Design Patterns in the Android Framework

8/10/2019 Design Patterns in the Android Framework

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Design Patterns in the Android

Framework

Prof. Sheng-De Wang

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What are Design Patterns? In software engineering, a design patternis a

general reusable solution to a commonly

occurring problem in software design.

is not a finished design

is a description or template for how to solve aproblem that can be used in many different

situations.

Obect-orienteddesign patterns typically showrelationships and interactions between

classes or obects.

Design patterns are widely used in the process

of designing component-based systems 2


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Definitions of Software Engineering

!he application of engineering to software

"ield of computer science dealing withsoftware systems

large and comple#

built by teams

e#ist in many versions

last many years undergo changes

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$ore definitions %pplication of a systematic,

disciplined, &uantifiable

approach to the

development, operation, and

maintenance of software

'I((( )**+ $ulti-person construction

of multi-version software

'Parnas )*


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Software Deelopment !ife "ycle

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#b$ect-#riented !ife "ycle

$ore time is spent in analysis and design,

less time in implementation, testing and

maintenance.

/ife cycle phases intermi#

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#b$ect-oriented %##& design

!he system is viewed as a collection of

interacting obects.

!he system state is decentrali0edand each

obect manages its own state.

Obects may be instances of an obect class

and communicate by e#changing messages.

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## Design "oncepts

Design classes

(ntity classes

1oundary classes

2ontroller classes

Inheritance3all responsibilities of a superclass is immediately

inherited by all subclasses

$essages3stimulate some behavior to occur in the receiving

obect

Polymorphism3a characteristic that greatly reduces the effort

re&uired to e#tend the design

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Android Framework

%ndroid framewor4 is based on obect-orienteddesign

Part of %ndroid Operating Systems

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AndroidOperating

Systems

What is a

framework?


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An important component

Web5it

%n open source web page layout engine

6sed in %pple7s Safaribrowser, 8oogle7s 2hrome

1rowser, 9

2oded in 2::

Ported to ;o4ia Symbian OS, iPhone OS, %ndroid

OS, Palm7s WebOS, 9

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First android application

e'ample

Inheritance enables software reuse.

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pac4age com.android.webviewselloWeb=iew2lient'ow they distribute responsibility

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Design Pattern Space

Purpose

"reational Structural (ehaioral

Scope "lass Factory 4ethod Adapter %class& 3nterpreter2emplate 4ethod

#b$ect Abstract Factory(uilderPrototypeSingleton

Adapter %ob$ect&(ridgeCompositeDecoratorFacade

FlyweightProxy

"hain of1esponsibility"ommand3terator4ediator

4ementoObserverStateStrategyVisitor

Defer ob$ect creation to

another class

Defer ob$ect creation to

another ob$ect

Describe algorithms and

flow control

Describe ways to

assemble ob$ects

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Factory 4ethod Pattern!he "actory method lets a class defer

instantiation to subclasses.

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View

Activity

GraphicView

OnCreate()

yDraw

OnCreate()

!!!

View" #actoryetho$()

!!!

ret%rn new GraphicView

Android E'ample of Factory

4ethod Pattern

A d id E l f F


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Android E'ample of Factory

4ethod Pattern- code

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public class MyDraw extends Activity { @Override public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(new GraphicView(this));

}

}

public class GraphicView extends View{ private Paint paint= new Paint();

GraphicView(Context ctx) {

super(ctx);

}

@Override protected void onDraw(Canvas canvas) {

int line_x = 10;

int line_y = 50; canvas.drawColor(Color.WHITE);

paint.setColor(Color.GRAY);

paint.setStrokeWidth(3);

canvas.drawLine(line_x, line_y, line_x+120, line_y, paint);

}

}


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"omposite Pattern2omposite allows a group of obects to be treated in the

same way as a single instance of an obect.

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Android 5iew and 5iew6roup

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"


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4ore Android "omposite Pattern

E'ample

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#bserer Pattern

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A subset of the asynchronous publish/subscribe pattern


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#bserer Pattern E'ample

Subect

Observers

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4ore #bserer Pattern E'ample

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E'ample+ Stock 7uote Serice

Ceal time

$ar4et Data

"eed

Stoc4 Muotes

2ustomer

2ustomer

2ustomer 2ustomer

2ustomer

Observers

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4odel-5iew-"ontroller Pattern

%pplication of Observer Pattern

1enefits Design reuse, /oose 2oupling

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!he solid line representsa direct association, the

dashedan indirect

association via an

observer 'for e#ample.


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45" Pattern in Android

2ursorG model

/ist=iewG view

%ctivityG control

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SimpleCursorAdapter

ListAdapter

2ursor !e#t=iew /ist=iew

control


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4odel

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5iew

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"ontrol

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4 45" l


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4ore 45" e'ample@@ 8et a Spinner and bind it to an %rray%dapter that

@@ references a String array.

Spinner s) 'Spinner find=iew1yId'C.id.spinner)

Documents

Design Patterns in the Android Framework