Smalltalk Best Practice Patterns Volume 1: Coding - Free

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Class: SuperFindingClientsuperclass: InstructionClientinstance variables: foundSuperFindingClient class>>isSuperSentBy: aMethod^self new isSuperSentBy: aMethodSuperFindingClient>>isSuperSentBy: aMethod| stream |found := false.stream := InstructionStream on: aMethod.[stream atEnd] whileFalse: [stream interpretNextInstructionFor: self].^foundSuperFindingClient>>sendSuper: aSymbol numArgs: anIntegerfound := trueI sometimes get into trouble using Send Back when I get several levels of Delegation and SendBack going at once. When I start to get confused, I sit down a draw a diagram that shows all theobjects and all the protocols they expect of each other. I invariably find that I am misusing SendBack or Delegation in at least one case, and I can clarify the situation by removing the gratuitouspattern.You can use Cunningham diagrams [ref OOPSLA 86], OORam Message Scenario diagrams [refReenskaug], or Object Interaction diagrams [ref Jacobsen] for this kind of analysis.Give your messages Intention Revealing Selectors.Coding Patterns page 46 of 147 9/30/2006
SuperYou are sending a Message.How can you invoke superclass behavior?An object executes in a rich context of state and behavior, created by composing together thecontexts of its class and all of its class’ superclasses. Most of the time, code in the class can bewritten as if the entire universe of methods it has available is flat. That is, take the union of all themethods up the superclass chain and that’s what you have to work with.Working this way has many advantages. It minimizes any given method’s reliance on inheritancestructure. If a method invokes another method on self, as long as that method is implementedsomewhere in the chain, the invoking method is happy. This gives you great freedom to refactorcode without having to make massive changes to methods that assume the location of some method.There are important exceptions to this model. In particular, inheritance makes it possible tooverride a method in a superclass. What if the subclass method wants some aspect of the superclassmethod? Good style boils down to one rule: say things once and only once. If the subclass methodwere to contain a copy of the code from the superclass method, the result would no longer be easyto maintain. We would have to remember to update both (or potentially) many copies at once. Howcan we resolve the tension between the need to override, the need to retain the illusion of a flatspace of methods, and the need to factor code completely?Invoke code in a superclass explicitly by sending a message to “super” instead of “self”. Themethod corresponding to the message will be found in the superclass of the classimplementing the sending method.Always check code using “super” carefully. Change “super” to “self” if doing so does not changehow the code executes. One of the most annoying bugs I’ve every tried to track down involved ause of super that didn’t do anything at the time I wrote it, and that invoked a different selector thanthe one for the currently executing method. I later overrode that method in the subclass and spenthalf a day trying to figure out why it wasn’t being invoked. My brain had overlooked the fact thatthe receiver was “super” instead of “self”, and I proceeded on that assumption for severalfrustrating hours.Extending Super adds behavior to the superclass. Modifying Super changes the superclass’behavior.Coding Patterns page 47 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 48 and 49: Extending SuperYou are using Super.
Page 50 and 51: DelegationA Composed Method needs w
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
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ByteArrayYou need to represent an A
Page 100 and 101:
(1 to: anArray size) collect:[:each
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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
Page 108 and 109:
Select/RejectHow do you execute cod
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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
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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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