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.
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.
It's the method each
.
Object delegation is a way of composing 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 Ruby Delegator
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.
The delegate.rb
library provides two mechanisms for allowing an object to forward messages to a delegate.
DelegateClass
, passing the name of the class to be delegated as a parameter (Example 1). Then, in your class's initialize
method, you'd call the superclass, passing in the object to be delegated. For example, to declare a class Fred
that also supports all the methods in Flintstone
, 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.
SimpleDelegator
(Example 2). You can also add delegation capabilities to an existing object using SimpleDelegator
(Example 3). In these cases, you can call the __setobj__
method in SimpleDelegator
to change the object being delegated at runtime. Example 1. Use the DelegateClass
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 subclass SimpleDelegator
when you want an object that both has its own behavior and delegates to different objects during its lifetime. This is an example of the State pattern. Objects of class TicketOffice
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. Create SimpleDelegator
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"
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 the Observable
module, which provides the methods for managing the associated observer objects.
add_observer(obj) | Add obj as an observer on this object. obj will now receive notifications. |
delete_observer(obj) | Delete obj 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 is true . |
changed? | Query the changed state of this object. |
notify_observers(*args) | If this object's changed state is true, invoke the update method in each currently associated observer in turn, passing it the given arguments. The changed state is then set to false . |
The observers must implement the update
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
The Singleton design pattern ensures that only one instance of a particular class may be created.
The singleton
library makes this simple to implement. Mix the Singleton
module into each class that is to be a singleton, and that class's new
method will be made private. In its place, users of the class call the method instance
, which returns a singleton instance of that class.
In this example, the two instances of MyClass
are the same object.
require 'singleton'
class MyClass
include Singleton
end
a = MyClass.instance → #<MyClass:0x401b4ca8>
b = MyClass.instance → #<MyClass:0x401b4ca8>
Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide"
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/).
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Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.