A Smalltalk Patterns Safari

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A Smalltalk Patterns Safari

• Brian Foote
• The Refactory, Inc.

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Why Patterns?

• Whats New Here is that Nothing is New Here
• Patterns are about what works
• Patterns give us a way to talk about what works
• Patterns distill experience
• Patterns give us a pithy design vocabulary

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Alexander on Patterns

• Patterns in solutions come from patterns in
  problems.
• "A pattern is a solution to a problem in a
  context."
• "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."
• Christopher Alexander -- A Pattern Language

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Why Patterns in the Wild?

• Learn where to look
• Learn how to spot em and recognize em
• See how they evolve
• Appreciate their history, and our history
• Understand their ecosystem and interdependence
• Helps us discern and discover new ones

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The Gang of FourThe Essential Field Guide

• Design Patterns Elements of Reusable
  Object-Oriented Software
• Erich Gamma, Richard Helm, Ralph Johnson, and
  John Vlissides
• Addison Wesley, 1995
• A landmark book that changed the way programmers
  think about building object-oriented programs

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Dr. Johnson, I presume?

• Great explorers, like Livingstone and Johnson
  neither necessarily create nor invent, nor, do
  they even always discover
• Often, their greatest skill is communication...

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Your Guide

• Over Twenty Years in the Bush...
• Stalking Smalltalk since 81 or 85 Patterns?
• An long-time associate of Dr. Johnson...

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Field Guides (cont.)

• The Design Patterns Smalltalk Companion
• Alpert, Brown, and Woolf
• Addison Wesley, 1998
• Pattern-Oriented Software Architecture
• Buschmann, Meunier, Rohnert, Sommerlad, and
  Stahl
• Wiley, 1996
• The Pattern Languages of Program Design Series
• Volumes 1-4
• Addison Wesley, 1995-2000

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VisualWorksThe Image A Virtual Veldt

• The Cradle of Object-OrientedArchitecture
• An Olduvai Gorge Teeming with Patterns

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GoF Design Patterns

• Creational patterns
• Abstract Factory
• Builder
• Factory Method
• Prototype
• Singleton
• Structural patterns
• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

• Behavioral Patterns
• Chain of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Template Method
• Visitor

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The Big Five

• Composite
• Proxy
• Strategy
• Observer
• Visitor

• Lion
• Leopard
• Elephant
• Buffalo
• Rhino

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Objects within Objects
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Composite

• Context
• Developing OO software
• Problem
• Complex part-whole hierarchy has lots of similar
  classes.
• Example document, chapter, section, paragraph.
• Forces
• simplicity -- treat composition of parts like a
  part
• power -- create new kind of part by composing
  existing ones
• safety -- no special cases, treat everything
  the same

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Composite
Idea make abstract "component"
class. Alternative 1 every component has a
(possibly empty) set of components.
Component
Children
Paragraph
Chapter
...
Problem many components have no components
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Composite
Component
Container
Children
Composite
Leaf
Composite and Component have the exact same
interface. interface for enumerating children
Component implements Children() by returning
empty set interface for adding/removing
children?
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Composite Pattern
Component
Container
Children
Composite
Leaf
Composite and Component have the exact same
interface. interface for enumerating children
Component implements Children() by returning
empty set interface for adding/removing
children?
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Two Design Alternatives
Component does not know what it is a part
of. Component can be in many composite. Component
can be accessed only through composite.
Component knows what it is a part of. Component
can be in only one composite. Component can be
accessed directly.
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Component Knows its Composite

• Rules when component knows its single composite.
• A is a part of B if and only if B is the
  composite of A.
• Duplicating information is dangerous!
• Problem how to ensure that pointers from
  components to composite and composite to
  components are consistent.

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Ensuring Consistency

• The public operations on components and
  composites are
• Composite can enumerate components.
• Component knows its container.
• Add/remove a component to/from the composite.
• The operation to add a component to a composite
  updates the container of the component. There
  should be no other way to change the container of
  a component.
• Smalltalk does not enforce private methods.

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Example Equipment
Equipment
weight
cost
CompositeEq.
FloppyDisk
...

• weight
• total
• total 0.
• self childrenDo each total total each
  weight.
• total

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VisualComponentgtgtadd
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Composite Pattern in VisualWorks
VisualComponent
Children
VisualPart
ComposedText
Container
CompositePart
ListView

• Most operations in CompositePart simply iterate
  over the children and repeat the operation on
  them.

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CompositePart

• addComponent aVisualComponent
• self isOpen
• ifTrue ...
• ifFalse components addLast aVisualComponent.
• aVisualComponent container self
• displayOn aGraphicsContext
• "Display each of the receiver's components."
• clipBox
• clipBox aGraphicsContext clippingBounds.
• components do
• component
• (component intersects clipBox)
• ifTrue component displayOn aGraphicsContext
  copy

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Summary of Composite

• Composite is a kind of Component
• Permits arbitrary hierarchies
• Add/remove Component from Composite
• Operations on Composite iterate over Components

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Template Method

• As plentiful as sawgrass...
• Often associated with scavengers
• e.g. printOn aString

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Factory Method

• Plentiful in the image...
• View defaultControllerClass
• UILookPolicy
• XxxClass methods

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Abstract Factory
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Builder

• ClassBuilder
• UI Builders

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Prototype

• This isnt Self, after all...
• VisualWorks TextAttributes
• Look for copy
• Thinglab, Morphic...

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Singleton

• All the Metaclasses...
• ProcessScheduler
• SourceFileManager
• true, false, nil

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Strategy

• Sundry Policies...

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Observer Pattern

• Intent Define a one-to-many dependency between
  objects so that when one object changes state,
  all its dependents are notified and updated
  automatically.

observer/dependent
Observer Update
Subject AddDependent RemoveDependent Changed
LineFigure Update
endPoint
RectangleFigure
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Iterator

• Provide a way to access the elements of an
  aggregate object sequentially without exposing
  its underlying implementation.

Iterator
Aggregate
First() Next() IsDone() CurrentItem()
CreateIterator()
ConcreteAggregate
ConcreteIterator
CreateIterator()
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Iterator in Smalltalk

• 1) Internal iterator - iterate inside the
  Iterator
• easy to use, not as powerful as external iterator
• works well because of blocks
• employees do each each name printOn
  aStream
• 2) Combine Next() and CurrentItem() (Smalltalk
  Stream)
• employeeStream GeneralMotors
  employeeStream.
• employeeStream atEnd
• whileFalse
• employeeStream next name
  printOn aStream

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Proxy

• Provide a surrogate or placeholder for another
  object to control access to it
• represent an object in a remote address space
• create expensive objects on demand
• check access rights
• Proxy has same interface as real subject, and
  forwards operations to it

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Proxy
Subject
Client
request
...
realSubject
Proxy
RealSubject
request
request
realSubject request
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Proxy

• Remote proxy - first package arguments, then make
  remote procedure call.
• Virtual proxy - compute objects, then forward
  request.
• Protection proxy - check access rights, then
  forward request.

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Creating an Orphan

• nil subclass Future
• instanceVariableNames semaphore '
• classVariableNames '
• poolDictionaries ' '
• category Reflection-Examples
• In VisualWorks, ClassBuilder does the rest.
  Default implementations of doesNotUnderstand and
  class are provided.

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Locating Orphans

• allBehaviorsDo aBlock
• "Evaluate the argument, aBlock, for each kind of
  Behavior in the system (that is, Object and its
  subclasses,
• plus other classes that have no superclass)."
• Class rootsOfTheWorld do
• cls
• aBlock value cls.
• cls allSubclassesDo aBlock

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Terminology and Taxonomy

• Is this a Wildebeest?
• A Gnu?
• Class MammaliaOrder ArtiodactylaFamily
  BovidaeGenus species Connochaetes (flowing
  beard) taurinus (like a bull) albojubatus (white
  mane)

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Decorator

• Object ()
• VisualComponent ()
• VisualPart ()
• Wrapper ()
• GeometricWrapper ()
• FillingWrapper ()
• StrokingWrapper ()
• GraphicsAttributesWrapper ()
• PassivityWrapper ('controlActive')
• ReversingWrapper ()
• StrikeOutWrapper ()
• ScalingWrapper ()
• TranslatingWrapper ()
• LayoutWrapper ()
• BoundedWrapper ()
• BorderedWrapper ()
• MenuBarWrapper ()
• BoundingWrapper ()

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Adaptor

• Aspect Adaptor
• Pluggable Adaptor

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Flyweight

• Character

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Chain of Responsibility

• VisualPart

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Command

• MenuItem
• Blocks are often used instead in Smalltalk, so
  this pattern is not as common in its pure form in
  this environment...

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Interpreter

• The WindowSpec Walker
• Though its not clear how to classify this
• The simulator?

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Observer
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Parse Tree
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Visitor

• ProgramNodeEnumerator
• This class provides a framework for recursively
  processing a program node tree. Program nodes of
  type ltNodeTypegt should implement
• nodeDo anEnumerator
• anEnumerator doltNodeTypegt self
  ...arguments...
• by analogy with the ProgramNodeBuilder messages
• newltNodeTypegtarguments
• The old node is the first argument, so that
  enumerators can preserve the comment and source
  information if they wish.
• To rebuild each non-leaf node, the enumerator
  sends the message
• self doNode ltargumentgt
• which is implemented by default as
• doNode aNode
• aNode nodeDo self

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Visitor

• ProgramNodeEnumerator
• Subclasses must implement the following messages
• enumerating-non-leaves
• doAssignmentvariablevalue
• doBlockargumentsbody
• doCascadereceivermessages
• doMessagereceiverselectorarguments
• doMethodselectorprimitiveblock
• doParametervariabletype
• doReturnvalue
• doSequencetemporariesstatements
• enumerating-leaves
• doLiteralvalue
• doVariablename

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ProgramNodeEnumerator

• ProgramNodeEnumerator is a visitor for the
  Smalltalk-80 compiler.
• Smalltalk compiler parse tree hierachy rarely
  changes.
• There are lots of potential programs that need to
  traverse trees
• consistency checkers
• program transformation
• metrics
• It makes sense to keep these applications out of
  the parse tree.
• Easy to make minor variations on tree walking
  applications.

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Facade

• Rarely seen in the image...
• Perhaps the Smalltalk Compiler itself qualifies

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Bridge

• Elusive Quarry
• Yellands paper from OOPSLA 96 discusses how
  Bridges might improve the image...

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State

• Have you seen one?

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Memento

• Nocturnal?
• Seen in applications, like the Refactoring
  Browser
• Difficult to find in the image...

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Mediator

• Case can be made for models...

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GoF Design Patterns

• Creational patterns
• Abstract Factory
• Builder
• Factory Method
• Prototype
• Singleton
• Structural patterns
• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

• Behavioral Patterns
• Chain of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Template Method
• Visitor

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Bird on Patterns

• Learn the patterns and then forget em
• -- Charlie Parker
• http//www.hillside.net

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Conserving Infodiversity

• The Image A Cyberworld Heritage Site

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A Victory over Entropy

• A stunning collective achievement
• A temple to design
• Incubator for architecture
• A pattern hunters paradise

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Contact Information

• Brian Foote
• The Refactory, Inc.
• 209 W. Iowa
• Urbana, IL 61801
• [phone]
• http//www.laputan.org (H)
• http//www.refactory.com (O)
• foote_at_refactory.com