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Android Development - 2

Prabhaker Mateti

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• Android.os classes, debug• java.io refresher• Activities and Tasks• Processes and Threads• Android Location Service• Google Maps External Library• Code Example: SimpleNetworking• Code Example: Android Location Service

Agenda

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• Provides basic operating system services, message passing, and inter-process communication on the device.

• Handler.Callback• Ibinder interface for a remotable object RPC• IBinder.DeathRecipient• Iinterface Base class for Binder interfaces. • Parcelable cf. serializable, marshalled result• RecoverySystem.ProgressListener

android.os interfaces

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• AsyncTask<Params, Progress, Result> enables proper use of the UI thread.

• BatteryManager• Binder remotable object; RPC mechanism defined by

IBinder. • Bundle A mapping from values to Parcelable types. • ConditionVariable a locking paradigm. • Environment• MemoryFile wrapper for the Linux ashmem (Anonymous

SHared MEMory) driver.

android.os classes

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• Parcel Container for a message (data and object references) that can be sent through an IBinder.

• PatternMatcher A simple pattern matcher; not full reg-exp, only simple globbing.

• RecoverySystem the separate partition that can be used to install system updates, wipe user data, etc.

• StrictMode while developing …

android.os classes

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• debugging functions for Android applications, including tracing and allocation counts.

• startMethodTracing(String traceName, int bufferSize, int flags)

• dumpService (String name, FileDescriptor fd, String[] args)

• getMemoryInfo (Debug.MemoryInfo memoryInfo)

• threadCpuTimeNanos ()

android.os.Debug

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• public FileOutputStream (File file, boolean append)

• public void write (byte[] buffer, int offset, int byteCount)

• public void close ()• BufferedOutputStream

java.io.FileOutputStream

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• FileInputStream(String path) Equivalent to new FileInputStream(new File(path)).

• public int read (byte[] buffer, int offset, int byteCount)

• public void close ()• BufferedInputStream.

java.io.FileInputStream

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• Activity – represents a single screen with a user interface.

• Service – runs in the background; Long-running; for remote processes– no user interface.

• Content provider – manages a shared set of application data.

• Broadcast receiver – responds to broadcast announcements.

• An application can have multiple instances of the above four types.• Each component is a different point through which the system can enter

an application.• Every component has a managed lifecycle.

Application Components

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• App Widgets are miniature application views that can be embedded in other applications (such as the Home screen) and receive periodic updates.

• App Widgets provide users access to some of your application features directly from the Home screen (without the need to launch an activity)

• App Widgets are backed by a special kind of broadcast receiver that handles the App Widget lifecycle

package android.widget

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• similar to a regular Android service. • consumes CPU time, memory, battery

• onCreateEngine() whose goal is to create a WallpaperService.Engine.

• source code example: android-sdk-linux_x86/ docs/resources/samples/CubeLiveWallpaper

Live Wallpapers

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Activity

• An Activity is an application component that provides a UI screen – e.g., dial the phone, take a photo, send an email,

or view a map. • Each activity is given a window.– typically fills the screen, but – may be smaller than the screen– float on top of other windows

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Activities

• One activity can start another, including one defined in a different application.

• Context.startActivity(Intent)• Activity.startActivityForResult (Intent,

Request_Code)• Asynchronous Message (Intent)

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• extends java.lang.Object– Interface to global information about an application environment.– implementation is provided by the Android system.

• Selected methods1. File getExternalFilesDir(String type)2. FileOutputStream openFileOutput(String name, int mode)3. Intent registerReceiver (BroadcastReceiver, IntentFilter, String

broadcastPermission, Handler scheduler)4. void sendBroadcast(Intent intent, String receiverPermission)5. void startActivities(Intent[] intents)6. Object getSystemService(String name)7. ComponentName startService(Intent service)

public abstract class Context

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Activities vs Tasks (Apps)• A concrete class in the API• An encapsulation of a

particular operation• They run in the process of

the .apk which installed them

• Optionally associated with a window (UI)

• Have an execution Context

• More of a notion than a concrete API entity

• A collection of related Activities

• Capable of spanning multiple processes

• Associated with their own UI history stack

• What users on other platforms know as “applications”

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• public static final int FIRST_APPLICATION_UID • public Process ()• public static final void killProcess (int pid)• public static final void sendSignal (int pid, int

signal)• public static final int getThreadPriority (int tid)• public static final void setThreadPriority (int

priority)

android.os.Process

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Processes

• When the first of an application's components needs to be run, Android starts a Linux process for it with a single thread of execution (Main Thread).

• Android may decide to kill a process to reclaim resources.

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Processes• We can specify a process where an individual component

should run by setting a process name to “process” attribute of <activity>, <service>, <receiver>, or <provider>.– Each component can run in its own process.– Some components share a process while others do not.– Components of different applications also can run in the same

process.• We can set a default value that applies to all components

by setting a default process to “process” attribute of <application>.

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Main Thread

• All components are instantiated in the main thread (aka UI thread) of the specified process.

• System calls to the components are dispatched from the main thread.

• Methods that respond to those calls always run in the main thread of the process.

• Main thread components should not perform long or blocking operations (e.g., network downloads, heavy computation loops)

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Worker Threads

• Anything that may not be completed quickly should be assigned to a different thread.

• Threads are created in code using standard Java Thread objects.

• android.os.Looper for running a message loop within a thread

• android.os.Handler for processing messages• android.os.HandlerThread for starting a new

thread that has a looper.CEG436: Mobile Computing (PM)

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• If the UI thread is blocked for more than a few seconds– "application not responding" (ANR) dialog

• Andoid UI toolkit is not thread-safe. • Two rules to follow:– Do not block the UI thread– Do not access the Android UI toolkit from outside

the UI thread

Thread Issues

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• Perform asynchronous work on user interface. – Does 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.• taskA extends AsyncTask• implement the doInBackground()

– runs in a pool of background threads.– implement onPostExecute(), which delivers the result from the

above and runs in the UI thread• run the taskA by calling execute() from the UI thread.

AsyncTask

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Linux v Android Process Basics

• Android process == Linux process– w/ its own unique UID

• By default, 1 process per .apk• By default, 1 thread per process• Most components interleave events into the

main thread

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Process Lifecycles

• Android tries to maintain a process for as long as possible, but eventually it may have to remove some processes when memory runs low.

• To determine candidates to be killed, Android places each process into an "importance hierarchy“ of 5 levels based on the components running in it and the state of those components.

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Foreground process• A process that is required for what the user is currently doing. • A process is considered to be in the foreground if it hosts:

– an Activity that the user is interacting with– a Service that's bound to the activity that the user is interacting with.– a Service that has called startForeground(). – a Service that's executing one of onCreate(), onStart(), or onDestroy().– a BroadcastReceiver that's executing its onReceive() method.

• Generally, only a few foreground processes exist at any given time.– They are killed only as a last resort—if memory is so low that they cannot

all continue to run. – Generally, at that point, the device has reached such a state that killing

some foreground processes is required to keep the user interface responsive.

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• A process that does not have any foreground components, but still can affect what the user sees on screen.

• A process is considered visible if it hosts:– an Activity that is not in the foreground, but is still visible

to the user (its onPause() method has been called).– a Service that's bound to a visible (or foreground) activity.

• A visible process is considered extremely important and will not be killed unless doing so is required to keep all foreground processes running.

Visible process

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• A process that is running a service that has been started with the startService() method and does not fall into either of the two higher categories.

• Service processes are not directly tied to anything the user sees. However, they are generally doing things that the user cares about (such as playing music in the background or downloading data on the network), so the system keeps them running unless there's not enough memory to retain them along with all foreground and visible processes.

Service process

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• A process holding an activity that's not currently visible to the user (the activity's onStop() method has been called).

• These processes have no direct impact on the user experience, and the system can kill them at any time to reclaim memory for a foreground, visible, or service process.

• Usually there are many background processes running, so they are kept in an LRU (least recently used) list to ensure that the process with the activity that was most recently seen by the user is the last to be killed. If an activity implements its lifecycle methods correctly, and saves its current state, killing its process will not have a visible effect on the user experience, because when the user navigates back to the activity, the activity restores all of its visible state.

Background process

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• A process that doesn't hold any active application components.

• The only reason to keep this kind of process alive is for caching purposes, to improve startup time the next time a component needs to run in it. The system often kills these processes in order to balance overall system resources between process caches and the underlying kernel caches.

Empty process

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Activities and Tasks

• A task is a collection of related Activities.• It is capable of spanning multiple processes.

Application1 (.apk)

Process

Activity Activity

Activity Activity

Content Provider

Service Service

Application2 (.apk)

Process

Activity Activity

Activity Activity

Content Provider

Service Service

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Activities and Tasks• All activities in a task are arranged in a stack.

• If one activity starts another, the new activity is pushed on the stack and it becomes the running activity.

• When the user presses the BACK key, the current activity is popped from the stack and the previous one resumes.

Instance of Activity B

Instance of Activity C

Instance of Activity B

Instance of Activity A

A Stack

The one that began the task (typically, an activity the user selected in the

application launcher)

The one that's currently running

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Affinities

• An affinity means a preference for each activity to belong to a certain task.

• An individual affinity can be set for each activity:

• By default, a new activity is launched into the task of the activity that called startActivity().

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Affinities

• If the Intent object passed to startActivity() contains the FLAG_ACTIVITY_NEW_TASK flag,– If there's already an existing task with the same

affinity as the new activity, the activity is launched into that task.

– If not, it begins a new task.• allowTaskReparenting == true it can move

from the task it starts in to the task it has an affinity for when that task comes to the fore.

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Launch Modes

• standard (default)• singleTop• singleTask• singleInstance

• A launch mode can be set for each activity• The modes differ from each other on four

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Launch Mode Differences-1

• Which task will hold the activity that responds to the intent

New Activity

Activity A

Root Activity

Original Task

Activity A

Root Activity

Original Task

New Activity

New Task

standard/singleTopwithout

FLAG_ACTIVITY_NEW_TASK

singleTask/singleInstance

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Launch Mode Differences-2

• Whether there can be multiple instances of the activity

• A "standard" or "singleTop" activity can be instantiated many times.• A "singleTask" or "singleInstance" activity is limited to

just one instance.

Activity B

Activity A

Task A

Activity D

Task B

Activity B and Activity C are

standard/singleTop

Activity C

Activity B

Activity C Activity B

Activity A

Task A

Activity C

Task B

Activity C is singleTask or singleInstance

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Launch Mode Differences-3

• Whether the instance can have other activities in its task

"standard" • These modes permit multiple activities to belong to the task.• A "singleTask" activity will always be the root activity of the task.

"singleTop"

"singleTask"

"singleInstance" • An activity stands alone as the only activity in its task.

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Launch Mode Differences-4a

• Whether a new instance of the class will be launched to handle a new intent

Original Task

Activity B

Activity A

Activity C

Activity DAn intent arrives for an activity of

type D

Activity B

Activity A

Activity C

Activity D

Activity D

If D is"standard"

Activity B

Activity A

Activity C

Activity D

If D is"singleTop"

The existing instance D is expected to handle the new intent (since it's at the top of the stack)

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Launch Mode Differences-4b

• Whether a new instance of the class will be launched to handle a new intent (Cont)

Original Task

The existing instance B is not expected to handle the new intent (since it's not at the top of the stack)

Activity B

Activity A

Activity C

Activity DAn intent arrives for an activity of

type B

Activity B

Activity A

Activity C

Activity D

If B is"standard"

Activity B

Activity A

Activity C

Activity D

If B is"singleTop"

Activity B Activity B

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Launch Mode Differences-4c

• Whether a new instance of the class will be launched to handle a new intent (Cont)

Activity B

Original Task

An intent arrives for an activity of

type B

If B is"singleInstance"

A "singleInstance"

activity is always at the

top of the stack, so it is always in

position to handle the

intent.

Activity B

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Launch Mode Differences-4d

• Whether a new instance of the class will be launched to handle a new intent (Cont)

Activity B

Original Task

An intent arrives for an activity of

type B

If B is"singleTask"

Activity B can handle the

intent since it is in position.

Activity B

Activity A Activity A

Activity B

Original Task

An intent arrives for an activity of

type B

If B is"singleTask"

Activity B cannot handle the

intent since it is not in position

and the intent is dropped.

Activity B

Activity A Activity A

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Clearing the Stack

• Default Control– If the user leaves a task for a long time, the system clears the

task of all activities except the root activity.• If alwaysRetainTaskState is set to the root activity– The task retains all activities in its stack even after a long

period.• If clearTaskOnLaunch is set to the root activity– The stack is cleared down to the root activity whenever the

user leaves the task and returns to it.– The user always returns to the task in its initial state, even

after a momentary absence.

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Clearing the Stack

• If finishOnTaskLaunch is set to an activity of a task– The activity remains part of the task only for the current

session.– If the user leaves and then returns to the task, it is no

longer present.• If an intent includes the

FLAG_ACTIVITY_CLEAR_TOP flag and the target task already has an instance of the type of activity that should handle the intent in its stack, all activities above that instance are cleared away.

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Activity Lifecycle• Running state: An activity is in the foreground of the screen

(at the top of the activity stack for the current task).• Paused state: An activity has lost focus but is still visible to

the user.• Stopped state: An activity is completely obscured by

another activity.– It still retains all state and member information.

• If an activity 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.

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Activity Lifecycle• onCreate()

– when the activity is first created, or – when the activity was killed

• onStart()– just before the activity becomes visible to user

• onRestart()– after the activity has been stopped, just prior to it being

started again• onResume()

– just before the activity starts interacting with the user– At this point, the activity is at the top of the activity stack,

with user input going to it.• onPause()

– when the system is about to start resuming another activity– This method is typically used to commit unsaved changes to

persistent data, stop animations and other things that may be consuming CPU, and so on.

• onStop()– when the activity is no longer visible to the user– This may happen because it is being destroyed, or because

another activity has been resumed and is covering it.• onDestroy()

– Called before the activity is destroyed

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Activity Lifecycle• Three nested loops for the entire lifecycle

• Visible Lifetime– During this time, the user can see the activity on screen– onStart() and onStop() can be called multiple times, as the activity

alternates between being visible and hidden to the user.• Foreground Lifetime

– During this time, the activity is in front of all other activities on screen and is interacting with the user.

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Saving activity state

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Service Lifecycle

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Service Lifecycle

• The service is started by calling – Context.startService()

• Runs until someone, including itself, calls– Context.stopService()

• Clients establish a connection to the Service object and use that connection to call into the service.– established by Context.bindService()– closed by Context.unbindService()

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Broadcast Receiver Lifecycle

• Only single callback method• onReceive(currentContext, Intent broadcastMsg)• When a broadcast message arrives for the

receiver, Android calls the method and passes it the Intent object containing the message.

• A process with an active broadcast receiver is protected from being killed but a process with only inactive components can be killed by the system at any time.

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Android Location Service

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Determining User Location• Multitude of location sources GPS, Cell-ID, and Wi-Fi can

each provide a clue to users location. Determining which to use and trust is a matter of trade-offs in accuracy, speed, and battery-efficiency.

• User movement Account for movement by re-estimating user location every so often.

• Varying accuracy Location estimates coming from each location source are not consistent in their accuracy. A location obtained 10 seconds ago from one source might be more accurate than the newest location from another or same source.

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android.location

• Address A class representing an Address, i.e, a set of Strings describing a location.

• Geocoder A class for handling geocoding and reverse geocoding.

• GpsStatus.NmeaListener Used for receiving NMEA sentences from the GPS.

• LocationManager This class provides access to the system location services.

• For more information, read the guide to Obtaining User Location.

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android.location.LocationManager• LocationManager lcm = (LocationManager)

getSystemService(Context.LOCATION_SERVICE);• listener = new LocationListener() { … }• lcm.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, a, b,

listener);– NETWORK_PROVIDER == cell phone tower and Wi-Fi– GPS_PROVIDER– a == minimum time interval between notifications; 0 is ok– b == minimum change in distance between notifications; 0 is ok

• public void onLocationChanged(Location ltn) { makeUseOfNewLocation(ltn); // your method}

• public void onStatusChanged(String provider, int status, Bundle extras) {…}• public void onProviderEnabled(String provider) {…}• public void onProviderDisabled(String provider) {…}

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Location Use Model for Performance

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Location Trade-Offs

• Long windows of listening for location fixes can consume a lot of battery power, but short periods might not allow for sufficient accuracy.

• locationManager.removeUpdates(listener);• locationManager.getLastKnownLocation(pro);• The most recent fix is not always the best.– Choose location fixes based on several criteria.

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Tagging content with location

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Help decide where to go

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Providing Mock Location Data

• Eclipse– Window > Show View > Other > Emulator Control

• DDMS– Using the Emulator Console send coordinates:• geo fix -121.45356 46.51119 4392• geo nmea $GPRMC,…

– Use a GPX file describing a route for playback.– Use a KML file describing individual place marks.

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geocoding

• Geocoder gcd = new Geocoder( context, Locale. getDefault());List<Address> addresses = gcd. getFromLocation(latit, longi, 1);if (addresses.size() > 0) System.out.println( addresses.get(0).getLocality());

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GPS

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• com.google.android.maps• Set up a new Android project — or reconfigure an existing one — to build

against the installed Google APIs add-on• Set up an Android Virtual Device configuration that uses a the Google APIs add-

on• Add a uses-library element to your application's manifest file, to reference the

Maps library. – <uses-library android:name= "com.google.android.maps" />

• Use the Maps classes in your application• Get a Maps API Key, so that your application can display data from the Google

Maps service.• Sign your application properly, using the certificate that matches your API Key.• Example: <sdk>/add-ons/google_apis-<api-level>/samples/MapsDemo

Google Maps External Library

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Selected APIs

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android.hardware

• Camera class is used to set image capture settings, start/stop preview, snap pictures, and retrieve frames for encoding for video.

• GeomagneticField estimates magnetic field at a given point on Earth, and to compute the magnetic declination from true north.

• Sensor Class representing a sensor. SensorEvent holds information such as the sensor's type, the time-stamp, accuracy and sensor's data. SensorManager lets you access the device's sensors.

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android.bluetooth

• BluetoothAdapter Represents the local Bluetooth radio. Entry-point for all Bluetooth interaction. Instantiate a BluetoothDevice using a known MAC address, and create a BluetoothServerSocket.

• BluetoothProfile.ServiceListener An interface for notifying BluetoothProfile IPC clients when they have been connected or disconnected to the service.

• BluetoothSocket A connected or connecting Bluetooth socket.

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android.nfc

• NfcManager This is the high level manager, used to obtain this device's Near Field Communication (NFC) NfcAdapter. Acquire an instance: getSystemService(String)

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android.net

• ConnectivityManager Class that answers queries about the state of network connectivity.

• LocalSocketAddress A UNIX-domain (AF_LOCAL) socket address.

• NetworkInfo Describes the status of a network interface of a given type (currently either Mobile or Wifi).

• Uri.BuilderHelper class for building or manipulating URI references.

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android.net.wifi

• WifiConfigurationA class representing a configured Wi-Fi network, including the security configuration.

• WifiManagerThis class provides the primary API for managing all aspects of Wi-Fi connectivity.

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Device Administration

• DeviceAdminReceiver Can interpret raw intent actions that are sent by the system.

• DevicePolicyManager manages policies for one or more DeviceAdminReceivers

• DeviceAdminInfo specify metadata for a device administrator component.

• Device Administration API sample

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References

• http://developer.android.com•

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