CSC/ECE 517 Fall 2009/wiki2 11 sv

Design patterns

The idea of design pattern was first introduced by the architect Christopher Alexander in the field of architecture. Later it has been adapted for various other disciplines, including computer science. Christopher Alexander says, "Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice". Even though Alexander was talking about patterns in buildings and towns, what he says is true about object-oriented design patterns. Our solutions are expressed in terms of objects and interfaces instead of walls and doors, but at the core of both kinds of patterns is a solution to a problem in a context [1].

In general, a pattern has four essential elements:

1. The pattern name is a handle used to describe a design problem, its solutions, and consequences in a word or two.
2. The problem explains the problem and its context and also describes when to apply the pattern.
3. The solution describes the elements that make up the design, their relationships, responsibilities, and collaborations.the pattern provides an abstract description of a design problem and how a general arrangement of elements solves it.
4. The consequences are the results and trade-offs of applying the pattern.

A design pattern is a formal way of documenting a solution to a design problem in a particular field of expertise. In software engineering, a design pattern is a solution to a general and commonly occurring problem in software design. A design pattern is not a finished design that can be transformed directly into code. But it is a description or template for how to solve a problem that can be used in many different situations.

---

Design patterns in ruby

Almost every design patterns of ruby are borrowed from the GOF book. Following is the list of common pattern used in ruby.

1. Template
2. Strategy
3. Observer
4. Composite
5. Iterator
6. Commands
7. Adapter
8. Proxy
9. Decorator
10. Singleton
11. Factory
12. Builder
13. Interpreter

---

Observer

---

The observer pattern (sometimes known as publish/subscribe) is a software design pattern in which an object, called the 'subject', maintains a list of its dependents, called 'observers', and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems.

Example Scenario where Observer Pattern is Used:
Consider a personnel system where an employee's salary changes and the payroll department needs to know when these changes take place. Here, How can one make the Employee object spread the news about salary changes without tangling it up with the payroll system? In such a situation, Observer pattern is used. Initially, an object is created that is interested in the state of a person's(any employee's) finances. This object then needs to simply register with that person's 'Employee Object' ahead of time. Once registered, that object would receive timely updates about the ups and downs of the person's paycheck.

Here is a basic version of an Employee object that tracks an employee. It does not have any code that tells about any salary updates.

<source lang="java">
class Employee
attr_reader :name
attr_accessor :title, :salary
def initialize( name, title, salary )
@name = name
@title = title
@salary = salary
end
end
</source>

The employees can get raises because we made the salary field accessible with attr_accessor.

jim = Employee.new("Jim Flintstone", "Crane Operator", 5000.0)
# Give Jim a raise
jim.salary=8000.0

Now, adding some code to keep the payroll department informed of pay changes:

class Payroll
def update( changed_employee )
puts("Cut a new check for #{changed_employee.name}!")
puts("His salary is now #{changed_employee.salary}!")
end
end

class Employee
attr_reader :name, :title
attr_reader :salary
def initialize( name, title, salary,payroll)
@name = name
@title = title
@salary = salary
@payroll = payroll
end
def salary=(new_salary)
@salary = new_salary
@payroll.update(self)
end
end

We can now change Jim's wages as follows:

payroll = Payroll.new
jim = Employee.new('Jim', 'Crane Operator', 5000, payroll)
jim.salary = 8000

And the payroll department will know about these changes. The output of the above code is:
Cut a new check for Jim! His salary is now 8000!

---

---

See Also

[1]

[2]


[3]

[4]

[5]

[6]

---

References

[1]

[2]

[3]

---