Squeak

Squeak is based on principles of Live and Exploratory Programming

• direct code changes $\Rightarrow$ instant visual / auditory effects

Object Model

• Everything is an object
  • classes are objects too
  • class is the single instance of an anonymous class (meta-class)
    • e.g. Rectangle is sole instance of Rectangle class
• Only message passing between objects
• (Very) late binding
• All methods are public
  • method lookup: starting at receiver class, then bubbling up the inheritance tree
• Instance variables are private to the object
• Class-based single inheritance
• Garbage collection

A few but powerful ideas and language constructs $\Rightarrow$ Small and extensible language

Syntax (excerpt)

• Messages
  • unary: receiver selector
  • binary: receiver binary-selector argument
  • keyword: receiver keyword1: arg1 keyword2: arg2
  • Priority: unary > binary > keywords (same level from left to right | workaround: parantheses)
• Arrays
  • literal array: #(1 2 3 (1 3) $a 4)
  • byte array: #[1 2 3]
  • array: { Date today . Time now }
    • syntactic sugar mapped to Array with: with:
• Fractions: 1/33
• Comments: "a comment"
• Block: [:var | | tmp | expr... ]
  • Anonymous function objects (storeable and passable)
  • higher order functions: Closure / Lambda
  • Lexically scoped
• Method annotation (pragma): <...>
  • Primitive: <primitive: ...>

Class Definition

= a message sent to another class

Morph subclass: #CalculatorMorph
	instanceVariableNames: 'first second result'
	classVariableNames: ''
	poolDictionaries: ''
	category: 'Calculator'

Method Definition

• usually defined in a browser (directly invoking the compiler also possible)
• always returns an object (default is self)

ObjectSubclass>>descriptionForPartsBin
	^ self
 
		partName: 'Calculator'
		categories: #('Useful')
		documentation: 'Not yet...'

Instance Creation

• Literal: e.g. 1 or 'abc'
• General: new which uses basicNew and calls initialize
• Class specific: e.g. Color orange or Color r: 1.0 g: 0.6 b: 0.0

---

Self and Super

Similarities

• Refer to same object when used in same context
  • { self. super. self == super } $\to$ { sameObject. sameObject. true }
• can be passed as arguments to a message
  • rarely used for super
• can be assigned to variables
  • rarely used
• can receive messages

Method Lookup

self:

• dynamic lookup at run-time
• search for method begins in instance’s (receiver) class
  • even if self is used by superclass method

super:

• statically determined at compile-time
• search for method begins in superclass of the class containing the method, in which super was used
  • not necessarily direct superclass of instance (reveiver)

---

Live and Exploratory Programming

Benefits of short feedback loops:

• understand code and its effect on the system
• check whether the program works as expected
• tests (unit, integration, acceptance)
• investigate bugs
• explore unfamiliar code

Programming system vs. programming language:

• Integration von Sprache und Werkzeugen in einer IDE

---

Framework

Morphic

• #drawOn: aCanvas | draws an Morph or similar onto a canvs
• #handlesMouseDown: evt and #mouseDown: evt | override to handle mouse events
• #handlesKeyboard: evt and #keyDown: enEvent | override to handle keyboard events
• #addMorph: aMorph | adds a submorph

Observer

• #changed: aParameter or #changed: anAspect with: anObject | inform all the dependents about a change on the receiver
• #update: aParameter or #update: anAspect with: anObject | override to receive a change notice from an object of whom the receiver is a dependent
• #addDependent: anObject | make the given object one of the receiver’s dependents

Prototype

• #copy | executes #shallowCopy and then #postCopy
  • #shallowCopy | returns a copy of the receiver which shares the receiver’s instance variables (sharing by reference)
  • #postCopy | should be overridden by subclasses to complete the full copy
• #deepCopy | returns a copy of the receiver with its own copy of each instance variable
  • recursively calls #deepCopy
  • does not preserve object identities in cycles in the object graph
• #veryDeepCopy | does a complete tree copy using a Dictionary
  • an object appearing twice in the tree is only copied once
  • all references to the object in the copy of the tree will point to the new copy