Controller pattern assigns responsibility for system events

9-Oct-13 1Controller - K.INIYA CSE Prefinal Year

9-Oct-13 2
It is the first object beyond the UI Layer
responsible for receiving or handling a system
operation message.
The controller pattern assigns the
responsibility of dealing with system events to
a non class that represents the overall system
or a use case scenario.
Controller - K.INIYA CSE Prefinal Year

9-Oct-13 3
What first object beyond the UI layer receives
and coordinates a system operation?

Assigning the responsibility to a class
representing one of the following option
1)representing the overall
system a root object, a device that the
software is running within or a major
subsystem. These are the variations of facade
controller.

2)Represents a Use Case Scenario
within which the system event occurs, often
named
<usecaseName> Handler, <usecaseName>
Coordinator, or <usecaseName> Session.

(a) Use the same controller class for all system
events in the same usecase scenario.
(b) Informally, a session is an instance of a
conversion with an actor. Sessions can be of
any length but are often organized in terms
of usecases (use case Sessions).

9-Oct-13 7
Some System Operations of
NextGen Pos Application
endsale()
enterItem()
makeNewSale()
makePayment()
…………………….
System

ItemID
Quantity
ENTER SAVE
:SaleJFrame
:Regi st er
actionPerformed (actionEvent)
UI Layer
Domain
Layer
Presses Button
9-Oct-13 9
enterItem(itemID,qty)
System Operation
Messages
Controller
:Sale
1:makeLineitem(itemID,qty)
Desirable Coupling of UI to domain Layer

ItemID
Quantity
ENTER SAVE
:SaleJFrame
:Sal e
actionPerformed (actionEvent)
UI Layer
Domain
Layer
Presses Button
Sale Jframe should not
send this message
9-Oct-13 10
1:makeLineitem(itemID,qty)
Less Desirable Coupling of User Interface to domain
Layer

9-Oct-13 11
:Register
:ProcessSaleHandler
enterItem(id,quantity)
enterItem(id,quantity)

System Operations
Discovered during system
behaviour analysis
Allocation of system
operations during design
using one façade controller
9-Oct-13 12
endsale()
enterItem()
makeNewSale()
makePayment()
makeNewReturn()
enterReturnItem()
…………………….
System
endsale()
enterItem()
makeNewSale()
makePayment()
makeNewReturn()
enterReturnItem()
…………………….
Register
……………………
………….

UI Layer does not contain Application Logic
rather UI Layer Objects must delegate work
requests to another layer.
System receives external input events ,that
involves GUI operated by a Person.
1) Other inputs are external messages.
2) For example: in a call processing
telecommunications switch or signals from
sensors.

A controller then suffers from bad (low)
cohesion violating the principle of High
Cohesion.
Facade controller are suitable when there are
“too many” system events, or when the User
Interface (UI) cannot redirect system event
messages to alternating controllers, such as in
a message processing system.

Boundary, Entity, Control classes in UP.
(a)Boundary objects are abstractions of the
interfaces.
(b)Entity objects are the application
independent domain software objects.
(c)Control objects are use Case handlers as in
Controller Pattern.

Increased potential for reuse and pluggable
interfaces.
Opportunity to reason about the state of the
usecase.

Command- In a message handling system,
each message may be represented and
handled by a separate command object.
Façade- A façade controller is a kind façade.
Layers- Placing domain logic in the domain
Layer rather than the Presentation Layer is the
part of the Layers.

• Pure Fabrication- A use case Controller is a
kind of Pure Fabrication.
(Pure Fabrication is a class that
does not represent a concept in the problem
Domain, specially made up to achieve low
coupling, high cohesion and reuse potential)

Java Technologies are used for
implementation for the two reasons:
i. Rich Client in Java Swing.
ii. Web UI with struts on the Server.

Package.com.craiglarman.nextgen.ui.swing;
Public class ProcessSaleJFrame extends Jframe
{
Private Register register;
Public ProcessSaleJFrame(Register_register)
{
register=_register;
}
Private Jbutton BTN_ENTER_ITEM()
Private Jbutton getBTN_ENTER_ITEM()
{
If (BTN_ENTER_ITEM!=null)
return BTN_ENTER_ITEM;
else

BTN_ENTER_ITEM=new Jbutton();
BTN_ENTER_ITEM.setText(“ENTER”)
BTN_ENTER_ITEM.addActionListener(newActionListener()
{
Public void actionPerformed(ActionEvent e)
{
ItemID id=Transformer.toItemID(getTXT_ID().getText());
Int qty=Transformer.toInt(getTXT_QTY().getText());
register.enterItem(id,qty);
}
});
return BTN_ENTER_ITEM;
}
}

DEFINITION
Poorly designed, a controller class with have
low cohesion-unfocused and handling too
many areas of responsibility, this is called a
Bloated Controller.

There is only a Single Controller class
receiving all system events in the system. This
sometimes happens if a façade controller is
chosen.
The Controller itself performs many of the
tasks necessary to fulfill the system event. This
usually involves a violation of Information
Expert and High Cohesion.

A Controller has many attributes and it
maintains significant information about the
system or domain, which should have been
distributed to other objects, or it duplicates
information found elsewhere.

Among the cures for a bloated controller are
these two:
1)Add more controllers- a system does not have
to need only one.
For example:
consider the application with many system
events, such as an Airline Reservation System.

It may contain the following Controllers
2) Design the controller so that it primarily
delegates the fulfillment of each system
operation responsibility to other objects.
9-Oct-13 26
Use Case Controllers
Make Reservation Handler
Make Schedules Handler
Make Fares Handler

Controller pattern assigns responsibility for system events

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