android programming - introduction · introduction • android is built on top of more than 100...

ANDROID PROGRAMMING -INTRODUCTIONRoberto Beraldi

Introduction• Android is built on top of more than 100 open projects,

including linux kernel• To increase security, each application runs with a distinct

system identity (linux UID and GID)• Application are isolated from each other • Application are isolated from each other

• Use a quite efficient IPC mechanism

• To facilitate resource access from isolated application, android exploit a permission-based security mechanism• Each application needs permission to access system resources• Permissions are granted at installation time• There are 130 resources (android 4.2)

Android architecture

Kernel LinuxSet of drivers

The kernel provides preemptive multitasking,low level core system services, likeMemory,power,process management. Network stack, device drivers (e.g. for display)

Android architectureDalvik VM Specific LibrariesThis is a set of libraries used predominantly for interacting directly with an instance of the Dalvik VM and is unlikely to be used by most Android application developers.

• Dalvik VM: similar to the JVM• Designed by Googlemore efficient than JVM in terms of memory usage, designed torun under resource constraints• Act as a sandbox: each application runs inside a DVM

.dex format has footprint 50% smaller

Android architecture • Java Interoperability Libraries are an open source implementation (based on the Apache Harmony project) of a subset of the Standard Java core libraries that have been adapted and transformed for use by applications running within a Dalvik VM..

• Android LibrariesThis category encompasses those Java-based libraries that are specific to Android development. Examples of libraries in this category include the application framework libraries in addition to those that facilitate user interface building, graphics drawing and database access.

Dalvik VM..

Android libraries• android.app – Provides access to the application mode l and is the cornerstone of all Android

applications.• android.content – Facilitates content access, publish ing and messaging between applications and

application components.• android.database – Used to access data published by c ontent providers and includes SQLite

database management classes.• android.graphics – A low-level 2D graphics drawing AP I including colors, points, filters, rectangles

and canvases.• android.hardware – Presents an API providing access t o hardware such as the accelerometer and

light sensor.• android.opengl – A Java interface to the OpenGL ES 3D graphics rende ring API.• android.opengl – A Java interface to the OpenGL ES 3D graphics rende ring API.• android.os – Provides applications with access to sta ndard operating system services including

messages, system services and inter-process communi cation.• android.media – Provides classes to enable playback o f audio and video.• android.net – A set of APIs providing access to the network stack. Includes android.net.wifi, which

provides access to the device’s wireless stack.• android.provider – A set of convenience classes that provide access to standard Android content

provider databases such as those maintained by the calendar and contact applications.• android.text – Used to render and manipulate text on a device display.• android.util – A set of utility classes for performin g tasks such as string and number conversion,

XML handling and date and time manipulation.• android.view – The fundamental building blocks of application user interfaces.• android.widget - A rich collection of pre-built user interface components such as buttons, labels, list views,

layout managers, radio buttons etc.• android.webkit – A set of classes intended to allow web-browsing capabilities to be built into applications.


Surface Manager:Rendering of Views2D graphics

Media Framework:Manage different codec, e.g.mp3,H.264,MPEG4,etc.

Rendering of Font types

In process DB

Open GL ES2D and 3D graphicsFor Embedded systems

Web engine

C standard library

Many are wrappers of library written in C/C++


Application framework:Set of managers wrapping the native libraries, make them accessible to the programmer

Android frameworks (not complete list)• Activity Manager – Controls all aspects of the application lifecycle and activity stack.

• Content Providers – Allows applications to publish and share data with other applications.

• Resource Manager – Provides access to non-code embedded resources such as strings, color settings and user interface layouts.

• Notifications Manager – Allows applications to display alerts and notifications to the user.

• View System – An extensible set of views used to create application user interfaces.

• Package Manager – The system by which applications are able to find out information about other applications currently installed on the device.

• Telephony Manager – Provides information to the application about the telelphony services available on the device such as status and subscriber information.

• Location Manager – Provides access to the location services allowing an application to receive updates about location changes.


Application layer

Characteristics of android applications

• User interaction • touch screen based UI interface

• Variable screen size• From low, medium, high (smart TV)

• Resource • Resource • usage is an issue

• …but..• Sensors

• Position, orientation, magnetic field, light sensor, ..

• Portable • Context-awareness based applications (what’s around me, where

are my friends, …)

Bird’s eye view to application architecture

User Interface• Activity• Fragment

UI runs in a threadMain thread � it should respond fast

� responsiveness

Computation

• AsyncTask• Local service • Remote service• Broadcast receiver

• Separate thread • Need mechanism to interact with UI• Implements the “business logic”

Data

• Preference• File• SQLite• Network• Content provider

• Many ways to store data

What an application is composed of?

SW component

Resourcesapk

+

….

Resourcesapk

ManifestFile

What an application is composed of?• Software components

• Activity• Fragment

• Service• Broadcast receiverBroadcast receiver• Content provider• Intent

• Resources • Pictures, video, audio file, etc.

• Accessed via an ID• Accessed via a manager.

Android applications• Every application runs in its own linux process (receivers

its own User ID)

• A process is created when a component of the application needs to be runneeds to be run

• An unusual feature of Android is that an application process’s lifetime is not directly controlled by the application (more on this soon)

• For example, if the application is temporary not visible the system may decide to kill the process

Software components - activity • The simplest application is composed of

a single activity that ‘inflates’ a UI, defined by an XML file (some similarity with HTML)

• An activity is an event-triggered software

User Interface

• An activity is an event-triggered software component staying behind a UI and managed by the operating system via callbacks or hooks

• It also reacts to user generated events coming from UI via handlers (e.g., push a button)

Activity

Software components -activity • The response time of an activity should be

small (<5s) otherwise the ANR message appears

• Multithreading is required to do slow work in background

User Interface

Activity

background

Activity

Software components - activity • An Activity has a state, {running , paused, stop} • The system can kill an activity in the pause or stop state

to reclaim resources• To assure consistency when a killed activity restarts, user

may implement callback methods to manage the may implement callback methods to manage the information that must persist

• These methods are called before killing or restarting the activity

RUNNING STOPKILLED

RUNNING

Software components - activity User

InterfaceUser

Interface

• Usually, inside an application one activity is ‘marked’ as MAIN (in the manifest file) and launched when a user touches the launching icon in the Home screen Activities

• However, an activity A can start another activity B

Activity ActivityINTENT

Software components - IntentUser

InterfaceUser

Interface

Matching Filter

• The activity can start another activity using a mechanism based on Intent and Filters

• An intent is a message directed either explicitly to another activity (by class name), or implicitly to any activity whose filter matches the intent’s action and data

Activity ActivityINTENTIntent

Software components - IntentUser

InterfaceUser

Interface

Matching Filter

• An Intent contains in fact the action to be performed and optionally data upon which to work

• The task of finding the right activity that can perform the action is called intent resolution

Activity ActivityINTENTIntent

Broadcast intent• System wide intent received by special component named

broadcast receivers that has been registered with the intent• Low battery• Chage in connectivity • Chage in connectivity • Etc..

• Asynchronous transmission• Ordered transmission

• in that it is sent to one receiver at a time where it can be processed and then either aborted or allowed to be passed to the next Broadcast Receiver.

•

Broadcast receiver• Broadcast Receivers are the mechanism by which applications

are able to respond to Broadcast Intents. • A Broadcast Receiver must be registered by an application and

configured with an Intent Filter to indicate the types of broadcast in which it is interested.

• When a matching intent is broadcast, the receiver will be invoked by the Android runtime regardless of whether the application that registered the receiver is currently running.

• The receiver then has 5 seconds in which to complete any tasks required of it (such as launching a Service, making data updates or issuing a notification to the user) before returning.

• Broadcast Receivers operate in the background and do not have a user interface.

Software comp – broadcast receiver• No UI

• Receive and react to broadcast announcement, or broadcast intents

Filter

Broadcast intent

or broadcast intents• BOOT_COMPLETED• ..

• It may start an activity, a service, or it may use the notification service to alert the user

Broadcast receiver

Filter

Services• Android Services are processes that run in the

background and do not have a user interface.

• They can be started and subsequently managed from Activities, Broadcast Receivers or other Services. Activities, Broadcast Receivers or other Services.

• Android Services are ideal for situations where an application needs to continue performing tasks but does not necessarily need a user interface to be visible to the user.

Services• Although Services lack a user interface, they can still notify the user

of events through the use of notifications and toasts (small notification messages that appear on the screen without interrupting the currently visible Activity) and are also able to issue Intents.

• Services are given a higher priority by the Android runtime than many other processes and will only be terminated as a last resort by the other processes and will only be terminated as a last resort by the system in order to free up resources.

• In the event that the runtime does need to kill a Service, however, it will be automatically restarted as soon as adequate resources once again become available.

• Example situations where a Service might be a practical solution include the streaming of audio that should continue when the application is no longer active, or a stock market tracking application that needs to notify the user when a share hits a specified price.

Software components - service • A service runs in

background and has not a UI

• Used to perform a long-running operation or to supply functionality for

User Interface

supply functionality for other applications to use.

• Activated explicitly, or via the intent/filter mechanism

• Can issue intents, notifications, or Toast message

Activity

INTENT Service

Software components: Service• System-level service

• WINDOW_SERVICE• The top-level window manager• LOCATION_SERVICE• controlling location (e.g., GPS) updates• controlling location (e.g., GPS) updates• CONNECTIVITY_SERVICE• Handling management of network connections• ….

• User defined• Intent Service (execute inside its own thread and dies)• Started Service• Bound Service

Notification• A service, running in the background, needs a way to let

users know something of interest has occurred, such as when email has been received.

• Moreover, the service may need some way to steer the • Moreover, the service may need some way to steer the user to an activity where they can act upon the event –reading a received message, for example.

• For this, Android supplies status bar icons, flashing lights, and other indicators collectively known as "notifications".

Software comp – content provider• The content provider is the data tier for Android applications

• Android ships with many content providers• File — Stores data such as browser bookmarks• Contacts — Stores user contacts• Contacts — Stores user contacts• SQLite db• …

SQLiteFile

CONTENT PROVIDER

Remote

Data store

Activity

Content providers• Content Providers implement a mechanism for the sharing of

data between applications.• Any application can provide other applications with access to

its underlying data through the implementation of a Content Provider including the ability to add, remove and query the data (subject to permissions). (subject to permissions).

• Access to the data is provided via a Universal Resource Identifier (URI) defined by the Content Provider. Data can be shared in the form a file or an entire SQLite database.

• The native Android applications include a number of standard Content Providers allowing applications to access data such as contacts and media files.

• The Content Providers currently available on an Android system may be located using aContent Resolver.

Resources • XML files defining:

• Layout (by tar the most important resource)• String• String array• Integer array• Integer array• Color• Styles…

• Binary image file (icon.png)• Stored in the /res/ directory• Accessed from the code through a symbolic ID • The mapping resource symbolic ID and resource is done

through a special class, called R

Assets• Accessed via an Asset Manager• Files that maintain their original raw format• Read the file as a stream of bytes.

Context• When an application is compiled, a class named R is

created that contains references to the application resources.

• The application manifest file and these resources combine to create what is known as the Application combine to create what is known as the Application Context .

• This context, represented by the Android Context class, may be used in the application code to gain access to the application resources at runtime.

• In addition, a wide range of methods may be called on an application’s context to gather information and make changes to the application’s environment at runtime.

Demo: my first application

My first application

Target API

Lowest API level

… from here, accept all the default options

Lowest API level required

Questions?


onCreate: Called when the activity is starting.

setContentView(): inflates the ‘layout’ inflates the ‘layout’


string.xml

style.xml

My first applicaition

activity_main.xml


@+id: creates an id called menu_settings

android programming - introduction · introduction • android is built on top of more than 100...

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