A CRC Description of HotDraw
CRC Cards are a way of describing objects. One writes on index cards the responsibilities
and collaborators one expects for each class used in a program. These are the first CRC
Cards ever made. I wrote them early one morning years ago after I had been sleeplessly
considering how to give students a feel for object design. The cards document HotDraw,
a drawing editor written in Smalltalk by myself and Kent Beck.
We start with the usual model, view and controller.
Drawings hold Figures. Here is the abstract class and a few refinements.
I chose these particular kinds of Figures to illustrate additional
responsibilities assumed in refinement.
Note that BoxFigure is defined by attributes. I knew that these were weaker than
responsibilities, but, wanted to show the versitility of paper in handling the
The DrawingController collaborates with Tools to make changes. I didn't make a card for
abstract Tool since it didn't carry any responsibility I felt needed recording. The class
naming conventions implies its existence.
The SelectionTool can touch and drag both Figures and Handles. Tools and Handles both
manipulate Figures. That makes Handles a wonderful little unit of extension that looks
like a Figure to a Tool and like a Tool to a Figure.
Finally, Locator is a Point substitute that knows how to propagate changes between related
figures. For example, when a Line connects to the corner of a BoxFigure and the BoxFigure
moves, an update message propagates through a Locator to the Line.
Actually HotDraw didn't
have Locators. Instead it had special cases that dealt with the change propagation.
When I was drawing the cards the idea of a Locator just came to me. I've since used
them effectivly in reimplementations of HotDraw.
So these are the original CRC Cards. I could tell as I was writing them that I was
on to something good.
I've recently reflected on this early experience in a
position statement for the OOPSLA '94
Workshop on Teams & Objects. There's also related information accessable from
our home page.
Copyright 1994, Ward Cunningham
All Rights Reserved.