Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Self DelegationYou are using Delegation.How should you implement delegation to an object that needs reference to the delegatingobject?The issues are the same for Self Delegation as for Simple Delegation. Do you need the identity ofthe original delegating object? Do you need state from the delegating object?If the answer to either of these questions is “yes”, Simple Delegation won’t work. The delegateneeds access to the delegating object somehow.One way to give the delegate access is to include a reference from the delegate back to thedelegating object. This approach has a number of drawbacks. The backwards reference introducesadditional programming complexity. Every time the delegate changes, the reference in the olddelegate has to be destroyed and the reference in the new delegate set. More importantly, eachdelegate can only be used by one delegating object at a time. If creating multiple copies of thedelegate is expensive or impossible, this simply won’t work.The other approach, the one suggested here, is to pass the delegating object along as an additionalparameter. This introduces a variant of the original method, which isn’t great, but the additionalflexibility of this approach is worth the cost.Pass along the delegating object in an additional parameter called “for:”The Digitalk Visual Smalltalk 3.0 image has an excellent example of Self Delegation. Theimplementation of hashed collections like Sets and Dictionaries is divided into two parts. The firstis the Set or Dictionary, the second is a HashTable. There are variants of HashTable that areefficient in different circumstances. The same collection might delegate to different HashTables atdifferent times, depending on its characteristics (how big, how full, etc.)The hash value of an object is implemented differently for different kinds of Collections.Dictionaries compute hash by sending “hash”, IdentityDictionaries compute it by sending“basicHash”.This is implemented using Self Delegation. When the Collection sends a message to the HashTableto add an element, it passes itself along:Dictionary>>at: keyObject put: valueObjectself hashTableat: keyObjectput: valueObjectfor: selfThe HashTable computes the hash value by sending back a message to the Collection:HashTable>>at: keyObject put: valueObject for: aCollection| hash |hash := aCollection hashOf: keyObject....Dictionaries and IdentityDictionaries implement this message differently:Coding Patterns page 52 of 147 9/30/2006
Dictionary>>hashOf: anObject^anObject hashIdentityDictionary>>hashOf: anObject^anObject basicHashSelf Delegation allows the hierarchy of hashed Collections to be independent of the hierarchy ofHashTables.If the delegate needs different logic depending on who is delegating, use Double Dispatch.Coding Patterns page 53 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 50 and 51: DelegationA Composed Method needs w
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
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ConcatentationHow do you put two co
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DoHow do you execute code for each
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Select/RejectHow do you execute cod
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Inject:into:You need an Enumeration
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Duplicate Removing SetYou have a Co
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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
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Parsing StreamHow do you write a si
Page 122 and 123:
ClassesThere is probably no coding
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Qualified Subclass NameYou have cre
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FormattingNo other topic generates
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Type Suggesting Parameter NameYou a
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Rectangular BlockYou are writing an
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Conditional ExpressionHow do you fo
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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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Smalltalk Best Practice Patterns Volume 1: Coding - Free

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