Running in Background

Question 1

The main thread

Answer 1

Independent path of execution in a running program
Code is executed line by line
App runs on Java thread called “main” or “UI thread”
Draws UI on the screen
Responds to user actions by handling UI events

Question 2

Two rules for Android threads

Answer 2

1. Do not block the UI thread. The Main thread must be fast
   Complete all work in less than 16 ms for each screen
   Run slow non-UI work on a non-UI thread

If the UI waits too long for an operation to finish, it becomes unresponsive
The framework shows an Application Not Responding (ANR) dialog

2. Do not access the Android UI toolkit from outside
   the UI thread
   Do UI work only on the UI thread

Question 3

Background threads

Answer 3

Execute long running tasks on a background thread

AsyncTask
The Loader Framework
Services

Question 4

AsyncTask

Answer 4

Use AsyncTask to implement basic background tasks

Override two methods:
doInBackground()—runs on a background thread
All the work to happen in the background
onPostExecute()—runs on main thread when work done
Process results
Publish results to the UI

onPreExecute()
Runs on the main thread
Sets up the task

onProgressUpdate()
Runs on the main thread receives calls from
publishProgress() from background thread

Question 5

Limitations of AsyncTask

Answer 5

When device configuration changes, Activity is destroyed
AsyncTask cannot connect to Activity anymore
New AsyncTask created for every config change
Old AsyncTasks stay around
App may run out of memory or crash

Question 6

When to use AsyncTask

Answer 6

Short or interruptible tasks
Tasks that do not need to report back to UI or user
Lower priority tasks that can be left unfinished
Use AsyncTaskLoader otherwise

Question 7

Loader

Answer 7

Provides asynchronous loading of data from a content provider or other data source

BENEFITS:

• Reconnects to Activity after configuration change. LoaderManager handles configuration changes for you
• Callbacks implemented in Activity
• Can monitor changes in data source and deliver new data

Many types of loaders available:
CursorLoader - data from a ContentProvider
Loader or AsyncTaskLoader - to load data from some other source

Question 8

LoaderManager

Answer 8

Manages one or more Loader instances

getSupportLoaderManager().initLoader(0, null, this);

To start loading data from a loader, call either initLoader() or restartLoader(). The system automatically determines whether a loader with the same integer ID already exists and will either create a new loader or reuse an existing loader.

Question 9

AsyncTaskLoader

Answer 9

A Loader subclass, used to load data from a data source.

DIFERENCESE with ASYNTASK :
doInBackground() loadInBackground()
onPostExecute() onLoadFinished()

Question 10

Steps for AsyncTaskLoader subclass

Answer 10

1) Subclass AsyncTaskLoader
2) Implement constructor - to call super()
3) loadInBackground()
4) onStartLoading() - When restartLoader() or initLoader() is called, the LoaderManager invokes the onStartLoading() callback

Question 11

Implement loader callbacks in Activity

Answer 11

onCreateLoader()
onLoadFinished()
onLoaderReset()

1) onCreateLoader() — Create and return a new Loader for the given ID:
@Override
public Loader> onCreateLoader(int id, Bundle args) {
    return new StringListLoader(this,args.getString("queryString"));
}

2)onLoadFinished() — Called when a previously created loader has finished its load. Results of loadInBackground() are passed to onLoadFinished() where you can display them

3) onLoaderReset() — Called when a previously created loader is being reset making its data unavailable
Only called when loader is destroyed
Leave blank most of the time

Question 12

broadcast

Answer 12

— Broadcast are send using Implicit Intents : sendBroadcast() method—asynchronous
sendOrderedBroadcast()—synchronously

• –Can be received by any application registered for the intent
• – Used to notify all apps of an event

Question 13

broadcast receiver

Answer 13

——Listens for incoming intents sent by sendBroadcast().(.from In the background)

—–Intents can be sent
> By the system, when an event occurs that might change the behavior of an app
> By another application, including your own

Question 14

Broadcast receiver always responds

Answer 14

Responds even when your app is closed
Independent from any activity
When a broadcast intent is received and delivered to onReceive(), it has 5 seconds to execute, and then the receiver is destroyed

Question 15

Steps for creating a broadcast receiver

Answer 15

Subclass BroadcastReceiver
Implement onReceive() method
Register to receive broadcast
Statically, in AndroidManifest
Dynamically, with registerReceiver()
registerReceiver(mReceiver, mIntentFilter)

Question 16

Controlling permission sender

Other apps can send broadcasts to your receiver—use permissions to control this

Answer 16

void sendBroadcast (Intent intent, 
                    String receiverPermission)

Receivers must request permission with in AndroidManifest.xml

Question 17

Controlling permission receiver

Other apps can respond to broadcast your app sends

Answer 17

registerReceiver(BroadcastReceiver,
IntentFilter, String, android.os.Handler)
or in tag

Senders must request permission with in AndroidManifest.xml

Question 18

Local Broadcast Manager

Answer 18

For broadcasts only in your app
No security issues since no cross-app communication

LocalBroadcastManager.sendBroadcast()
LocalBroadcastManager.registerReceiver()

Question 19

Service

Answer 19

A Service is an application component that can perform long-running operations in the background and does not provide a user interface

Question 20

What are services good for?

Answer 20

Network transactions
Play music
Perform file I/O
Interact with a content provider

Question 21

Characteristics of services

Answer 21

Started with an Intent
Can stay running when user switches applications
Lifecycle—which you must manage
Other apps can use the service—manage permissions
Runs in the main thread of its hosting process

Question 22

Forms of services: started

Answer 22

Started with startService()
Runs indefinitely until it stops itself
Usually does not update the UI

Question 23

Forms of services: bound

Answer 23

Offers a client-server interface that allows components to interact with the service
Clients send requests and get results
Started with bindService() with an explicit Intent
Ends when all clients unbind

Multiple components can bind to the service at once, but when all of them unbind, the service is destroyed.

Question 24

Creating a service

Answer 24

Manage permissions

Subclass IntentService or Service class
Implement lifecycle methods
Start service from activity
Make sure service is stoppable

Question 25

Stopping a service
A started service
A bound service
IntentService

Answer 25

A started service must manage its own lifecycle
If not stopped, will keep running and consuming resources
The service must stop itself by calling stopSelf()
Another component can stop it by calling stopService()

Bound service is destroyed when all clients unbound

IntentService is destroyed after onHandleIntent() returns

Question 26

IntentService

Answer 26

Simple service with simplified lifecycle
Uses worker threads to fulfill requests
Stops itself when done

Ideal for one long task on a single background thread

Question 27

IntentService Limitations

Answer 27

Cannot interact with the UI
Can only run one request at a time
Cannot be interrupted

Question 28

What is Job Scheduler

Answer 28

Used for intelligent scheduling of background tasks

Question 29

Job Scheduler components

Answer 29

JobService—Service class where the task is initiated
JobInfo—Builder pattern to set the conditions for the task
JobScheduler—Schedule and cancel tasks, launch service

Question 30

JobService

Answer 30

Runs on the main thread

JobService subclass
Override
&raquo_space; onStartJob() –Implement work to be done here.
Called by system when conditions are met
&raquo_space; onStopJob() –Called if system has determined execution of job must stop… because requirements specified no longer met

Question 31

JobScheduler - Scheduling the job

Answer 31

Obtain a JobScheduler object form the system
Call schedule() on JobScheduler, with JobInfo object