Smalltalk Best Practice Patterns Volume 1: Coding - Free

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DelegationA Composed Method needs work done by another object. A Message invoked computation inanother object.How do you share implementation without inheritance?Inheritance is the primary built-in mechanism for sharing implementation in Smalltalk. However,inheritance in Smalltalk is limited to a single superclass. What if you want to implement a newobject like A but also like B? Also, inheritance carries with it potentially staggering long-termcosts. Code in subclasses isn’t just written in Smalltalk, it is written in the context of every variableand method in every superclass. In deep, rich hierarchies, you may have to read and understandmany superclasses before you can understand even the simplest method in a subclass.Factored Superclass explains how to make effective use of inheritance at minimal developmentcost. You will encounter situations where you will recognize common implementation, but whereFactored Superclass is not appropriate. How can you respond?Pass part of your work on to another object. Other objects can also delegate to that object (oranother instance of the same class).Use Simple Delegation when the delegate need know nothing about the original object. Use SelfDelegation when the identity of the original object or some of its state is needed by the delegate.Coding Patterns page 50 of 147 9/30/2006
Simple DelegationYou need Delegation to a self-contained object. You may be implementing a method: CollectionAccessor Method, Equality Method, Ordering Method, or Hashing Method.How do you invoke a disinterested delegate?When you use delegation, there are two main issues that help clarify what flavor of delegation youneed. First, is the identity of the delegating object important? This might be true if a client objectpasses itself along, expecting to be notified of some part of the work actually done by the delegate.The delegate doesn’t want to inform the client of its existence, so it needs access to the delegatingobject. Second, is the state of the delegating object important to the delegate? Delegates are oftensimple, even state-less objects, in order to be as widely useful as possible. If so, the delegate islikely to require state from the delegating object to accomplish its job.There are many cases of delegation where the answer to these two questions is “no”. The delegatehas no reason to need the identity of the delegating object. The delegate is self-contained enough toaccomplish its job without additional state.Delegate messages unchanged.The typical example of this is an object that acts like a Collection (at least a little), but has lots ofother protocol. Rather than waste inheritance by subclassing one of the collection classes, yourobject refers to a Collection. From a client’s perspective, though, you respond to protocol like do:or at:put:.The Collection doesn’t care who invoked it. No state from the delegating object is required. Theidentity of the delegating object is irrelevant.Here’s an example, a Vector that holds only Numbers. We could implement it by subclassingCollection, but then we would lose the benefits of inheriting from the Number hierarchy. Clientswant to treat a Vector as a Collection of Numbers sometimes.Vectorsuperclass: Numberinstance variables: elementsWe create a Vector with a given number of elements:Vector class>>new: anInteger^self new setElements: (Array new: anInteger)Vector>>setElements: aCollectionelements := aCollectionWe’ll ignore the arithmetic nature of Vectors, and focus on how it delegates. When someoneiterates over a Vector, it delegates to its “elements” instance variable:Vector>>do: aBlockelements do: aBlockThis is an example of Simple Delegation. You can imagine implementing at:, at:put:, size, etc. thesame way.Coding Patterns page 51 of 147 9/30/2006
Page 1 and 2: Smalltalk Best Practice PatternsVol
Page 3 and 4: STATE..............................
Page 5 and 6: PrefaceThis preface will explain wh
Page 7: AcknowledgmentsI would like to than
Page 10 and 11: Many of the patterns tell you what
Page 12 and 13: day to day in development and docum
Page 14 and 15: What’s Missing?There are a few co
Page 16 and 17: AdoptionHow can you learn and use t
Page 18 and 19: Why Patterns WorkHere is the Big As
Page 20 and 21: Role of PatternsReadingThese patter
Page 22 and 23: BehaviorObjects model the world thr
Page 24 and 25: Composed MethodYou are implementing
Page 26 and 27: Complete Creation MethodA Composed
Page 28 and 29: ConversionHow do you convert inform
Page 30 and 31: Converter Creation MethodYou need t
Page 32 and 33: Query MethodA Composed Method has h
Page 34 and 35: Execute Around MethodHow do you rep
Page 36 and 37: MessagesMessages are the heartbeat
Page 38 and 39: Choosing MessageYou are using a Mes
Page 40 and 41: Decomposing MessageYou are using a
Page 42 and 43: Intention Revealing SelectorYou may
Page 44 and 45: Integer>>addFloat: aFloat^self asFl
Page 46 and 47: Class: SuperFindingClientsuperclass
Page 48 and 49: Extending SuperYou are using Super.
Page 52 and 53: Self DelegationYou are using Delega
Page 54 and 55: Pluggable BehaviorHow can you param
Page 56 and 57: Pluggable BlockYou need complex Plu
Page 58 and 59: marriedMenAndUnmarriedWomen^self ma
Page 60 and 61: Instance VariablesI wrote the secti
Page 62 and 63: Variable StateHow do you represent
Page 64 and 65: Timer>>defaultMillisecondPeriod^100
Page 66 and 67: Default Value MethodYou have a comp
Page 68 and 69: Direct Variable AccessYou need to a
Page 70 and 71: Getting MethodYou are using Lazy In
Page 72 and 73: Collection Accessor MethodYou are u
Page 74 and 75: Enumeration MethodYou are using Ind
Page 76 and 77: Role Suggesting Instance Variable N
Page 78 and 79: Temporary VariableA Composed Method
Page 80 and 81: Caching Temporary VariableA Perform
Page 82 and 83: Reusing Temporary VariableTemporary
Page 84 and 85: CollectionsThe collection hierarchy
Page 86 and 87: CollectionHow do you represent a on
Page 88 and 89: RunArrayYou are using an OrderedCol
Page 90 and 91: | s |s := Set new.Like an OrderedCo
Page 92 and 93: You may need Double Dispatch for co
Page 94 and 95: DictionaryYou need a Collection tha
Page 96 and 97: SortedCollectionYou have a Collecti
Page 98 and 99: ByteArrayYou need to represent an A
Page 100 and 101:
(1 to: anArray size) collect:[:each
Page 102 and 103:
IsEmptyHow do you test if a collect
Page 104 and 105:
ConcatentationHow do you put two co
Page 106 and 107:
DoHow do you execute code for each
Page 108 and 109:
Select/RejectHow do you execute cod
Page 110 and 111:
Inject:into:You need an Enumeration
Page 112 and 113:
Duplicate Removing SetYou have a Co
Page 114 and 115:
StackHow do you implement a stack?O
Page 116 and 117:
implementation of queues in Pascal
Page 118 and 119:
Lookup CacheHow do you optimize com
Page 120 and 121:
Parsing StreamHow do you write a si
Page 122 and 123:
ClassesThere is probably no coding
Page 124 and 125:
Qualified Subclass NameYou have cre
Page 126 and 127:
FormattingNo other topic generates
Page 128 and 129:
Type Suggesting Parameter NameYou a
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Rectangular BlockYou are writing an
Page 132 and 133:
Conditional ExpressionHow do you fo
Page 134 and 135:
Simple Enumeration ParameterWhat sh
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self listPane parentcolor: Color bl
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Point>>printOn: aStreamaStreamprint
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Development ExampleIn this chapter,
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Money>>setAmount: aNumber currency:
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MoneySum>>printOn: aStreammonies do
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| m1 m2 |m1 := Moneyamount: 5curren
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