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Android programming course

By Võ Văn Hải

Faculty of Information Technologies

Asynchronous Techniques

Session objectives

• Introduction

• Asynchronous Techniques

o Thread

o Executor

o HandlerThread

o AsyncTask

o Service & IntentService

o AsyncQueryHandler

o Loader

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Introduction

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Lifecycles

Activity Service

BroadcastReceiver

ContentProvider

Android Components

Java Objects

BroadcastReceiver

Activity Service

ContentProvider

LinuxProcess

Threads

BG

BG

BG

UI

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ProcessProcess

ApplicationStart

ApplicationEnd

LinuxProcess

Native Thread Lifecycle

UI

BG

Time

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Example: Activity Lifecycle

Time

ActivityComponent

onCreate() onDestroy() onCreate() onDestroy()

ActivityObject

new() GC

ActivityObject

new() GC

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Example: Activity Lifecycle

Time

ActivityComponent

onCreate() onDestroy() onCreate() onDestroy()

ActivityObject

new()

ActivityObject

new()

Start

BG

Reference

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Example: Activity Lifecycle

Time

ActivityComponent

onCreate() onDestroy() onCreate() onDestroy()

ActivityObject

new()

ActivityObject

new()

Start

BG

Reference

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Background Threads

• A thread is a GC root

• Implement cancellation policy for your background threads!

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Use Cases

Get off the UI thread

early!

Get off the UI thread

early!

Do long running

task

Do long running

task

Get off the UI thread

early!

Get off the UI thread

early!

Report result or

update UI

Report result or

update UI

Do long running

task

Do long running

task

UIThread

BGThread

UIThread

BGThread

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Asynchronous Techniques

• Thread

• Executor

• HandlerThread

• AsyncTask

• Service

• IntentService

• AsyncQueryHandler

• Loader

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ThreadPlain old Java Thread

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Creating Tasks and Threads (1)

• Tasks are objects. To create tasks, you have to first define a class

for tasks. A task class must implement the Runnable interface

• You need to implement run method to tell the system how your

thread is going to run

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Creating Tasks and Threads (2)

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Or you can use Callable<V> to improve some problems

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Thread - Example

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Dis-advantages and using

• Pitfalls

o Non-retained threads

o Missing cancellation policy (runnable)

o Starting over and over again

• Good Use Cases

o One-shot tasks

• Post messages to the UI thread at end.

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ExecutorPowerful task execution framework.

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Executor and ExecutorService

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ExecutorExecutor

ExecutorService

ExecutorService

ThreadPoolExecutor

ThreadPoolExecutor

Task submission:

executorService.submit(MyTask);executorService.invokeAll(Collection<Tasks>);executorService.invokeAny(Collection<Tasks>);

Lifecycle management:

executorService.shutdown();executorService.shutdownNow();

Lifecycle observation:

executorService.isShutdown();executorService.isTerminated();executorService.awaitTermination();

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Task / Execution Environment

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Task:Independent unit of work executed anywhere

Runnablerun()

Runnablerun()

Callablecall()

Callablecall()

Execution Environment:

Technique used to execute the task

Executorexecute(Runnable)

Executorexecute(Runnable)

Future future = executorService.submit(Callable);

FutureisDone()

isCancelled()cancel()

FutureisDone()

isCancelled()cancel()

Task manager/observer:

Thread Pools

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Fixed Thread Pool Executors.newFixedThreadPool(3)

Cached Thread Pool Executors.newCachedThreadPool()

Single Thread Pool Executors.newSingleThreadExecutor()

Custom Thread Pool new ThreadPoolExecutor(corePoolSize, maximumPoolSize, aliveTime, unit, workQueue)

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Dis-advantages and using

• Pitfalls

o Lost thread safety when switching from sequential to

concurrent execution

• Good Use Cases

o Execute tasks concurrently

• Multiple Http requests

• Concurrent image processing

• Use cases gaining performance from concurrent execution.

o Lifecycle management and observation of task execution.

o Maximum platform utilization

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Handler Class

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t = new HandlerThread("BgThread");t.start();

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Handler h = new Handler(t.getLooper()) {@Overridepublic void handleMessage(Message msg) {

//Process message}

};

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Handler Class

• Inherits from Thread and encapsulates a Looper-object.

• Thread with a message queue and processing loop.

• Handles both Message and Runnable.

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HandlerThread

Create and Start

Add Process

Message Queue

1

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h.sendEmptyMessage(42);3

Looper and Handler

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Handler

• Runnable/Message submission:

o post(Runnable);

o postDelayed(Runnable);

o postAtTime(Runnable)

o postAtFrontOfQueue(Runnable);

o sendMessage(Message);

o sendMessageDelayed(Message);

o sendMessageAtTime(Message);

o sendMessageAtFrontOfQueue(Message);

• Runnable/Message removal:

o removeCallbacks(Runnable);

o removeMessages(Message)

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Using sendMessage

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Main Thread

Background Thread

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Example – using messgae

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Using post

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Good Use Cases

• Keep a thread alive

• Sequential execution of messages

• Avoid concurrent execution on multiple button clicks

• State machine

• Detailed control of message processing.

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AsyncTask Class

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AsyncTask Class

• AsyncTask enables proper and easy use of the UI thread. This

class allows to perform background operations and publish

results on the UI thread without having to manipulate threads

and/or handlers.

• AsyncTask is designed to be a helper class around Thread and

Handler and does not constitute a generic threading

framework.

• An asynchronous task is defined by a computation that runs on

a background thread and whose result is published on the UI

thread.

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How it works

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onCancelled()4b4b

UIThread

BGThread

new AsyncTask.execute()1

onPreExecute()2

3doInBackground()

onPostExecute()4a AsyncTask.cancel()

onProgressUpdate()

publishProgress()

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AsyncTask class

AsyncTask's generic types

1. Params, The type of the parameters sent to the task upon execution.

2.Progress The type of the progress units published during the background computation.

3.Result The type of the result of the background computation.33

Example

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Service

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Services

• A Service is an application component that can perform long-

running operations in the background and does not provide a user

interface.

• Another application component can start a service and it will

continue to run in the background even if the user switches to

another application.

• Additionally, a component can bind to a service to interact with

it and even perform interprocess communication (IPC).

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Services’s states

State Description

Started

A service is started when an application component, such asan activity, starts it by calling startService(). Oncestarted, a service can run in the background indefinitely,even if the component that started it is destroyed.

Bound

A service is bound when an application component binds toit by calling bindService(). A bound service offers a client-server interface that allows components to interact withthe service, send requests, get results, and even do soacross processes with interprocess communication (IPC).

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Service lifecycle

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Service’s call-back methods

Callback Description

onStartCommand()

The system calls this method when another component, such asan activity, requests that the service be started, by callingstartService().

onBind()The system calls this method when another component wants tobind with the service by calling bindService().

onUnbind()The system calls this method when all clients have disconnectedfrom a particular interface published by the service.

onRebind()The system calls this method when new clients have connectedto the service, after it had previously been notified that all haddisconnected in its onUnbind(Intent).

onCreate()The system calls this method when the service is first createdusing onStartCommand() or onBind().

onDestroy()The system calls this method when the service is no longer usedand is being destroyed.

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Creating Service

There are two classes you can extend to create a started service

• Service

o This is the base class for all services. When you extend this class, it's

important that you create a new thread in which to do all the service's

work, because the service uses your application's main thread, by default,

which could slow the performance of any activity your application is

running.

• IntentService

o This is a subclass of Service that uses a worker thread to handle all start

requests, one at a time. This is the best option if you don't require that

your service handle multiple requests simultaneously. All you need to do is

implement onHandleIntent(), which receives the intent for each start

request so you can do the background work.

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Creating ServiceExtends the IntentService class

• 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.

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Creating ServiceExtends the Service class

If you require your service to perform multi-threading (instead of

processing start requests through a work queue), then you can

extend the Service class to handle each intent.

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• Define your service in AndroidManifest.xml file

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Service restart behavior

• In its onStartCommand() method call, the service returns an int which

defines its restart behavior in case the service gets terminated by the

Android platform. You can use the constants, the most common options are

described by the following table.

Option Description

Service.START_STICKY

Service is restarted if it gets terminated. Intent datapassed to the onStartCommand method is null. Used forservices which manages their own state and do not dependon the Intent data.

Service.START_NOT_STICKY

Service is not restarted. Used for services which areperiodically triggered anyway. The service is onlyrestarted if the runtime has pending startService() callssince the service termination.

Service.START_REDELIVER_INTENTSimilar to Service.START_STICKY but the original Intentis re-delivered to the onStartCommand method.

You can check if the service was restarted via the Intent.getFlags() method. 46

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Running a service in its own process

• You can also specify that your Service runs in a separate process via

the android:process=":process_description" attribute.

• Running a service in its own process will not block the application in case

the service performs long running operations in its main thread. But as

the services runs in its own process, you need to use some interprocess

communication (IPC) to communicate to your service from other parts.

• Even if the service runs in its own process, you need to use

asynchronous processing to perform network access, because Android

does not allow network access in the main thread of a process. 47

Communicating with services

• Using Intent data: The service receives the intent data from the starting

Android component and performs its work

• Using receiver: You can also use broadcast events and registered receivers for

the communication.

• Activity binding to local service: If the service is started in the same process as

the activity, the activity can directly bind to the service.

• Handler and ResultReceiver or Messenger : If the service should be

communicating back to the activity, it can receive an object of type Messenger

via the Intent data it receives from the activity. If the Messenger is bound to a

Handler in the activity, the service can send objects of type Message to the

activity.

• AIDL for services in a different process :To bind to a service which runs in a

different process, you need to use Inter Process Communication (IPC) as the

data needs to be sent between different processes. 48

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Service communicationUsing BroadcastReceiver

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Service communicationUsing BroadcastReceiver

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AsyncQueryHandler

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AsyncQueryHandler

• Asynchronous operations on a ContentResolver

o Query

o Insert

o Delete

o Update

• Wraps a HandlerThread

• Cons

• No cursor management

• No content observation

• No data retention on configuration changes

• Background thread can’t be forced to quit

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How it works

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UIThread

BGThread

new AsyncQueryHandler();1

startQuery();startInsert();startUpdate();startDelete();

2

onQueryComplete();onInsertComplete();onUpdateComplete();onDeleteComplete();

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DB operations

AsyncQueryHandler sample class

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AsyncQueryHandler demo

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AsyncQueryHandler demo

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Loader

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Loader

• API added in Honeycomb Available in compatibility package

• Load data in a background thread

• Observes data changes

• Retained on configuration changes

• Connected to the Activity and Fragment lifecycles

• Pitfalls

o Data loading will stop when Activity/Fragment is destroyed

o Http requests not finished

• Good Use Cases

o Loading data from Content Providers.

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Basics

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Data Source

Loader

1 Load data in BG 2 Observe data

3 Lifecycle management 4 Retained on configuration change

Any Data Source

ContentProvide

r

CustomLoader

CursorLoader

How it works

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Summary

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Concurrent executionConcurrent execution

Thread Executor

HandlerThread

One shot tasksOne shot tasks

AsyncTask

Subsequent executionSubsequent execution

AsyncTask<Void, Void, Void>AsyncTask.execute(Runnable)AsyncTask<Void, Void, Void>AsyncTask.execute(Runnable)

One task with UI callbackOne task with UI callback

Service

Activity lifecycle independent tasksActivity lifecycle independent tasks

IntentService

“On demand”“On demand”“Continuous”“Continuous”

AsyncQuery

Handler

Asynchronous CRUDAsynchronous CRUD

Loader

Load data from CPData observingLoad data from CPData observing

Full CRUDFull CRUD

“newSingleThreadExecutor”Execute Message and RunnableBetter clean up

“newSingleThreadExecutor”Execute Message and RunnableBetter clean up

Execution abstractionRepetitive tasksConcurrent executionSequential executionExecution managementTask management

Execution abstractionRepetitive tasksConcurrent executionSequential executionExecution managementTask management

Task controlTask control

CustomLoaderCustomLoader

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