"Egyke tervminta"

"Ebben a példában egy adatbáziskezelő osztályt valósítunk, melyből csak egyetlen egy létezhet. Az írások és az olvasások között zárolást használ az osztály."

Object subclass: #DatabaseAccessor
instanceVariableNames: 'lock'
classVariableNames: 'Instance'
poolDictionaries: '' 

DatabaseAccessor class>>singleton
 Instance isNil
  ifTrue: [Instance := self basicNew initialize].
 ^Instance 

DatabaseAccessor class>>new
 ^self error: 'DatabaseAccessor has only one instance. ','To retrieve it, send "DatabaseAccessor singleton".' 

DatabaseAccessor>>initialize
 lock := false.
"Open the file" 


DatabaseAccessor>>write: aDatabaseRecord
 "Set the lock and fork the 'real' write method."
 lock := true.
 [self writePrim: aDatabaseRecord] fork 

DatabaseAccessor>>writePrim: aDatabaseRecord
 "Write the record in aDatabaseRecord to the file."
  "Now that the write is complete, unlock:"
 lock := false. 

DatabaseAccessor>>read: aKey
 "Return the DatabaseRecord keyed by aKey."
 | record |
 "Don't read while a write is in progress."
 [lock] whileTrue: [Processor yield].
 "Now, read the record:"
 record := DatabaseRecord new.
 "Modify the Record"
 ^record 


“DatabaseAccessor singleton read: aKey” 

“DatabaseAccessor default.”