Introduction to Android

1. Introduction to Android • Overview of Android System • Android Components • Component lifecycles • Slides rely heavily on http://developers.android.com

2. 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

3. 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

4. Android OS Structure

5. 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

6. 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.

7. Views A contains B Contains C Contains H detail D detail I detail E detail J detail Fdetail K detail G detail L M • 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!

8. 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

9. 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.

10. 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.

11. 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)

12. 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>

13. 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()

14. Activity Lifecycle Activity Starts User navigates back to the activity OnCreate() OnStart() OnRestart() OnResume() Process is killed Activity comes to the Foreground Activity is running Another Activity in front Other Applications need memory OnPause() Activity is no longer visible Activity comes to the Foreground OnStop() OnDestroy() Activity is shut down

15. 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?