ac5vate components other than content providers –  Message consists of (for ac5vi5es and services)

•  Ac5on being requested (MAIN/VIEW/EDIT/PICK/DELETE/DIAL/etc) •  URI of the data to act upon

–  For broadcast receivers the message is the ac5on being announced –  Example

public void onClick(View v) { startActivity(new Intent(welcome.this,menu.class));}


Intents (Contd.)

•  Two kinds of intents: –  Explicit intent: explicitly specify the component’s name that should

respond to the intent –  Implicit intent: Let Android locate the best component that can

respond to the intent •  Uses intent filters for this (more on this later) •  Example

startActivity(new Intent(Intent.ACTION_VIEW,Uri.parse("http://course.cse.ust.hk/comp4521/Description.html")));


Shu�ng Down Components

•  Ac5vi5es and services need to be shut down explicitly •  No need to explicitly shut down content provider and

broadcast receiver –  A content provider ac5ve only while responding to a request from a

ContentResolver –  A broadcast receiver ac5ve only while responding to a broadcast

message

•  Components might also be shut down by the system –  When they are no longer being used –  When Android must reclaim memory for more ac5ve components


The Manifest File Revisited

•  Applica5ons declare their components in a manifest file –  Bundled into the.apk file –  Before Android can start an applica5on component, it must learn that

the component exists

•  Structured XML file –  Always named AndroidManifest.xml for all applica5ons –  Also names any libraries the applica5on needs to be linked against

(besides the default Android library) –  Iden5fies any permissions the applica5on expects to be granted


Intent Filters

•  A component's intent filters in the manifest file informs Android of the kinds of intents the component is able to handle. –  Example

<?xml version="1.0" encoding="u/-‐8"?> <manifest xmlns:android="h5p://schemas.android.com/apk/res/android" package="hkust.comp355.courseinfo” android:versionCode="1” android:versionName="1.0"> <applica5on android:icon="@drawable/icon" android:label="@string/app_name" android:debuggable="true"> <ac5vity android:name=".welcome" android:label="@string/app_name"> <intent-‐filter> <ac5on android:name="android.intent.acHon.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-‐filter> </ac5vity> . . .


Intent Filters (Contd.)

•  A component can have any number of intent filters, each one declaring a different set of capabili5es.

•  One filter may indicate that the ac5vity is the entry point for the applica5on.

•  Another filter may declare an ac5on that the ac5vity can perform on par5cular type of data

•  If it doesn't have any filters, it can be ac5vated only by intents that explicitly name the component as the target


Exercise: Intents and Ac5vi5es

Using Implicit Intents

•  Open the cseinfo.xml file and add a bu6on widget: <Bu6on android:id="@+id/bu6on1" android:layout_height="wrap_content" android:layout_width="wrap_content” android:text=“My Web Page” />


Using Implicit Intents

•  Open myinfo.java to add an onClick() listener to the bu6on as follows:

Bu6on bu6on= (Bu6on) findViewById(R.id.bu5on1); bu6on.setOnClickListener(new Bu6on.OnClickListener() { public void onClick(View v) { startAc5vity(new Intent(Intent.ACTION_VIEW, Uri.parse("h[p://www.cse.ust.hk/~muppala/"))); } });


Applica5on Components and

Component Lifecycle

Applica5ons and Components •  An Android applicaHon typically

consists of one or more related, loosely bound ac5vi5es for the user to interact with, typically bundled up in a single file (with an .apk suffix) –  One ac5vity designated as the

main ac5vity •  An Android applica5on is

composed of several components –  Every applica5on runs in its own

Linux process –  Process created whenever any one

of the components get ac5vated –  Every component has a managed

lifecycle

Applica5on

Ac5vity 1 Ac5vity 2

Service 1

Broadcast Receiver

Service 2

Content Provider

Ac5vity 3


Component Lifecycle

•  Component lifecycle –  From the beginning when Android instan5ates them to respond to

intents through to an end when the instances are destroyed –  In between, they may some5mes be ac5ve or inac5ve –  Ac5vi5es may be visible to the user or invisible


Component Lifecycles

•  Ac5vity Lifecycle –  Three states

•  Running, paused and stopped

–  If an ac5vity is paused or stopped, the system can drop it from memory either by:

–  asking it to finish (calling its finish() method)

–  simply killing its process.

• Ac5vity in the foreground of the screen (at the top of the ac5vity stack for the current task)

Resumed

• Ac5vity has lost focus but is s5ll visible to the user Paused

• Ac5vity is completely obscured by another ac5vity

• It s5ll retains all state and member informa5on

Stopped


Ac5vity Lifecycle

•  As an ac5vity transi5ons from state to state, it is no5fied of the change by calls to the following protected methods: void onCreate(Bundle savedInstanceState) void onStart() void onRestart() void onResume() void onPause() void onStop() void onDestroy()


Ac5vity Lifecycle •  Three nested loops for the en5re

lifecycle –  En're life'me

•  From onCreate() to onDestroy() •  An ac5vity does all its ini5al setup of

"global" state in onCreate() •  releases all remaining resources in

onDestroy() •  For example, create that thread in

onCreate() and then stop the thread in onDestroy().

–  Visible Life5me •  User can see the ac5vity on-‐screen,

though it may not be in the foreground and interac5ng with the user.

•  onStart() and onStop() can be called mul5ple 5mes, as the ac5vity alternates between being visible and hidden to the user.

–  Foreground Life5me •  Ac5vity is in front of all other ac5vi5es

on screen and is interac5ng with the user.

En5re Life5m

e

Visib

le Life

5me

Foregrou

nd

Life5m

e


Ac5vity Lifecycle

•  When are the methods called in an ac5vity's overall lifecycle? –  onCreate()

•  Called when the ac5vity is first created or when the ac5vity was killed –  onStart()

•  Called just before the ac5vity becomes visible to user

–  onRestart() •  Called a=er the ac5vity has been stopped, just prior to it being started again

–  onResume() •  Called just before the ac5vity starts interac5ng with the user •  At this point, the ac5vity is at the top of the ac5vity stack, with user input going to it


Ac5vity Lifecycle

•  When are the methods called in an ac5vity's overall lifecycle? –  onPause()

•  Called when the system is about to start resuming another ac5vity •  This method is typically used to commit unsaved changes to persistent data, stop anima5ons and other things that may be consuming CPU, and so on

–  onStop() •  Called when the ac5vity is no longer visible to the user •  This may happen because it is being destroyed, or because another ac5vity has been resumed and is covering it

–  onDestroy() •  Called before the ac5vity is destroyed


Ac5vity Lifecycle


Saving Ac5vity State

•  When system shuts down an ac5vity, user may expect to return to the ac5vity in its previous state

•  Make use of two methods implemented within the ac5vity to save and restore state: –  onSaveInstancestate(): Android calls this before making the ac5vity

vulnerable to being destroyed (i.e. before onPause()) •  Save state in a Bundle object

–  onRestoreInstanceState(): Bundle passed both to onCreate() and the onRestoreInstanceState() which is called just before onStart() to recreate the captured state

–  Both methods are not part of lifecycle, as they need not be called when the user inten5onally destroys the ac5vity


Saving Ac5vity State


Coordina5ng Ac5vi5es

•  When one ac5vity starts another, they both experience lifecycle transi5ons –  One pauses and may stop, while the other starts up. –  On occasion, you may need to coordinate these ac5vi5es, one with

the other.

•  The order of lifecycle callbacks is well defined, par5cularly when the two ac5vi5es are in the same process. For example if Ac5vity A starts Ac5vity B: 1.  A's onPause() method is called. 2.  B's onCreate(), onStart(), and onResume() methods are called in

sequence. 3.  Then, if A is no longer visible on screen, its onStop() method is called.


Service Lifecycle

•  A service can be used in two ways: –  The service can be started and allowed to run un5l someone stops it

or stops itself. •  Started by calling Context.startService() •  Stopped by calling Context.stopService()

–  The service can be operated programma5cally using an interface that it defines and exports

•  Clients establish a connec5on to the Service object and use that connec5on to call into the service.

•  Connec5on established by calling Context.bindService() •  Connec5on closed by calling Context.unbindService()


Service Lifecycle

•  Service lifecycle methods –  They are public, not protected: void onCreate() void onStart(Intent intent) void onDestroy()


Broadcast Receiver Lifecycle

•  Single callback method void onReceive(Context curContext, Intent broadcastMsg)

•  When a broadcast message arrives for the receiver, Android calls the method and passes it the Intent object containing the message

–  Ac5ve only when the onReceive() is being executed. Becomes inac5ve on exit of this method

–  A process with an ac5ve broadcast receiver is protected from being killed but a process with only inac5ve components can be killed by the system at any 5me.


Summary

•  Android Architecture: Layered; –  Applica5ons, framework, libraries and run5me, and kernel

•  Android applica5on components –  Ac5vity, service, content provider and broadcast receiver

•  Intents •  Component Lifecycle

–  Lifecycle methods


Android’Overview - HKUSTmuppala/aad/notes/Android_Overview.pdf · AAD Hands-On (Muppala) Android Overview 5 Name* Version* API*Level* Release*Date* LinuxKernel Base’ 1.0 1 Sep.2008’

Documents

Android’Overview - HKUSTmuppala/aad/notes/Android_Overview.pdf · AAD Hands-On (Muppala) Android Overview 5 Name* Version* APILevel ReleaseDate LinuxKernel Base’ 1.0 1 Sep.2008’