callbacks singelton and wrappers

7/25/2019 Callbacks Singelton and Wrappers

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Copyright W. Howde 1

Lecture 14: Callbacks,

Singletons and Wrappers


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Callback attern

! Conte"t: controlling entity # calls$uses controlled

entity %

! roble&:' % can ha(e no control o(er #

' How can % initiate so&ething that is under the control

o) # without taking control o) #*

! Solution: # gi(es % a pointer to one o) its

)unctions to call, or an ob+ect to e"ecute-, when

the situation occurs


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Copyright W. Howde

Callback /b+ects

! Controller # will call a &ethod %.b0a incontrollee % with an ob+ect a

! a &ust ha(e an e"pected &ethoda.callback0 that is known to % and whichwill be called by % when the callback

situation arises! # &ust know the na&e o) the &ethod %.b0in % to call


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Callbacks and 3 Separation

! odel 3iew separation architecture0discussed in

architecture lecture

' separation )acilitates change o) look and )eel- whereodel is retained but 3iew changed

' controller is (iew, controllee is &odel

' (iew posts callback ob+ect with &odel by calling

appropriate &ethod in &odel' callback ob+ect &ust ha(e e"pected callback &ethod


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Copyright W. Howde

5S Callback 6"a&ple

! 7re8uent 5atee 7eature

' nor&ally 9; will call get#5ate0 in 5L when user

indicates this user option' get#5ate0 will return date in)or&ation which 9; will

display

' i) the )ound date has been dated &any ti&es, we want

5L to display a 9; &essage 0i.e. e"ecute so&ething inthe 9;

' 9; registers a 7re8uent5atee call back ob+ect w


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Copyright W. Howde <


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Copyright W. Howde =

6(ent 5elegation and Callbacks

! Co&ponents and e(ents, e.g. button b

! /ptions

i i) b is pushed, b>s special response &ethod iscalled

ii other code tells b what to do when button ispushed. ?egister- a callback0actionListener ob+ect " with b by e"ecuting

b.add#ctionListener0"@ Callback pattern* sort o)


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Copyright W. Howde A

?elated atterns

! Can be used )or Call%ack

'ublisher$Subscriber

! 9eneral pattern

'/bser(er$/bser(able

! Supported by classes in Ba(a library


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Copyright W. Howde

ublisher@Subscriber

! Special 6(ent anager ob+ect

! /b+ects interested in an e(ent, register with

e(ent &anager! /b+ects who recogniDe e(ent, publish

occurrence o) e(ent with e(ent &anager

! 6(ent &anager in)or&s subscribers'6"pected to ha(e a speci)ic &ethod that will be

called


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Copyright W. Howde 1E

/bser(er$/bser(able

! /bser(ers register with /bser(able

! /bser(able recogniDes e(ent and calls

speci)ied &ethod in obser(er

! Ba(a /bser(er

';nter)ace class, i&ple&ented by obser(ers

! Ba(a /bser(able

'Class, e"tended by callback caller classes


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Ba(a /bser(er Class

! ;&ple&enters &ust ha(e an update0 &ethod

which the /bser(able ob+ect " will call

'update0/bser(able ", /b+ect y

! When /bser(able ob+ect " calls the update0

&ethod )or a registered obser(er, it passes

itsel) as an identi)ier and also a data ob+ect y


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Ba(a /bser(able Class

! sers o) this class e"tend it

! ;nherited &ethods )ro& /bser(able

'add/bser(er0/bser(er D

! #dds the ob+ect D to the obser(able>s list o)

obser(ers

'noti)y/bser(er0/b+ect y! Calls the update0 &ethod )or each o) the obser(ers

in its list o) obser(ers, passing y as the data ob+ect


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Singleton attern

! Conte"t: class )or which we only want a single

instance, and to which we &ay want global access

! Solution:' re)erence the ob+ect globally through its class de)inition

' class (ariable called instance- whose (alue is an

instance o) the ob+ect

' static &ethod called instance0 that

! Creates an instance i) not created yet

! ?eturns the instance o) the class


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5S Singleton 6"a&ple

! Want to ha(e a globally accessible ob+ect

that records session history statistics

! 9ets updated )ro& di))erent places, e.g.

when di))erent kinds o) &essages are

generated

! se the singleton pattern with a Historyclass


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5S Singleton History Class


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Copyright W. Howde 1s constructor takes an adaptee ob+ect as anargu&ent

' #daptor &ethods re)erence the adaptee ob+ect (ia itsadaptee class &ethods


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Copyright W. Howde 1A

#dapter @ ;nheritance

Client

+request'()

interface

Target

+request'()

Adaptor

+request()

Adaptee


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#daptor @ Co&position

+Adaptor(in adaptee)+request'()

-Adaptee: adaptee

Adaptor

+request()

Adaptee: adaptee

Client

+request'()

interface

Target


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Copyright W. Howde 2E

#daptor 6"a&ple '

ri&iti(e 5ata ?ei)ication! /b+ecti)ication o) pri&iti(e data types in

Ba(a, i.e. wrap an instance in a class

! Why*! When pri&iti(e is used as a para&eter, it will be

passed as call by (alue.

' Cannot change its (alue inside the &ethod

! ay want to add to )unctionality,

' e.g. ;nt is the ob+ecti)ication )or int

' ;nt.parse;nt0String s will return the int )or s.


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ser 5e)ined

?ei)ication 6"a&ple! ;nter)ace

%ool?e);nter)ace G set0boolean b boolean get0 I

! #daptee @ boolean (alue! #daptor

class %oolean?e) i&ple&ents %ool?e);nter)ace

G public boolean (al

public %oolean?e)0boolean "Gthis.(al J "Ipublic set0boolean b Gthis.(al J bI

public get0 Greturn this.(alI

I


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#daptor 6"a&ple '

;nter)ace #daptors! ;n order to carry out certain operations, a

class &ay be re8uired to i&ple&ent a

special inter)ace! ;nter)ace &ay contain &any )unctions only

a )ew o) which are nor&ally rele(ant

! Fedious to continually supply nullde)initions )or the unneeded )unctions


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;nter)ace #daptor Strategy

! Construct a utility class that i&ple&ents the

inter)ace and supplies null de)initions )or all

the &ethods! ser e"tends the utility adaptor class

instead o) i&ple&enting the inter)ace

! ser supplies de)initions )or the &ethods o)interest


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Ba(a ;nter)ace #daptor 6"a&ple

! WindowListener inter)ace' i&ple&ented by any ob+ect that wants to register itsel)

as a listener )or e(ents generated by a Window

' = &ethods in inter)ace

' window closing e(ent that occurs when user clicks " inupper right corner re8uires de)inition )orwindowClosing0 &ethod only

! Window#daptor i&ple&ents inter)ace with = null)unctions' e"tend Window#dapter and o(erride windowClosing0


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5ecorator Strategy

! 5ecorator wrapper@ e&phasis:'9oal is to alter the run ti&e beha(ior o) an

ob+ect'Wrapper should preser(e the inter)ace

! ossible adaptor class approach:'adaptor sublcasses adaptee so has sa&e inter)ace

'adaptor constructor takes wrappee as anargu&ent, so it is altering it at runti&e


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callbacks singelton and wrappers

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