Using Patterns to Document Frameworks

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Using Patterns to Document Frameworks

• Frameworks contain patterns
• Patterns are design decisions

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Design

• Design is a set of choices.
• Choices made one at a time.
• Ideally, all choices are correct.
• Each choice based on current information.

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Design Documentation

• Need to learn a bit at a time.
• Need to know why a decision was made.
• Know when to change it.
• Know how to adapt to it.

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Using Patterns for Documentation

• Patterns can describe how to use a system.
• Documenting Frameworks Using Patterns
• Patterns can describe the design of a system. A
  sequence of patterns, one for each design
  decision.
• Patterns Generate Architectures

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HotDraw A Framework for Graphics Editors
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Overview of Derivation of HotDraw

• Model-View-Controller user interface
• Composite Design Patterns
• State Design Patterns
• Editor (to operate on drawing)
• Observer Design Patterns
• Collect Damage user interface
• Update at User Speed user interface
• Editor (to operate on figure)
• Decorator Design Patterns

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Model-View-Controller
Drawing
DrawingView
DrawingController
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Composite
Drawing contains many figures. Make Drawing be a
composite figure.
Figure
Display
Drawing
Drawing
EllipseFigure
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State

• Allow an object to alter its behavior when its
  internal state changes.
• The object will appear to change classes.
• Make a class hierarchy that models the different
  kinds of states that client could be in.

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• Client points to its "current state".
• Client delegates all state-dependent behavior to
  current state.
• Tools are the "current state" of the
  DrawingController it delegates operations to the
  current state.

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Figure
Display
Drawing
Drawing
current tool
Tool
DrawingController
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Editor

• Who creates the tools that the DrawingController
  will use as states?
• Different drawings will need different Tools.
• Make Drawing responsible for knowing Tools that
  can operate on it.

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Figure
Display
Drawing
Drawing
Default tools
current tool
DrawingController
Tool
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What is a Pattern?

• Is Editor a pattern, or a combination of
  patterns?
• Con same as using Factory Method to produce an
  Adapter.
• Pro Editor has a unique intent to provide a
  common interface to a heterogenous collection of
  objects.

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What is a Pattern?

• Are two patterns with the same solution the same
  pattern?
• Are combinations of patterns also patterns?

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Observer

• Each Figure knows its dependents, and notifies
  them when it changes state.
• Dependents of a figure include its Drawing, so
  the Drawing can redisplay itself if a figure
  changes.

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Collect Damage

• How do you display as little as possible?
• Have the graphics system keep track of the part
  of the image that has changed, a damaged
  region. Every time a part of the image changes,
  add its area to the damaged region. Redisplay
  the changed region when possible.

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Figure
Display
Drawing
dependent model
Drawing
DrawingView
Default tools
Damaged region
current tool
DrawingController
Tool
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Update at User Speed

• Update too fast, and all the time is spent doing
  graphics.
• Update too slow, and animation is jerky.
• Update the drawing at the same speed that the
  user makes gestures. Redisplay the drawing once
  for each user event.

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Editor

• How to operate on Figures?
• Ask Figures for their handles and then operate on
  the handles.

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Figure
Display
Handles
Drawing
dependent
Drawing
DrawingView
model
Default tools
Damaged region
current tool
Tool
DrawingController
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Decorator

• Should handles go in the Drawing? No - 2 views of
  a drawing.
• In the View? This makes views complex.
• Make a Drawing Decorator class (SelectionDrawing)
  to hide and show sets of handles.

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Figure
Display
Handles
Drawing
Drawing
Handles
Default tools
dependent
DrawingView
SelectionD.
model
Damaged region
current tool
Tool
DrawingController
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Summary of Derivation of HotDraw

• How I wish I had done it.
• Rational design.
• Describes "why" of a feature, not just how it is
  implemented.
• Lots of different kinds of patterns are necessary.