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<html><title>Programming Ruby: The Pragmatic Programmer's Guide</title><head><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><STYLE TYPE="text/css"><!--
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</STYLE><table bgcolor="#a03030" cellpadding="3" border="0" cellspacing="0"><tr><td colspan="3"><table bgcolor="#902020" cellpadding="20"><tr><td><h1 class="header">Programming Ruby</h1><h3 class="subheader">The Pragmatic Programmer's Guide</h3></td></tr></table></td></tr><tr><td width="33%" align="left"><a class="subheader" href="lib_standard.html">Previous <</a></td><td width="33%" align="center" valign="middle"><a class="subheader" href="index.html">Contents ^</a><br></td><td width="33%" align="right"><a class="subheader" href="lib_network.html">Next ></a><br></td></tr></table></head><body bgcolor="white">
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material may be distributed only subject to the terms and
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<h1>Object-Oriented Design Libraries</h1><hr><br>
<P></P>
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.
<P></P>
To help in this process, Ruby has support for some design-level strategies.
<P></P>
<ul>
<li> <b>The Visitor pattern</b> (Design Patterns,
) is a way of traversing a collection
without having to know the internal organization of that collection.
</li><li> <b>Delegation</b> is a way of composing classes more flexibly and
dynamically than can be done using standard inheritance.
</li><li> <b>The Singleton pattern</b> is a way of ensuring that only
one instantiation of a particular class exists at a time.
</li><li> <b>The Observer pattern</b> implements a protocol allowing one
object to notify a set of interested objects when certain changes
have occurred.
</li></ul>
<P></P>
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.
<P></P>
Before we get into the proper library descriptions, let's get the
simplest strategy out of the way.
<P></P>
<h2>The Visitor Pattern</h2>
<P></P>
It's the method <code>each</code>.
<P></P>
<table border="0" width="100%" bgcolor="660066" cellpadding="10"><tr><td valign="center"><font color="white" size="7">
Library: delegate</font></td></tr></table><p></p>
<P></P>
Object delegation is a way of <em>composing</em> 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.
<P></P>
The Ruby <code>Delegator</code> 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.
<P></P>
The <code>delegate.rb</code> library provides two mechanisms for allowing an
object to forward messages to a delegate.
<P></P>
<ol>
<P></P>
<li> For simple cases where the class of the delegate is fixed,
arrange for the master class to be a subclass of
<code>DelegateClass</code>, passing the name of the class to be
delegated as a parameter (Example 1). Then, in your class's
<code>initialize</code> method, you'd call the superclass, passing in
the object to be delegated. For example, to declare a class
<code>Fred</code> that also supports all the methods in <code>Flintstone</code>, you'd
write
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
class Fred < DelegateClass(Flintstone)
def initialize
# ...
super(Flintstone.new(...))
end
# ...
end
</pre></td></tr></table>
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.
<P></P>
</li><li> For cases where the delegate needs to be dynamic, make the
master class a subclass of <code>SimpleDelegator</code> (Example 2). You
can also add delegation capabilities to an existing object using
<code>SimpleDelegator</code> (Example 3). In these cases, you can call the
<code>__setobj__</code> method in <code>SimpleDelegator</code> to
change the object being delegated at runtime.
<P></P>
</li></ol>
<P></P>
<b>Example 1.</b> Use the <code>DelegateClass</code> 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 <code>@sizeInInches</code> array is large, so we want only one copy
of it. We then define a class that accesses it, converting the
values to feet.
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
require 'delegate'
<P></P>
sizeInInches = [ 10, 15, 22, 120 ]
<P></P>
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
</pre></td></tr></table>
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>sizeInFeet = Feet.new(sizeInInches)</code></td>
</tr>
<tr>
<td valign="top"><code>sizeInInches[0..3]</code></td>
<td valign="top"></td>
<td valign="top"><code>[10, 15, 22, 120]</code></td>
</tr>
<tr>
<td valign="top"><code>sizeInFeet[0..3]</code></td>
<td valign="top"></td>
<td valign="top"><code>[0.8333333333, 1.25, 1.833333333, 10.0]</code></td>
</tr>
</table>
<P></P>
<P></P>
<b>Example 2.</b> Use subclass <code>SimpleDelegator</code> when you want an
object that both has its own behavior <em>and</em> delegates to
different objects during its lifetime. This is an example of the
State
pattern.
Objects of class <code>TicketOffice</code> sell tickets if a
seller is available, or tell you to come back tomorrow if there is
no seller.
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
require 'delegate'
<P></P>
class TicketSeller
def sellTicket()
return 'Here is a ticket'
end
end
<P></P>
class NoTicketSeller
def sellTicket()
"Sorry-come back tomorrow"
end
end
<P></P>
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
</pre></td></tr></table>
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>to = TicketOffice.new</code></td>
</tr>
<tr>
<td valign="top"><code>to.sellTicket</code></td>
<td valign="top"></td>
<td valign="top"><code>"Here is a ticket"</code></td>
</tr>
<tr>
<td valign="top"><code>to.allowSales(false)</code></td>
<td valign="top"></td>
<td valign="top"><code>false</code></td>
</tr>
<tr>
<td valign="top"><code>to.sellTicket</code></td>
<td valign="top"></td>
<td valign="top"><code>"Sorry-come back tomorrow"</code></td>
</tr>
<tr>
<td valign="top"><code>to.allowSales(true)</code></td>
<td valign="top"></td>
<td valign="top"><code>true</code></td>
</tr>
<tr>
<td valign="top"><code>to.sellTicket</code></td>
<td valign="top"></td>
<td valign="top"><code>"Here is a ticket"</code></td>
</tr>
</table>
<P></P>
<P></P>
<b>Example 3.</b> Create <code>SimpleDelegator</code> objects when you want
a single object to delegate all its methods to two or more other
objects.
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code># Example 3 - delegate from existing object</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>seller = TicketSeller.new</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>noseller = NoTicketSeller.new</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>to = SimpleDelegator.new(seller)</code></td>
</tr>
<tr>
<td valign="top"><code>to.sellTicket</code></td>
<td valign="top"></td>
<td valign="top"><code>"Here's a ticket"</code></td>
</tr>
<tr>
<td valign="top"><code>to.sellTicket</code></td>
<td valign="top"></td>
<td valign="top"><code>"Here's a ticket"</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>to.__setobj__(noseller)</code></td>
</tr>
<tr>
<td valign="top"><code>to.sellTicket</code></td>
<td valign="top"></td>
<td valign="top"><code>"Sorry-come back tomorrow"</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>to.__setobj__(seller)</code></td>
</tr>
<tr>
<td valign="top"><code>to.sellTicket</code></td>
<td valign="top"></td>
<td valign="top"><code>"Here's a ticket"</code></td>
</tr>
</table>
<P></P>
<P></P>
<P></P>
<table border="0" width="100%" bgcolor="660066" cellpadding="10"><tr><td valign="center"><font color="white" size="7">
Library: observer</font></td></tr></table><p></p>
<P></P>
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.
<P></P>
In the Ruby implementation, the notifying class mixes in the
<code>Observable</code> module, which provides the methods for managing the
associated observer objects.
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3">
<P></P>
<tr><td colspan="9" bgcolor="#ff9999" height="3"><img src="dot.gif" width="1" height="1"></td></tr><tr>
<td valign="top">add_observer(<i>obj</i>)</td>
<td valign="top">Add <i>obj</i> as an observer on this
object. <i>obj</i> will now receive notifications.</td>
</tr>
<tr>
<td valign="top">delete_observer(<i>obj</i>)</td>
<td valign="top">Delete <i>obj</i> as an observer on this
object. It will no longer receive notifications.</td>
</tr>
<tr>
<td valign="top">delete_observers</td>
<td valign="top">Delete all observers associated with this
object.</td>
</tr>
<tr>
<td valign="top">count_observers</td>
<td valign="top">Return the count of observers associated with this
object.</td>
</tr>
<tr>
<td valign="top">changed(<i>newState</i>=<code>true</code>)</td>
<td valign="top">Set the changed state of this
object. Notifications will be sent only if the changed state is
<code>true</code>.</td>
</tr>
<tr>
<td valign="top">changed?</td>
<td valign="top">Query the changed state of this object.</td>
</tr>
<tr>
<td valign="top">notify_observers(<i>*args</i>)</td>
<td valign="top">If this object's changed state is
true, invoke the <code>update</code> method in each currently associated
observer in turn, passing it the given arguments. The changed state is then
set to <code>false</code>.</td>
</tr>
<tr><td colspan="9" bgcolor="#ff9999" height="2"><img src="dot.gif" width="1" height="1"></td></tr></table>
<P></P>
The observers must implement the <code>update</code> method to receive
notifications.
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
require "observer"
<P></P>
class Ticker # Periodically fetch a stock price
include Observable
<P></P>
def initialize(symbol)
@symbol = symbol
end
<P></P>
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
<P></P>
class Warner
def initialize(ticker, limit)
@limit = limit
ticker.add_observer(self) # all warners are observers
end
end
<P></P>
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
<P></P>
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
<P></P>
ticker = Ticker.new("MSFT")
WarnLow.new(ticker, 80)
WarnHigh.new(ticker, 120)
ticker.run
</pre></td></tr></table>
<em>produces:</em>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="400"><tr><td><pre>
Current price: 83
Current price: 75
--- Sun Mar 04 23:26:31 CST 2001: Price below 80: 75
Current price: 90
Current price: 134
+++ Sun Mar 04 23:26:31 CST 2001: Price above 120: 134
Current price: 134
Current price: 112
Current price: 79
--- Sun Mar 04 23:26:31 CST 2001: Price below 80: 79
</pre></td></tr></table>
<P></P>
<P></P>
<table border="0" width="100%" bgcolor="660066" cellpadding="10"><tr><td valign="center"><font color="white" size="7">
Library: singleton</font></td></tr></table><p></p>
<P></P>
The Singleton design pattern ensures that only
one instance of a particular class may be
created.
<P></P>
The <code>singleton</code> library makes this simple to implement. Mix
the <code>Singleton</code> module into each class that is to be a singleton,
and that class's <code>new</code> method will be made private. In its
place, users of the class call the method <code>instance</code>, which
returns a singleton instance of that class.
<P></P>
In this example, the two instances of <code>MyClass</code> are the same object.
<P></P>
<table bgcolor="#fff0f0" cellspacing="0" border="0" cellpadding="3" width="500">
<tr>
<td colspan="3" valign="top"><code>require 'singleton'</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code></code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>class MyClass</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code> include Singleton</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code>end</code></td>
</tr>
<tr>
<td colspan="3" valign="top"><code></code></td>
</tr>
<tr>
<td valign="top"><code>a = MyClass.instance</code></td>
<td valign="top"></td>
<td valign="top"><code>#<MyClass:0x4018c924></code></td>
</tr>
<tr>
<td valign="top"><code>b = MyClass.instance</code></td>
<td valign="top"></td>
<td valign="top"><code>#<MyClass:0x4018c924></code></td>
</tr>
</table>
<P></P>
<P></P>
<P></P>
<p></p><hr><table bgcolor="#a03030" cellpadding="10" border="0" cellspacing="0"><tr><td width="33%" align="left"><a class="subheader" href="lib_standard.html">Previous <</a></td><td width="33%" align="center" valign="middle"><a class="subheader" href="index.html">Contents ^</a><br></td><td width="33%" align="right"><a class="subheader" href="lib_network.html">Next ></a><br></td></tr></table><p></p><font size="-1">Extracted from the book "Programming Ruby -
The Pragmatic Programmer's Guide"</font><br><font size="-3">
Copyright
©
2000 Addison Wesley Longman, Inc. Released under the terms of the
<a href="http://www.opencontent.org/openpub/">Open Publication License</a> V1.0.
<br>
This reference is available for
<a href="http://www.pragmaticprogrammer.com/ruby/downloads/book.html">download</a>.
</font></body></html>
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