an introduction to reactive programmingstg-tud.github.io/sedc/lecture/ws16-17/rp1.pdf ·...

GuidoSalvaneschi:introductiontoreactiveprogramming

AnIntroductiontoReactiveProgramming

Prof.GuidoSalvaneschi


Outline

• Introtoreactiveapplications• TheObserverpattern• Event-basedlanguages• Reactivelanguages


INTROTOREACTIVEAPPLICATIONS


• Atransformational system– Acceptsinput,performscomputationonit,producesoutput,andterminates

– Compilers,shelltools,scientificcomputations

• Areactive system– Continuouslyinteractswiththeenvironment– Updatesitsstate

SoftwareTaxonomy


• Transformationalsystems:– Expresstransformationsasincrementalmodificationsoftheinternaldatastructures

• Reactivesystems:– Representthecurrentstateofinteraction– Reflectchangesoftheexternalworldduringinteraction

UseofState

Stateisnotnecessarytodescribethesystem

Stateisessentialtodescribethesystem


ReactiveApplications

InteractiveApplicationsUI

Monitoring/ControlSystems


ReactiveApplications• Manyotherexamples

– Webapplications– Mobileapps– Distributedcomputations

• Cloud– …

• Typicaloperations– Detect events/notificationsandreact– Combinereactions– Propagateupdates/changes


ReactiveApplicationsWhyshouldwecare?

• Eventhandling:– 30%ofcodeindesktopapplications– 50%ofbugsreportedduringproductioncycle


ReactiveProgramming

Now…– Reactiveapplicationsareextremelycommon– Canwedesignnewlanguagefeaturestospecificallyaddressthisissue?

• Thinkabouttheproblemssolvedbyexceptions,visibilitymodifiers,inheritance,…


REACTIVEPROGRAMMING


Definition…?

Forexample,abstractionsto:RepresenteventstreamsAutomaticallypropagatechangesinthestateCombineevents…

ReactiveProgramming

“Programminglanguageabstractions(techniquesandpatterns)

todevelopreactiveapplications”


• Haskell:Fran,Yampa• FrTime,Flapjax,REScala,Scala.react,…

• Angular.js,Bacon.js,Reactive.js,…• MicrosoftReactiveExtensions(Rx)

• Books2014-16

ReactiveProgramming


ReactiveProgramming


THEOBSERVERPATTERN


The(good?old)ObserverPattern


The(good?old)ObserverPatternbooleanhighTemp;booleansmoke;

voidInit(){tempSensor.register(this);smokeSensor.register(this);

}

voidnotifyTempReading(TempEvente){highTemp=e.getValue()>45;if(highTemp&&smoke){

alert.start();}

}

voidnotifySmokeReading(SmokeEvente){smoke=e.getIntensity()>0.5;if(highTemp&&smoke){

alert.start();}

}

Callbackfunctions

State

Controlstatements

Registration

Callbackfunctions

Controlstatements


TheObserverPattern

• WhataboutJavaSwing?– javax.swing

BEEEEP!


public class Beeperextends JPanel implements ActionListener {JButton button;

public Beeper(){super(new BorderLayout());button=new JButton("ClickMe");button.setPreferredSize(new Dimension(200,80));add(button,BorderLayout.CENTER);button.addActionListener(this);

}public void actionPerformed(ActionEvent e){Toolkit.getDefaultToolkit().beep();

}private static void createAndShowGUI(){//CreatetheGUIandshowit.JFrame frame=new JFrame("Beeper");//Createandsetupthewindow.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);JComponent newContentPane =new Beeper();//Createandsetupthecontentpane.newContentPane.setOpaque(true);frame.setContentPane(newContentPane);frame.pack();//Displaythewindow.frame.setVisible(true);

}public static voidmain(String[]args){javax.swing.SwingUtilities.invokeLater(new Runnable(){public void run(){createAndShowGUI();}});

}}

BEEEEP!


EVENT-BASEDLANGUAGES


Event-basedLanguages

Language-levelsupportforevents

• Eventsasobjectattributes– Describechangesoftheobject'sstate– Partoftheinterface

• Event-basedlanguagesarebetter!– Moreconcise,clearprogrammingintention,…– C#,Ptolemy,EScala,EventJava,…


ExampleinC#publicclassDrawing{Collection<Figure>figures;publiceventNoArgs Changed();publicvirtualvoidAdd(Figurefigure){figures.Add(figure);figure.Changed +=OnChanged;OnChanged();

}publicvirtualvoidRemove(Figurefigure){figures.Remove(figure);figure.Changed -=OnChanged;OnChanged();

}protectedvirtualvoidOnChanged(){if (Changed!=null){Changed();}

}...}


EVENTSINSCALA


REScala

• www.rescala-lang.com– Anadvancedevent-basedsystem– Abstractionsfortime-changingvalues– Bridgingbetweenthem

• Philosophy:fosteramoredeclarativeandfunctionalstylewithoutsacrificingthepowerofOOdesign

• PureScala


AddingEventstoScala

• C#eventsarerecognizedbythecompiler– Scala doesnotsupporteventsbyitself,but…

• CanweintroduceeventsusingthepowerfulScalasupportforDSLs?

• CanwedoevenbetterthanC#?– E.g.,eventcomposition?


REScala events:Summary

• Differenttypesofevents:Imperative,declarative,…

• Eventscarryavalue– Boundtotheeventwhentheeventisfired– Receivedbyallthehandlers

• Eventsareparametrictypes.– Event[T],Evt[T]

• AlleventsaresubtypeofEvent[T]


ImperativeEvents

• Valideventdeclarations

val e1=Evt[Unit]()val e2=Evt[Int]()val e3=Evt[String]()val e4=Evt[Boolean]()

val e5:Event[Int]=Evt[Int]()

class Fooval e6=Evt[Foo]()


ImperativeEvents

• Multiplevaluesforthesameeventareexpressedusingtuples

val e1=Evt[(Int,Int)]()val e2=Evt[(String,String)]()val e3=Evt[(String,Int)]()

val e4=Evt[(Boolean,String,Int)]()

val e5:Evt[(Int,Int)]=Evt[(Int,Int)]()


Handlers

• Handlersareexecutedwhentheeventisfired– The+=operatorregistersthehandler.

• Thehandlerisafirstclassfunction– Theattachedvalueisthefunctionparameter.

var state=0val e=Evt[Int]()e+={println(_)}e+=(x=>println(x))e+=((x:Int)=>println(x))e+=(x=>{//Multiplestatementsinthehandlerstate=xprintln(x)})


Handlers

• Thesignatureofthehandlermustconformtheevent– E.g.,Event[(Int,Int)]requires(Int,Int)=>Unit– Thehandler:

• receivestheattachedvalue• performssideeffects.

val e=Evt[(Int,String)]()e+=(x=>{println(x._1)println(x._2)})e+=(x:(Int,String)=>{println(x)})


Handlers

• EventswithoutargumentsstillneedaUnitargument inthehandler.

val e=Evt[Unit]()e+={x=>println(“Fired!”)}e+={(x:Unit)=>println(“Fired!”)}


MethodsasHandlers

• Methodscanbeusedashandlers.– Partiallyappliedfunctionssyntax– Typesmustbecorrect

defm1(x:Int)={val y=x+1println(y)}

val e=Evt[Int]e+=m1_e(10)


FiringEvents

• Methodcallsyntax• Thevalueisboundtotheeventoccurrence

val e1=Evt[Int]()val e2=Evt[Boolean]()val e3=Evt[(Int,String)]()

e1(10)e2(false)e3((10,"Hallo"))


FiringEvents

• Registeredhandlersareexecutedeverytimetheeventisfired.– Theactualparameterisprovidedtothehandler

val e=Evt[Int]()e+={x=>println(x)}e(10)e(11)

-- output----1011


FiringEvents

• Allregisteredhandlersareexecuted– Theexecutionorderisnondeterministic

val e=Evt[Int]()e+={x=>println(x)}e+={x=>println("n:" +x)}e(10)e(11)

-- output----10n:1011n:11


FiringEvents

• The.removeoperatorunregistersahandlerviaitshandle

• The+=operatoralsoreturnsthehandlethatwillbeusedforunregistration

val e=Evt[Int]()val handler1={x:Int =>println(x)}val handler2={x:Int =>println("n:"+x)}

val h1=e+=handler1val h2=e+=handler2e(10)h1.removee(10)h2.removee(10)

-- output----10n:10n:10


ImperativeEvents

• Eventscanbereferredtogenerically

val e1:Event[Int]=Evt[Int]()


DECLARATIVEEVENTS


TheProblem

• Imperativeeventsarefiredbytheprogrammer• Conceptually,certaineventsdependonotherevents

• Examples:– mouseClickE ->museClickOnShape– mouseClose,keyboardClose ->closeWindow

• Canwesolvethisproblemenhancingthelanguage?

?


DeclarativeEvents

• Declarativeeventsaredefinedbyacombinationofotherevents.

• Somevaliddeclarations:

val e1=Evt[Int]()val e2=Evt[Int]()

val e3=e1||e2val e4=e1&&((x:Int)=>x>10)val e5=e1map((x:Int)=>x.toString)


ORevents

• Theevente1||e2isfiredupontheoccurrenceofoneamonge1ore2.– Theeventsintheeventexpressionhavethesameparametertype

val e1=Evt[Int]()val e2=Evt[Int]()val e1_OR_e2=e1||e2e1_OR_e2+=((x:Int)=>println(x))e1(10)e2(10)

-- output----1010


PredicateEvents

• Theevente&&pisfiredifeoccursandthepredicatepissatisfied.– ThepredicateisafunctionthatacceptstheeventparameterasaformalandreturnsBoolean.

– &&filterseventsusingaparameterandapredicate.

val e=Evt[Int]()val e_AND:Event[Int]=e&&((x:Int)=>x>10)e_AND +=((x:Int)=>println(x))e(5)e(15)-- output----15


MapEvents

• Theeventemapfisobtainedbyapplyingftothevaluecarriedbye.– Themapfunctiontakestheeventparameterasaformal.– Thereturntypeofmapisthetypeparameteroftheresultingevent.

val e=Evt[Int]()val e_MAP:Event[String]=emap((x:Int)=>x.toString)e_MAP +=((x:String)=>println("Here:" +x))e(5)e(15)-- output----Here:5Here:15


EXAMPLESOFRESCALA EVENTS


Example:Figures

abstractclassFigure{valmoved[Unit]=afterExecMoveByval resized[Unit]val changed[Unit]=resized||moved||afterExecSetColorval invalidated[Rectangle]=changed.map(_=>getBounds())...val afterExecMoveBy =newEvt[Unit]val afterExecSetColor =newEvt[Unit]…defmoveBy(dx:Int,dy:Int){position.move(dx,dy);afterExecMoveBy()}def resize(s:Size){size=s}def setColor(col:Color){color=col;afterExecSetColor()}def getBounds():Rectangle

...}


Example:Figures

class Connector(val start:Figure,val end:Figure){val h1=start.changed +=updateStart _val h2=end.changed +=updateEnd _...def updateStart(){...}def updateEnd(){...}...def dispose{h1.removeh2.remove

}}


Example:Figures

• Inheritedevents– Maybeoverridden– Arelatebound

class RectangleFigure extends Figure{val resized=afterExecResize ||afterExecSetBoundsoverride valmoved=super.moved ||afterExecSetBounds...val afterExecResize =new Evt[Unit]val afterExecSetBounds =new Evt[Unit]…def resize(s:Size){...;afterExecResize()}def setBounds(x1:Int,y1:Int,x2:Int,y2:Int){...;afterExecSetBounds }...}

abstractclassFigure{valmoved[Unit]=afterExecMoveByval resized[Unit]…}


Example:TemperatureSensor

class TemperatureSensor {val tempChanged[Int]=new Evt[Int]

...def run{var currentTemp =measureTemp()while(!stop){val newTemp =measureTemp()if (newTemp !=currentTemp){tempChanged(newTemp)currentTemp =newTemp

}sleep(100)

}}

}


REACTIVELANGUAGES


EventsandFunctionalDependencies

Eventsareoftenusedforfunctionaldependenciesboolean highTemp :=(temp.value >45);

val update=Evt[Unit]()var a=3var b=7var c=a+b//Functionaldependency

update+=(_=>{c=a+b})

a =4update()b =8update()

var a=3var b=7val c=a+b

a =4b =8


Constraints

• Whataboutexpressingfunctionaldependenciesasconstraints?

val a=3val b=7val c=a+b// Statementprintln(c)>10a=4println(c)>10

val a=3val b=7val c:=a+b// Constraintprintln(c)>10a=4println(c)>11


EMBEDDINGREACTIVEPROGRAMMINGINSCALA


ReactiveValues

• Vars:primitivereactivevalues– Updated“manually”

• Signals:reactiveexpressions– Theconstraints“automatically”enforced

val a=Var(3)val b=Var(7)val c=Signal{a()+b()}println(c.now)>10a()=4println(c.now)>11


ReferenceModel

• Changepropagationmodel– Dependencygraph– Push-drivenevaluation

val a=Var(3)val b=Var(7)val c=Signal{a()+b()}val d =Signal{2*c()}…

a b

c

d

f

d

h

e

i

g

…


SIGNALSANDVARS


Vars

• Varswrapnormal Scalavalues

• Var[T]isaparametrictype.

– TheparameterTisthetypethevar wrapsaround

– Vars areassignedbythe“()=“operator

val a=Var(0)val b=Var("HelloWorld")val c=Var(false)val d:Var[Int]=Var(30)val e:Var[String]=Var("REScala")val f:Var[Boolean]=Var(false)

a()=3b()=“New World”c()=true


Signals

• Syntax:Signal{sigexpr}– Sigexprshouldbeside-effect free

• Signalsareparametrictypes.– AsignalSignal[T]carriesavalueoftypeT


Signals:CollectingDependencies

• AVar oraSignal calledwith()inasignalexpressionisaddedtothedependenciesofthedefinedsignal

//Multiplevars//inasignalexpressionval a=Var(0)val b=Var(0)val s=Signal{a()+b()} a b

c


Signals:Examples

val a=Var(0)val b=Var(0)val c=Var(0)

val r:Signal[Int]=Signal{a()+1}//Explicittypeinvar decl

val s=Signal{a()+b()} //Multiplevars isasignalexpression

val t=Signal{s()*c()+10} //Mixsignalsandvars insignalexpressions

val u=Signal{s()*t()} //Asignalthatdependsonothersignals


Signals:Examples

val a=Var(0)val b=Var(2)val c=Var(true)val s=Signal{if (c())a()else b()}

def factorial(n:Int)=...val a=Var(0)

val s:Signal[Int]=Signal{ //Asignalexpressioncanbeanycodeblockval tmp =a()*2val k=factorial(tmp)k+2 //ReturnsanInt}


Signals

• Accessingreactivevalues:now– Oftenusedtoreturntoatraditional computation

val a=Var(0)val b=Var(2)val c=Var(true)val s:Signal[Int]=Signal{a()+b()}val t:Signal[Boolean]=Signal{!c()}

val x:Int =a.nowval y:Int =s.now

val z:Boolean=t.nowprintln(z)


EXAMPLESOFSIGNALS


Example


Example:Observer

/*Createthegraphics*/title="ReactiveSwingApp"val button=newButton{text="Clickme!"}val label=new Label{text="Nobuttonclicksregistered"}contents=new BoxPanel(Orientation.Vertical){contents+=buttoncontents+=label}

/*Thelogic*/listenTo(button)var nClicks =0reactions+={case ButtonClicked(b)=>nClicks +=1label.text ="Numberofbuttonclicks:"+nClicksif (nClicks >0)button.text ="Clickmeagain"

}


Example:Signals

title="ReactiveSwingApp"val label=new ReactiveLabelval button=new ReactiveButton

val nClicks =button.clicked.fold(0){(x,_)=>x+1}

label.text =Signal{(if (nClicks()==0)"No"else nClicks())+"buttonclicksregistered"}

button.text =Signal{"Clickme"+(if (nClicks()==0)"!" else "again" )}

contents=new BoxPanel(Orientation.Vertical){contents+=buttoncontents+=label

}


Example:SmashingParticles

66

class Oval(center:Signal[Point],radius:Signal[Int]){…}

val base=Var(0)//Increasesindefinitelyval linearTime =base()val cyclicTime =Signal{linearTime()%200}

val point1=Signal{new Point(20+cyclicTime (),20+cyclicTime ())}new Oval(point1,cyclicTime )…//4times


BASICCONVERSIONFUNCTIONS


• Signals(andevents)areobjectsfields– Inheritance,latebinding,visibilitymodifiers,…

• Conversionfunctionsbridgesignalsandevents

REScala designprinciples

EVENTS

SIGNALS


BasicConversionFunctions

• Changed :: Signal[T] -> Event[T]• Latest :: Event[T] -> Signal[T]


Example:Changedval SPEED=10val time=Var(0)val space=Signal{SPEED*time()}

while (true){Threadsleep20time()=time.now +1}

space.changed +=((x:Int)=>println(x))

-- output--10203040...


Example:Latest

val senseTmp =Evt[Int]()//Fahrenheitval threshold=40

val fahrenheitTmp =senseTmp.latest(0)val celsiusTmp =Signal{(fahrenheitTmp()– 32)/1.8 }

val alert=Signal{if (celsiusTmp()>threshold)“Warning” else “OK”}


Quiz1

val v1=Var(4)val v2=Var(2)val s1=Signal{v1()+v2()}val s2=Signal{s1()/3}

assert(s2.now==2)v1()=1assert(s2.now==1)

val v1=Var(4)val v2=Var(2)val s1=Signal{v1()+v2()}val s2=Signal{s1()/3}

assert(s2.now==v1()=1assert(s2.now==


Quiz2

var test=0val v1=Var(4)val v2=Var(2)val s1=Signal{v1()+v2()}s1.changed+=((x:Int)=>{test+=1})

assert(test==0)v1()=1assert(test==1)

var test=0val v1=Var(4)val v2=Var(2)val s1=Signal{v1()+v2()}s1.changed+=((x:Int)=>{test+=1})

assert(test==v1()=1assert(test==


Quiz3

val e=Evt[Int]()val v1=Var(4)val v2=Var(2)val s1=e.latest(0)val s2=Signal{v1()+v2()+s1()}

assert(s2.now==6)e(2)assert(s2.now==8)e(1)assert(s2.now==7)

val e=Evt[Int]()val v1=Var(4)val v2=Var(2)val s1=e.latest(0)val s2=Signal{v1()+v2()+s1()}

assert(s2.now==e(2)assert(s2.now==e(1)assert(s2.now==


TRUBLESHOOTING


Commonpitfalls

• Establishingdependencies– ()createsadependency.Useonlyinsignalexpressions

– now returnsthecurrentvalue

• Signalsarenot assignable.– Dependonothersignalsandvars– Areautomaticallyupdated

val a=Var(2)val b=Var(3)val c =Signal{a.now +b()}

a

c

b


Commonpitfalls

• Avoidsideeffectsinsignalexpressions

• Avoidcyclicdependencies

var c=0val s=Signal{val sum=a()+b();c=sum*2}...foo(c)

val c=Signal{val sum=a()+b();sum*2}...foo(c.now)

val a=Var(0)val s=Signal{a()+t()}val t=Signal{a()+s()+1}


ReactiveAbstractionsandMutability

• Signalsandvars holdreferencestoobjects,nottheobjectsthemselves.class Foo(init:Int){var x=init}val foo=new Foo(1)

val varFoo=Var(foo)val s=Signal{varFoo().x+10}assert(s.now==11)foo.x=2assert(s.now ==11)

class Foo(x:Int)//Immutableval foo=new Foo(1)

val varFoo =Var(foo)val s=Signal{varFoo().x+10

}assert(s.now ==11)varFoo()=new Foo(2)assert(s.now ==12)

class Foo(init:Int){var x=init}val foo=new Foo(1)

val varFoo =Var(foo)val s=Signal{varFoo().x+10}assert(s.now ==11)foo.x =2varFoo()=fooassert(s.now ==11)


QUESTIONS?

an introduction to reactive programmingstg-tud.github.io/sedc/lecture/ws16-17/rp1.pdf ·...

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