Class Methods At Work

What You Will Learn

• Class methods are normal methods
• Most class methods create new instances
  • but they can be used for other things

Parsing Lines

Imagine we want to parse

!Section Title
- list item
-- subitem

Any text here

A Possible Design

• Document item classes know
  • if they can parse a line (canParse:)
  • how to create instances (newFromLine:)

Parsing Lines

Parser >> documentClasses
   ^ DocumentItem allSubclasses
      sorted: [ :class1 :class2 | class1 priority < class2 priority ]

Parser >> parse: line
   self documentClasses
      detect: [ :subclass |
         (subclass canParse: aLine)
            ifTrue: [ ^ subclass newFromLine: line ] ]

The Command-Line Handler

• the Pharo command-line interface (CLI) uses the same approach
• each subclass of CommandLineHandler knows how to deal with one command
• the correct subclass is selected by sending messages to the class

$ pharo Pharo.image eval "10 factorial"
3628800

The Command-Line Handler

CommandLineHandler class >> isResponsibleFor: arguments
   ^ arguments includesSubCommand: self commandName

EvaluateCommandLineHandler class >> commandName
   ^ 'eval'

CommandLineHandler class >> allHandlers
   ^ self allSubclasses
      reject: [ :handler| handler isAbstract ]

CommandLineHandler class >> handlersFor: arguments
   ^ self allHandlers
      select: [ :handlerClass |
         handlerClass isResponsibleFor: arguments ]

Conclusion

• Classes are objects and can be sent messages
• Method lookup is exactly the same as for all objects:
  • go to the class of the receiver
  • follow inheritance chain
• More during the lecture Understanding Metaclasses
• Pharo makes it easy to iterate over subclasses