Brief Introduction to ObjectOriented Frameworks

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Brief Introduction to Object-Oriented Frameworks

• Kurt Stirewalt

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Overview of talk

• OO Frameworks
• Definitions and characteristics
• Example Java AWT
• Issues in framework design
• Usability
• Extensibility, efficiency, and maintenance
• Frameworks and code generation

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Object-Oriented Frameworks

• A.k.a Object-oriented abstract design
• Salient features
• Corpus of (partially implemented) classes
• Framework classes expect to collaborate with
  others
• Instantiated by subclassing framework classes and
  providing or over-riding polymorphic operations
• Many well-known examples
• Modern UI toolkits (e.g., Java AWT, subArctic,
  MFC).
• Distributed computing toolkits (e.g., ACE).

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Brief example (and notation)
Button
ActionListener
addActionListener()
actionPerformed()
MyApplicationClass
actionPerformed()
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Frameworks programming model

• Often
• User-defined methods invoked by framework code.
• Framework plays the role of main program''.
• This inversion of control allows frameworks to
  serve as extensible code skeletons.
• User-supplied methods tailor generic framework
  algorithms for a specific application.

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Example Java AWT Framework
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Example instantiation Online orders

Applet
Button
TextBox
Panel
OrderEntryApplet
StoreFrontDisplay
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Overview of talk

• OO Frameworks
• Definitions and characteristics
• Example Java AWT
• Issues in framework design
• Usability
• Extensibility, efficiency, and maintenance
• Frameworks and code generation

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Issues in framework design

• Usability
• Some frameworks require more implementation
  knowledge to use than others
• White- vs. black-box distinction
• Extensibility/maintenance
• Successful frameworks get large and bulky
• Monolithic vs. collaboration-stratified frameworks

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Framework usability

• The AWT1.0 event model is the son of Satan!
• -- Ian Smith, subArctic developer
• White-box
• Override to customize
• Internal structure of classes exposed
• Black-box
• Configure to customize
• Only interfaces exposed

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Example AWT event handling

• White box (version 1.02)
• Every UI component had handleEvent method
• To catch events, define subclass and override
  method
• Black box (version 1.1)
• UI components fire events to interested
  listeners
• Objects that implement a listener interface can
  register for event notification from a UI
  component
• Instantiation in 1.1 uses knowledge of an
  abstract protocol, not UI-component implementation

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Black-box evolution of AWT
AWT v1.1
JFC/Swing
AWT v1.0
Black-box
White-box

• Observe Can take a long time to migrate a
  white-box to a black-box framework

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Why is black-box better?

• Possible answers
• Maturity of product
• Separation of concerns
• Better conceptual abstractions
• Better understanding of key collaborations
• Observe All of these features are important for
  code generation

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Maintenance Evolution

• Problem Framework classes can get bulky
• Function of dependencies among the classes
• Also backward compatibilty in presence of new
  features.
• But, not all classes communicate with all others!
• Rather, classes involved in smaller
  collaborations.
• Classes play roles in a collaboration
• Role often involves only subset of class
  operations
• Solution GenVoca style layering/composition
  useful for organizing frameworks

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Overview of talk

• OO Frameworks
• Definitions and characteristics
• Example Java AWT
• Issues in framework design
• Usability
• Extensibility, efficiency, and maintenance
• Frameworks and code generation

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Frameworks and code generation

• Easier to build tool to specialize well-defined
  classes than to build a general-purpose code
  generator
• Johnson Foote88.
• Frameworks provide
• (partial) specification for new components and
• template for implementing them
• UML class/state diagrams good at representing
  framework classes and behaviors
• Meridian Can we automatically instantiate
  framework classes from UML-model specifications