Case Study

1

• Case Study
• GUI

2
Case study

• GUI (Graphical User Interface) toolkit
• A typical GUI has many different objects
• Menu, window, button, scrollbar, . . .
• Hierarchical (tree-like) structure
• Like a file directory
• A window has several windows and buttons
• OO provides an elegant solution to GUI problems
• Most successful example Apple Macintosh

3
How to draw GUI objects?

• Put all concrete objects into a Glyph container
  ask each Glyph to draw itself

Glyph container
Glyph (System provided) Draw()
Glyph Draw()
Add
Glyph Draw()
Concrete Glyph (user provided) Draw()
Glyph Draw()
Glyph Draw()
4
How to draw GUI objects?

• Glyph window new Button(),
• new ScrollBar()
• foreach (g in Window) g.Draw()

Glyph virtual void Draw()
Button void Draw()
ScrollBar void Draw()
Panel void Draw()
5
Upcasting and polymorphism

• Create concrete objects
• new Button(), new ScrollBar()
• Add to an abstract container
• Container only knows abstract base class Glyph
• Upcasting
• Button is upcasted to Glyph
• Polymorphism
• All concrete objects are treated uniformly (same)
  as Glyph
• foreach (g in Window) g.Draw()

6
Composition

• Is Window a kind of glyph?
• Yes, window shares many common properties with
  buttons, scroll bars, panel, . . .
• Is-a relationship inheritance

Glyph
is-a
Window
Button
ScrollBar
Panel
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Composition

• But Window can have some buttons, scroll bars,
  etc.
• Has-a relationship composition
• Window has a number of glyphs

has-a
Window
Glyph
is-a
Button
ScrollBar
Panel
8
Is-a and Has-a relationships

• Is-a
• Inheritance
• Window is-a Glyph
• class Window Glyph
• . . .

• Has-a
• Composition
• Window has-a few buttons, windows
• class Window
• Glyph g
• . . .

9
Composite pattern

• Combining is-a and has-a
• class Window Glyph //inheritance
• Glyph g //composition
• . . .
• So that we can have a window inside a window

Glyph
is-a
has-a
Button
ScrollBar
Panel
Window
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An even more general form of composite pattern

• Can support any complex structure imaginable
• e.g. a Window inside a Button
• Glyph has-a Glyph container
• class Glyph
• Glyph g
• . . .

has-a
Glyph

is-a
Button
ScrollBar
Panel
Window
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An even more general form of composition pattern

• class Window Glyph
• . . .
• Window inherits the Glyph container from Glyph
• Support Window inside a Button

has-a
Glyph

is-a
Button
ScrollBar
Panel
Window
12
Limitation of inheritance

• How to change to a different Button at run time?

Glyph container
Glyph (System provided) Draw()
Glyph Draw()
Add
Glyph Draw()
Concrete Glyph (user provided) Draw()
Glyph Draw()
Glyph Draw()
13
Limitation of inheritance

• Base classs Draw() is overrided by the subclass
• The actual Draw() is done by the concrete
  subclass
• To draw a different look-and-feel button?
• Tedious, has to replace the old Button object in
  the container by a new one that has a different
  Draw()

14
Delegation

• Outsource the drawing action to a drawing object
• So that we can change the look-and-feel by
  changing the drawing object at run-time

Glyph container
Glyph DrawStyle obj Draw()
obj.Draw() ChangeStyle( DrawStyle newobj)
obj newobj
Add
Glyph Draw()
DrawStyle Draw()
Glyph Draw()
Glyph Draw()
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The essential ideas of OO

• INHERITANCE (is-a)
• Similar types of objects that share the same
  interface
• COMPOSITION (has-a)
• An object that is made up from other objects
• DELEGATION
• To outsource work to another object
• Delegation has the same syntax as composition

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Design of Callback

• What is callback?
• The mechanism of sending messages between objects
• e.g. a mouse button is clicked, how to send this
  mouse-up event message to a mouse-up handler?
• Window Server
• Contains GUI objects (Button, ScrollBar, )
• If mouse is clicked, window server finds the
  clicked object in its container, and calls the
  objects OnClick() virtual function
• You provide the implementation of OnClick()

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Callback by inheritance

• Build the MyWindow class inherited from Form
  (Form is like Glyph in prev example)
• Override OnClick()
• Add concrete MyWindow object to container as Form

Window Server
Container
Form virtual OnClick()
Form (Button) virtual OnClick()
Form (ScrollBar) virtual OnClick()
MyWindow override OnClick()
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Get callback by inheritance

• using System
• using System.Windows.Forms
• class MyWindow Form
• static void Main()
• Application.Run(new MyWindow())
• //start a window and wait for input event
• protected override void OnClick(EventArgs e)
• Console.WriteLine(I(the Form) am clicked)
• base.OnClick(e) //forward to parent (Form)
• //in case there are more things to
  do

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OO in one picture
www.vec.ca/images/Gift-Basket.JPG
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OO is like a basket of fruits

• The basket is a container of fruits
• Important
• You have just made an abstraction
• Physically the container has different concrete
  objects (apples and oranges), but because of your
  abstraction, you have generalized them as fruits
• You dont see apples, oranges, only fruits
• Program to an interface, not implementation
• You use the objects as fruits (interface), not
  apples and oranges (implementation)

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Your abstract view

• Program to an interface, not an implementation
• Abstract and flexible programming

Container
Fruit Eat()
Fruit Eat()
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Your abstract view

• What if you eat a fruit?
• Pick a fruit object from the container and eat it
• fruit.Eat()
• What happens next?
• If fruit is an Apple, it tastes like an apple
• If it is an Orange, it tastes like an orange
• Your leave the actual implementation to the
  hidden concrete objects

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The concrete view

• What actually happens

Container
Fruit Eat()
Fruit Eat()
Fruit Eat()
Banana Eat()
Orange Eat()
Apple Eat()
Orange Eat()
Apple Eat()
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Library application
Container
Lendable Borrow()
Lendable Borrow()
Journal Borrow()
DVD Borrow()
Book Borrow()
Lendable Borrow()
Lendable Borrow()
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Bank application
Container
Account Deposit()
Account Deposit()
USD Deposit()
Current Deposit()
Saving Deposit()
Account Deposit()
Account Deposit()
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Game application

• Attack() is done by collision detection of
  rectangles

Container
Fighter Attack()
Fighter Attack()
Sasuke Attack()
Sakura Attack()
Naruto Attack()
Fighter Attack()
Fighter Attack()
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Callback by delegation

• Store a handler pointer into an object of type
  EventHandler
• Add the EventHandler object to Form

Window server
Container
obj Handler()
EventHandler obj.Handler()
Form
Point to a handler function
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Callback by delegation

• class MyWindow Form
• static void Main()
• Application.Run(new MyWindow())
• public MyWindow()
• this.MouseUp new EventHandler(this.Handler())
  
• //MouseUp is a container
• private void Handler(object sender,System.EventAr
  gs e)
• Console.WriteLine(call from 0,sender.ToStrin
  g())

object
function pointer
Handler function
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Differences between the two approaches

• Callback by inheritance is easier to understand
• The mechanism is more direct and simple
• Callback by delegation is more powerful
• Can callback to multiple handlers (multicasting)
  by adding several EventHandler objects to the
  MouseUp container in Form
• Can change the callback handler at run-time by
  changing the EventHandler object

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Delegate how to Build your own handler object

• delegate keyword
• A built-in C keyword that helps you to create
  your own EventHandler object easily
• public delegate void MyHandler(Student s)
• This instruction creates the class MyHandler for
  you
• A MyHandler object can encapsulate any function
  pointer with a matched signature (void and
  Student)

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Example of using delegate to generate handler

• public delegate void CallbackDelegate(String s)
• //a Callback class is generated automatically
• Main()
• ArrayList list new ArrayList()
• Client c new Client()
• list.Add(new Callback(c.Notify())
• foreach ( (Callback) obj in list)
• obj(server)
• Class Client
• private void Notify(String s)
• Console.WriteLine(call from 0,s)

The function pointer
This object function is called from list
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Game application

• Implementation reuse is best done by delegation

Container
Fighter Attack()
Fighter Attack()
ConcreteFighter Image _image Weapon
_w Draw() Attack() _w.Attack()
Fighter Attack()
Weapon Attack()
Fighter Attack()
Fire Attack()
Sand Attack()
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Visual Studio Window Forms

• Tool that helps you to design a GUI easily
• Open a new project and choose Window Forms
• Drag and drop GUI into the Form

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Add a Button

• In ToolBox window, drag a Button object and drop
  it to the position you want in the Form

35
What Visual Studio has done

• Initially Visual Studio has an empty container
• Your action (dragging a button) causes
• A button is created and added to the container
• Visual Studio then asks objects in the container
  to draw themselves, that is why you see the button

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Change Buttons properties

• Can resize and move the button, and use the
  Properties window of the button to change the
  name.

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What Visual Studio has done

• Your action (select the button) causes
• Collision detection
• Visual Studio checks the mouse pointer position
  and sees which object in its container is hit
• Find the button object
• Your further action acts on the button invokes
  methods inside the button which change the
  attributes of the button

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The generated code (view by right click on the
form)

• public class Form1 System.Windows.Forms.Form
• private System.Windows.Forms.Button button1
• static void Main()
• Application.Run(new Form1())
• public Form1()
• InitializeComponent()
• private void InitializeComponent()
• this.button1 new System.Windows.Forms.Button()
  
• //. . .
• protected override void Dispose( bool disposing
  )
• //. . .

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Notes

• Application.Run(new Form1())
• Run() is a static method that has a loop to
  process the event messages
• The Main() is very small !
• Constructor of Form1() calls InitializeComponent()
  
• InitializeComponent() simply create all the GUI
  controls and add them to the container of Form1
• Dispose()
• Call by application in exit to free unmanaged
  resources (more about this later)

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InitializeComponent()

• region Windows Form Designer generated code
• private void InitializeComponent()
• this.button1 new System.Windows.Forms.Button()
  
• this.button1.Text "Click Me !"
• this.Controls.Add(this.button1) //add to
  container
• . . .
• endregion
• Controls is a Property
• this.Controls return a reference to a container

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What Visual Studio has done

• InitializeComponent() is the main part in the
  code generation (the rest are just boilerplate
  code)
• Visual Studio simply asks each object in its
  container to output code based on its stored
  information
• A button would simply generate the constructor
  code and the Text code, then adds itself to the
  container this.Controls
• this.Controls is used by window server for
  collision detection, etc

42
Class diagram of Windows Form
Object
MarshalByRefObject
Control.ControlCollection
1
Component
Control

ScrollableControl
ButtonBase
PictureBox
Button
Panel
ContainerControl
Form
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Class diagram of Windows Form

• Where is the container?
• How does this class structure supports
  composition (has-a) relationship?
• Each Control has a ControlCollection (container)
• Each ControlCollection can have many Controls
• What is the equivalent of class Glyth in this
  structure?
• Control

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Invalidate()

• A single command that invokes the action of
  redrawing all the GUI items in Form
• Invoke whenever the window is dirty
• e.g. resize, expose,
• Window server calls this.Invalidate(), which
  sends a message to the message queue that will be
  processed by the Application.Run() loop
• Form ask all Controls in its container to draw
  itself
• Each Control asks Controls in its container to
  draw itself (recursion)

45
Unmanaged resource

• Unmanaged resource is resource that is not
  managed by the garbage collector (e.g. memory
  obtained from Cs malloc(), must be deleted by
  the user)
• Managed resource is a key advantage of C/Java
  over C/C
• When garbage collector is invoked, it calls
  Finalize() automatically
• Finalize() is the C destructor (opposite of
  constructor)
• Can have user code to remove unmanaged resource
• If you dont have unmanaged resource, then you
  dont need to write object.Finalize()

46
Dispose()

• Problem with Finalize()
• Finalize()can only be invoked by GC, but GC is
  only called when system runs out of memory
• For critical resources, this is too late
• e.g. opened file, opened database connection,
  want to close as soon as possible
• Solution
• Called Dispose() explicitly
• If you use Dispose(), then you should call
  GC.SuppressFinalize()to suppress Finalize()
• Otherwise the object will be clear twice

47
Dispose()

• Finalize() is a method in System.Object
• Supported by all classes
• Dispose() is NOT a method in System.Object
• For an object to support Dispose(), must inherit
  the IDisposable interface and implement the only
  function Dispose()
• Base class Form support IDisposable , it provides
  a do-nothing default
• You cannot use Dispose() to remove managed
  resource, managed resource can only be released
  by garbage collector

48
Using

• The special construct Using calls Dispose()
  automatically whenever it is out of scope
• Standard usage
• static void Main()
• using (Form1 frm new Form1())
• Application.Run(frm)
• //frm.Dispose() called here

49

• SOA
• Web Services

50
.Net Remoting layer

• How to simplify network programming?
• Some key terms
• Object serialization
• Proxies
• Marshaling by reference / value
• Server activated objects

51
Serialization

• Convert the state of an object into a linear
  sequence of byte stream, and send it to another
  computer
• To make an object serializable, label the class
  with Serializable attribute
• Serializable
• public class Student
• . . .
• Default is no serialization, for security reason

52
Serialization Formatter

• Different formats for sending the serialized
  objects
• Binary format
• SOAP (Simple Object Access Protocol) format
• XML format
• using System.Runtime.Serialization.Formatters.Bina
  ry
• using System.Runtime.Serialization.Formatters.Soap
  
• Using System.Xml.Serialization.XmlFormatter
• SOAP is XML-based
• SOAP and XML use ascii characters, can be read by
  human

53
Saving objects to file by serialization

• //save object to file
• Student s new Student()
• FileStream fs File.Create(student.dat)
• BinaryFormatter format new BinaryFormatter()
• format.Serialize(fs, s)
• //retrieve the object from file
• FileStream fs File.OpenRead(student.dat)
• Student s (Student)format.Deserialize(fs)

54
.NET Remoting

• Client
• The one who wants to use a remote object
• Server
• The one who provides the remote object
• Marshaling
• Describes how a remote object is passed to client
• Marshal-by-value (MBV)
• Client gets a copy of the remote object
• Marshal-by-reference (MBR)
• Client gets a proxy of the remote object

55

• MBV
• Server passes a copy of remote object to client
• Any modification by the client wont affect the
  original object in the server side
• MBR
• Server passes a proxy of the object to the
  client, the proxy acts as a reference to the
  object
• Any modification by the client changes the
  original object in the server side

56
Key elements in Remoting

• Proxy
• The local object in the client side that
  impersonates the remote object
• Formatter
• Channel TCP / HTTP

Client object
Remote object
Proxy
Channel
Formatter
Formatter
57
Client-server programming using .NET Remoting

• A simple client-server program in OO
• Client gets a copy or a proxy of the remote
  object
• Use the object to send a message to server
• Server returns a message to client
• Example creates 3 projects
• SimpleClient.cs
• SimpleServer.cs
• Remote.cs
• Class for the remote object
• Use by both SimpleClient and SimpleServer

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Remote.cs

• //The MBR object at the server side
• using System
• namespace RemoteLib
• public class RemoteHello MarshalByRefObject
• public void SendMsg(string str) //from
  client
• Console.WriteLine("0",str)
• public string ReceiveMsg() //servers
  response
• return "hello world"

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SimpleServer.cs

• using System
• using System.Runtime.Remoting
• using System.Runtime.Remoting.Channels
• using System.Runtime.Remoting.Channels.Http
• using RemoteLib
• namespace SimpleServer
• class Server
• static void Main()
• HttpChannel c new HttpChannel(12345)
• ChannelServices.RegisterChannel(c)
• RemotingConfiguration.RegisterWellKnownServiceT
  ype(
• typeof(RemoteHello), "RemoteHelloObj.soap",
• WellKnownObjectMode.Singleton) //start the
  server
• Console.ReadLine() //to quit, just type
  something

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SimpleClient.cs

• using System
• using System.Runtime.Remoting
• using System.Runtime.Remoting.Channels
• using System.Runtime.Remoting.Channels.Http
• using RemoteLib
• class Client
• static void Main()
• HttpChannel c new HttpChannel()
• ChannelServices.RegisterChannel(c)
• object remoteObjActivator.GetObject(typeof(Remot
  eHello),
• "http//localhost12345/RemoteHelloObj
  .soap")
• RemoteHello simple (RemoteHello) remoteObj
  //casting
• simple.SendMsg("hello from client")
• Console.WriteLine(From server0",
  simple.ReceiveMsg())

61
History of Distributed Computing

• 1980 Remote Procedure Call (RPC)
• (to invoke functions in a remote machine)
• 1990 ORPC (Object-RPC)
• (to invoke methods in a remote object)
• 1990 CORBA by OMG (Object Management Group)
• Common Object Request Broker Architecture
• To support communication at object level in a
  
• heterogeneous environment

62
Key Components in CORBA

• IDL Interface Definition Language
• ORB The middleware that takes care of the
• communication between objects, e.g.
• method invocation, parameters passing

client
IDL interface
IDL interface
server
ORB
stub
skeleton
Network
63
Other similar models

• DCOM by Microsoft
• (Distributed Component Object Model)
• RMI by Java
• (Remote Method Invocation)
• Limitations
• DCOM and Java are platform specific
• For CORBA, both sides must run the same ORB
• ORBs from different vendors may have
  interoperability problems

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• Interoperability issue
• Advent of e-commerce, want distributed computing
  to be more widely available
• With DCOM, RMI, or CORBA, the chance that both
  ends run the same platform is not high
• 2000 IBM and Microsoft proposed the web
• services which forms the basis for SOA
• (Service Orientated Architecture)

65
SOA

• Power plant example
• A hundred years ago, each family had its own
  power generator
• Complex, you need to know how to fix the machine
• What happened next
• Development of power transmission grid
• Simple interface at client side (just a switch)
• Complexity of power generation at the power plant
  side (the server)
• Web services plan to follow the same development
• Complex server, thin clients

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• The ultimate web service

67
Key Issues in SOA

• How to describe the interface of a remote object?
• By WDSL (Web Services Description Language)
• Describes the interface in XML (replace IDL)
• How to communicate with the remote object?
• By SOAP (Simple Object Access Protocol)
• To invoke a method, send a message that has the
  name of the method and parameters
• How to find objects that provide the services?
• By UDDI (Universal Description, Discovery and
  Integration)
• A directory service to search for web services

68
Web services model
Server
Client
HTTP/SOAP
Concrete Object
Proxy object
URI
Interface
69
WSDL is just a XML file
portType is similar to class

• name"getPrice" / on

Method in a class
70

• WSDL
• Use XML to describe a class
• .NET can generate the WSDL file automatically
  from a class definition
• Client can download the WSDL file, and use it to
  create proxy objects
• Many companies already provide the web services,
  e.g. Amazon, Google

71
A SOAP request to the server

• POST /InStock HTTP/1.1 Host www.stock.org
• xmlnssoap"http//www.w3.org/2001/12/soap-envelop
  e"
• 
    
• HSBC     

Schema
Namespace of SOAP
Schema of your namespace m
parameter
method
72
A SOAP response from server

• HTTP/1.1 200 OK
• xmlnssoap"http//www.w3.org/2001/12/soap-envelop
  e"
• "
• 34.5

73
XML-Scheme

• Schema is a document that describes the abstract
  structure of a data set
• Like what you have done to describe the structure
  of a class
• XML schema provides a list of names (types) that
  are used to define complex data structure
  (similar to how you define a class or structure)
• e.g a string type can be described by xsstring

74
Why use XML?

• Syntax-wise XML file is similar to HTML
• The main reason of using XML is that it supports
  namespaces
• Namespace solved the problem of name collision
• An XML namespace is a collection of names,
  identified by the URI reference
• Elements in the XML namespace is defined by the
  schema

75
Key advantages of web services

• Free (Web Services) vs fee economic model (
  CORBA)
• Communication via HTTP port 80 channel
• Firewall friendly
• Use widely accepted standards
• Method invocation is via SOAP messages encoded in
  XML
• Interface definition is by WSDL, also in XML
• Interoperability problem solved by using protocol
  based on SOAP and XML
• Any node that supports HTTP, WSDL, and SOAP can
  be connected to SOA