Introduction to Android Development

Owain Lewisowainrlewis@googlemail.com

@Ow_Zone

Goals of this session

• Give an overview of the Android Ecosystem• Explain the core components of an Android

App• Drink beer

• Create a simple working Android Application• Drink more beer

Order of Service

Part 1: Introduction to Android Development (45mins)

Beer Break

Part 2: Live Coding – (45mins - or until it compiles)

Socialise

Android VM != JVM

Android Apps are:• Written in java (mostly)• Converted to .dex files

• Compiled to bytecode• Run on the Dalvik VM

What’s inside an App

• Compiled Java source code• Library dependencies e.g. MIME type lib• XML Files – UI layouts, fragments etc.

• Images, sounds, videos etc.• AndroidManifest.xml

Application Security

• Each App is assigned a user in Android linux • Each linux process has it’s own VM• Each Application runs in 1 process.

• Applications can request permissions to system resources etc. - but user must agree.

App Fundamentals: UI

App Fundamentals: UI

UI Design... Wrong talk

Application Resources

• Additional files and static content that your code uses

• Layouts for different types of devices and device orientations.

• Images for different screen sizes and densities

• Referenced by unique ID within code• Allow easy localization

Application Components

ApplicationManifest.xml

• Defines application components• Identifies required permissions• Defines required API level

• Definers hardware/software features required• API libs to link against (e.g. maps API)

• Components not in the manifest aren’t usable

Application Components

Intro to Activities

• Single screen with UI• Multiple activities exist in same app

• Activities work together but each is stand alone

• Any app can trigger any activity in any app (if allowed)

• More on that later…

Intro to Services

• Services generally run in the background• Perform work that a user isn’t aware of• Multiple components can make use of a

running service• Example: GPS service, mp3 playing

Intro to Content Providers

• Mechanism to allow apps to interact with data• Create, Query, Store, Update

• Can store data in multiple ways File System, DB, network storage, or anywhere else the App can

get to.

• Apps can query any Content Provider if they have permission

Intro to Broadcast Receivers

• A listener for system wide events• No UI component

• Tend to be lightweight and do little work• Example: Low battery handler

Intro to Intents

• Intent binds components to each other at runtime (e.g. Event mechanism)

• Explicit Intent = message to activate specific component by name

• Implicit Intent = message to activate type of component

• Components define which Intent Types they pick up with Intent Filters

Component Interoperability

• Any app can start any other apps component e.g. use the camera app activity to capture pictures– Don't need that code in your app– Components run in the process of the holder app (e.g.

camera take pic activity runs under camera process). – Android apps don't have single entry point (main

method)• Activating a component in another app is not

allowed by system permissions - so use Intent.

Activating Components

• Activity: pass Intent to startActivity(…) startActivityForResult() - gets a result

• Service: pass Intent to startService() or bindService() if running already

• Broadcast: pass Intent to sendBroadcast(), sendOrderedBroadcast() sendStickyBroadcast()

• Content: call query() on a ContentResolver

Make sense so far?

Activities

• One main per app – identified by Intent type• Each new Activity pushes previous to back

stack • Back button destroys current and pops

previous• Callback methods are how the system

interacts with Activities – can be triggered by Intents.

Activities – States

• Resumed/Running: up front and has focus• Paused: Another Activity is on top of this one -

but this is still visible.• Stopped: Backgrounded - still alive, not

attached to window manager• The OS can kill activities without focus – but

will restart if possible.

Activities - Lifecycle

Activities

• Must be declared in Manifest• Can declare IntentFilters – how other apps can

use the activity• E.g. Action=MAIN and Category=LAUNCHER

tells Android this can be launched.

Activities - Manifest

Activities – Starting

• Explicit Intent to start Activity:Intent intent = new Intent(this, SignInActivity.class);startActivity(intent);

• Implicit Intent to start Activity:Intent intent = new Intent(Intent.ACTION_SEND);intent.putExtra(Intent.EXTRA_EMAIL, recipientArray);startActivity(intent);

• Need a result?– Call startActivityForResult() then implement onActivityResult() callback– Get the result in an Intent

Activities - Stopping

• Call finish()

• To shutdown an activity you start call finishActivity(…) – but Android will do this for you.

• Saving State: onSaveInstanceState()

• Rotating the screen destroys current Activity - so good way to test restore logic.

Activities - Advanced

• Fragments– Sub Activity with own lifecycle– Can be an invisible worker

• Loaders (3.0 +)– Async pull data into Activity Views

Services

• Long running operations - no UI.– e.g. play music

• Service can run forever, but should know how to stop itself

• Started service = runs in background - does it's own thing until it stops

• Bound service = allows interaction/feedback. Destroyed when all components unbind.

• Determine type of Activity by implementing callbacks

Services - Callbacks

• onCreate()• onStartCommand()• onBind()– which returns IBinder = the defined interface for

comms.• onDestroy()

Services – Start/Stop

• startService(Intent)• stopSelf()

• bindService (Intent service, ServiceConnection conn, int flags)

• System destroys bound service – when clients disconnect.

• Note that system can kill service without warning - but will restart it

Services - Manifest

• Must declare all services in your application's manifest file.

• Supports IntentFilters and permissions.

Services - Threads

• Services use the Main UI thread – this is bad.• Your service should create threads it needs to

handle work– If you want to handle multiple thread

simultaneously you need to manage this here.– You need to manage stopping the thread too

• Alternative is to extend IntentService – offers worker per request – one at a time.

Services - IntentService• Creates a default worker thread that executes all intents

delivered to onStartCommand() separate from your application's main thread.

• Creates a work queue that passes one intent at a time to your onHandleIntent() implementation, so you never have to worry about multi-threading.

• Stops the service after all start requests have been handled, so you never have to call stopSelf().

• Provides default implementation of onBind() that returns null.• Provides a default implementation of onStartCommand() that

sends the intent to the work queue and then to your onHandleIntent() implementation.

Services – User Comms

• Services can communicate with the user in a limited way.

• Once running, a service can notify the user of events using Toast Notifications or Status Bar Notifications.

• Toast notification is a message that appears on the surface of the current window for a moment then disappears.

• Status bar notification provides an icon in the status bar with a message, which the user can select in order to take an action (such as start an activity).

Services- Advanced

• Messengers• AIDL

Broadcast Receivers

• Component that responds to system-wide broadcast announcements

• Many broadcasts originate from the system• Register for intents via Manifest• Meant to do very little work

Broadcast Receivers

• Receiving a broadcast involves:– Registering receiver in Manifest or dynamically– Implementing:

• onReceive(Context, Intent)

Broadcast Receivers

• Normal broadcasts: Context.sendBroadcast()

– Asynchronous. – All receivers of the broadcast are run in an undefined

order, often at the same time. • Ordered broadcasts:

Context.sendOrderedBroadcast()– Delivered to one receiver at a time. – Each receiver executes in turn, and can propagate a

result to the next receiver, or abort the broadcast so that it won't be passed to other receivers. Similar to Filters in JEE

Content Providers

• Manage secure access to data - across process boundaries– Don't need one if you're just providing data to

own app– Use ContentResolver to access data in a

ContentPtorvider• -Android includes ContentProviders for

contacts, audio, video, images etc

Content Providers

• Data can be file (video etc) or Structured (tabular)• Data is provided as tables (similar to relational dbs) with

primary keys (if needed)• Content is referenced using a URI scheme

– e.g. content://user_dictionary/words– Need to ask for permission to use a provider - this is done in

the manifest• Define a query to perform against the URI.

– Returns a Cursor– Cursor can be wrapped in a CursorAdaptor and bound to a UI

element.

Content Providers - Query

Content Providers

• Also can insert, update and delete.

• Batch access = process lots of data at a time

• Contract Classes = Constants used in accessing a ContentProvider.

Content Providers

Intent and IntentFilter• Activities, Services and Broadcast receivers are activated through Intents• Intent is a passive bundle of information. OS figures out how to get it to

the correct place.• Intent is:

– Optional component name– Action (e.g. ACTION_BATTERY_LOW) - you can invent your own– Data - URI of data and MIME type– Category - type of Intent (e.g. CATEGORY_LAUNCHER)– Extras - key/value pairs– Flags - how to deal with the intent

• Explicit Intent = named• Implicit Intent = category of intents

Intent and IntentFilter

• IntentFilter tells the system which type of Intent a component can handle

• Component can specify multiple filters• Filter has fields that parallel the action, data,

and category fields of an Intent object• Intent must match all 3 fields to be delivered– Note that Intent may not specify a field…

IntentFilter Matching

• Action: Intent action must match at least one Filter action. Intent can leave action blank and match

• Category: Every category in the Intent object must match a category in the filter. Filter can list additional categories, but it cannot omit any that are in the intent.– android.intent.category.DEFAULT must be implimented

• Data: – No data = match– More complex rules for data matching if specified

IntentFilter Matching

• Multiple filter match: Ask the user

Intent and IntentFilter

Processes and Threads

• One process per app. One Main thread is created for the App process – this talks to the UI aka UIThread

• General Rules:– Do not block the UI thread – Do not access the Android UI toolkit from outside the UI

thread • Workarounds: Create own threads and use

Activity.runOnUiThread(Runnable)View.post(Runnable)View.postDelayed(Runnable, long)

Processes and Threads

AsyncTask

• AsyncTask allows you to perform asynchronous work on your user interface. It performs the blocking operations in a worker thread and then publishes the results on the UI thread, without requiring you to handle threads and/or handlers yourself.

• Subclass AsyncTask and implement:– doInBackground()– onPreExecute, onPostExecute(), onProgressUpdate()

• Run Task from UIThread by calling execute()

AsyncTask

Development Tools

• Android Platform + SDK

• Android Developer Tools (ADT) for Eclipse

• Android Studio - Based on IntelliJ IDEA• Currently in Early Access release

• Uses Gradle

• Emulators and Hardware debugging

Questions?

Beer!

Live Coding

Introduction to Android Development

Owain Lewisowainrlewis@googlemail.com

@Ow_Zone