cs2340: Smalltalk 101

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cs2340 Smalltalk 101

• Summer 2007

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Advanced Topics in Smalltalk

• Loading/Unloading Code
• Inheritance / Exception Handling
• File IO
• Threading
• Networking

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Code Import

• Bundles - Packages
• FileIn / FileOut
• Parcels (binary deployment)
• Store
• Changesets Tools-gtChange List

See App Developers Guide, Managing Smalltalk
Source Tool Guide, Change Sets
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Inheritance

• shouldNotImplement (refuse to implement an
  inherited method)
• subclassResponsibility (force subclasses to
  implement this method)
• These are not reserve words, they are messages to
  instances of class Object

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Exceptions

• error a string.
• Old exception handling mechanism common to all
  objects (like try/catch)
• handle do sent to instance of Signal
• ArithmeticValue divisionByZeroSignal
• handle ex Transcript cr show ex
  errorString
• do 4 // 0
• Throwing DivisionSignal raise.

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New ANSI Exception Handling

• Replaces Signal with Exception class hierarchy
• send the on do message to a block
• x y
• x 7.
• y 0.
• x / y
• on ZeroDivide
• do ex Transcript show 'zero divide
  detected' cr.
• Avoid grabbing all exceptions/errors
• on Exception or on Error
• Method will return the result of exception block
  (in above example, the Transcript).

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New ANSI Exception Handling

• Can specify multiple exceptions
• comma separated
• handle ArithmeticException, AllocationFailure
• make an Exception Set
• specialExceptions ExceptionSet
• with ArithmeticException
• with AllocationFailure

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Create New Custom Exceptions

• Subclass Exception or Error
• Throwing the exception
• MyException raiseSignal
• MyException raiseSignal OOPS
• the string OOPS can be retrieved by sending
  the description message to the exception.

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Do Overs??

• What if I want to retry something with a new
  value? just sent retry to the exception
  instance.
• x / y
• on ZeroDivide
• do exception
• "make the divisor very small but gt
  0"
• y 0.00000001.
• exception retry

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Do Over (but try something new)!

• I want to try again, but use a different
  algorithm
• self doTaskQuickly
• on LowMemory
• do exception
• exception retryUsing self
  doTaskEfficiently
• Note Fail a second time passes exception up chain

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Streams

• Stream metaphor
• Streams can be on anything, not just files
• In-Memory ReadStream, WriteStream,
  ReadWriteStream
• File FileStream
• Streams have position (next, end, reset)

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Filename

• Utilities for accessing files
• filename bar.xml asFilename.
• filename Filename named bar.xml.
• check if file exists
• filename exists

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Reading/Writing to File
newFile stream newFile 'testFile'
asFilename. stream newFile writeStream. stream
nextPutAll A text message. stream
close. newFile contentsOfEntireFile
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Binary Storage of Objects (BOSS)

• colorNames newColor bos
• "First create a file containing color names."
• colorNames ColorValue constantNames.
• bos BinaryObjectStorage
• onNew 'colors.b' asFilename writeStream.
• bos nextPutAll colorNames
• ensure bos close.
• "Then append a new color name."
• newColor mudBrown.
• bos BinaryObjectStorage
• onOld 'colors.b' asFilename readAppendStream.
• bos setToEnd.
• bos nextPut newColor
• ensure bos close.

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XML

• Classes in XML namespace
• XML.Document, XML.XMLParser
• XML.Element, XML.Attribute, XML.Text
• BasicLibrary Reference

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Process Control

• Creating a new thread (or lightweight process) in
  Smalltalk is easy
• send fork message to a block. to start
  immediately
• send newProcess message to a block to create but
  not schedule the process.
• Creates instance of the Process class
• Valid states Suspended, Waiting, Runnable,
  Running

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Process Control

• To pass arguments use
• proc index emp code
  newProcessWithArguments (2 employee a).
• proc resume. start it up
• proc suspend. stop till I say otherwise
• proc terminate. die you monster
• proc yield. give up the rest of my time

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Synchronization

• Semaphore class provides thread synchronization
• sema Semaphore new.
• sema signal. Up got something
• sema wait. down wait for something

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Sharing Data

• SharedQueue class provides for thread-safe data
  transfer.
• q SharedQueue new.
• q next. get next item or wait till item
  available
• q nextPut a. put an item in queue, folks
  waiting will be notified

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Sleeping or Waiting a specific time period

• Create a Delay object and call wait
• d Delay forSeconds 30.
• d wait.
• (Delay forSeconds30) wait