Object-Oriented Design Libraries

One of the interesting things about Ruby is the way it blurs the distinction between design and implementation. Ideas that have to be expressed at the design level in other languages can be implemented directly in Ruby.
To help in this process, Ruby has support for some design-level strategies.

• The Visitor pattern (Design Patterns, ) is a way of traversing a collection without having to know the internal organization of that collection.
  
• Delegation is a way of composing classes more flexibly and dynamically than can be done using standard inheritance.
  
• The Singleton pattern is a way of ensuring that only one instantiation of a particular class exists at a time.
  
• The Observer pattern implements a protocol allowing one object to notify a set of interested objects when certain changes have occurred.

Normally, all four of these strategies require explicit code each time they're implemented. With Ruby, they can be abstracted into a library and reused freely and transparently.
Before we get into the proper library descriptions, let's get the simplest strategy out of the way.

The Visitor Pattern

It's the methodeach.

Library: delegate

Object delegation is a way ofcomposing objects---extending an object with the capabilities of another---at runtime. This promotes writing flexible, decoupled code, as there are no compile-time dependencies between the users of the overall class and the delegates.
The RubyDelegator class implements a simple but powerful delegation scheme, where requests are automatically forwarded from a master class to delegates or their ancestors, and where the delegate can be changed at runtime with a single method call.
Thedelegate.rb library provides two mechanisms for allowing an object to forward messages to a delegate.

1. For simple cases where the class of the delegate is fixed, arrange for the master class to be a subclass ofDelegateClass, passing the name of the class to be delegated as a parameter (Example 1). Then, in your class'sinitialize method, you'd call the superclass, passing in the object to be delegated. For example, to declare a classFred that also supports all the methods inFlintstone, you'd write

   class Fred < DelegateClass(Flintstone)
   
     def initialize
   
       # ...
   
       super(Flintstone.new(...))
   
     end
   
     # ...
   
    end
   
   

   This is subtly different from using subclassing. With subclassing, there is only one object, which has the methods and the defined class, its parent, and their ancestors. With delegation there are two objects, linked so that calls to one may be delegated to the other.

2. For cases where the delegate needs to be dynamic, make the master class a subclass ofSimpleDelegator (Example 2). You can also add delegation capabilities to an existing object usingSimpleDelegator (Example 3). In these cases, you can call the__setobj__ method inSimpleDelegator to change the object being delegated at runtime.
   

Example 1. Use theDelegateClass method and subclass the result when you need a class with its own behavior that also delegates to an object of another class. In this example, we assume that the@sizeInInches array is large, so we want only one copy of it. We then define a class that accesses it, converting the values to feet.

require 'delegate'

sizeInInches = [ 10, 15, 22, 120 ]

class Feet < DelegateClass(Array)
def initialize(arr)
super(arr)
end
def [](*n)
val = super(*n)
case val.type
when Numeric; val/12.0
else; val.collect {|i| i/12.0}
end
end
end

sizeInFeet = Feet.new(sizeInInches)
sizeInInches[0..3]
»
[10, 15, 22, 120]
sizeInFeet[0..3]
»
[0.8333333333, 1.25, 1.833333333, 10.0]

Example 2. Use subclassSimpleDelegator when you want an object that both has its own behaviorand delegates to different objects during its lifetime. This is an example of the State pattern. Objects of classTicketOffice sell tickets if a seller is available, or tell you to come back tomorrow if there is no seller.

require 'delegate'

class TicketSeller
def sellTicket()
return 'Here is a ticket'
end
end

class NoTicketSeller
def sellTicket()
"Sorry-come back tomorrow"
end
end

class TicketOffice < SimpleDelegator
def initialize
@seller = TicketSeller.new
@noseller = NoTicketSeller.new
super(@seller)
end
def allowSales(allow = true)
__setobj__(allow ? @seller : @noseller)
allow
end
end

to = TicketOffice.new
to.sellTicket
»
"Here is a ticket"
to.allowSales(false)
»
false
to.sellTicket
»
"Sorry-come back tomorrow"
to.allowSales(true)
»
true
to.sellTicket
»
"Here is a ticket"

Example 3. CreateSimpleDelegator objects when you want a single object to delegate all its methods to two or more other objects.

# Example 3 - delegate from existing object
seller = TicketSeller.new
noseller = NoTicketSeller.new
to = SimpleDelegator.new(seller)
to.sellTicket
»
"Here's a ticket"
to.sellTicket
»
"Here's a ticket"
to.__setobj__(noseller)
to.sellTicket
»
"Sorry-come back tomorrow"
to.__setobj__(seller)
to.sellTicket
»
"Here's a ticket"

Library: observer

The Observer pattern, also known as Publish/Subscribe, provides a simple mechanism for one object to inform a set of interested third-party objects when its state changes.
In the Ruby implementation, the notifying class mixes in theObservable module, which provides the methods for managing the associated observer objects.

add_observer(obj)
Addobj as an observer on this object.obj will now receive notifications.
delete_observer(obj)
Deleteobj as an observer on this object. It will no longer receive notifications.
delete_observers
Delete all observers associated with this object.
count_observers
Return the count of observers associated with this object.
changed(newState=true)
Set the changed state of this object. Notifications will be sent only if the changed state istrue.
changed?
Query the changed state of this object.
notify_observers(*args)
If this object's changed state is true, invoke theupdate method in each currently associated observer in turn, passing it the given arguments. The changed state is then set tofalse.

The observers must implement theupdate method to receive notifications.

require "observer"

class Ticker # Periodically fetch a stock price
include Observable

def initialize(symbol)
@symbol = symbol
end

def run
lastPrice = nil
loop do
price = Price.fetch(@symbol)
print "Current price: #{price}\n"
if price != lastPrice
changed # notify observers
lastPrice = price
notify_observers(Time.now, price)
end
end
end
end

class Warner
def initialize(ticker, limit)
@limit = limit
ticker.add_observer(self) # all warners are observers
end
end

class WarnLow < Warner
def update(time, price) # callback for observer
if price < @limit
print "--- #{time.to_s}: Price below #@limit: #{price}\n"
end
end
end

class WarnHigh < Warner
def update(time, price) # callback for observer
if price > @limit
print "+++ #{time.to_s}: Price above #@limit: #{price}\n"
end
end
end

ticker = Ticker.new("MSFT")
WarnLow.new(ticker, 80)
WarnHigh.new(ticker, 120)
ticker.run
produces:

Current price: 83

Current price: 75

--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 75

Current price: 90

Current price: 134

+++ Sun Jun 09 00:10:25 CDT 2002: Price above 120: 134

Current price: 134

Current price: 112

Current price: 79

--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 79

Library: singleton

The Singleton design pattern ensures that only one instance of a particular class may be created.
Thesingleton library makes this simple to implement. Mix theSingleton module into each class that is to be a singleton, and that class'snew method will be made private. In its place, users of the class call the methodinstance, which returns a singleton instance of that class.
In this example, the two instances ofMyClass are the same object.

require 'singleton'

class MyClass
include Singleton
end

a = MyClass.instance
»
#<MyClass:0x401b4ca8>
b = MyClass.instance
»
#<MyClass:0x401b4ca8>