Smalltalk and Object Orientation: an Introduction - Free

about a set of classes, than to attempt to understand the class and work from there (particularly when
you are learning Smalltalk). For example, if a method takes a block as an argument, what is that block
supposed to look like? An example or two are often invaluable.
Every HotDraw application is comprised of a number of elements, these are the Drawing Editor, the
Drawing V iew, the Drawing Controller, the Drawing, figures in the drawing, handles, constraints
between drawings and creation tools. Each of these is explained briefly below:
DrawingEditor DrawingEditor is a subclass of Model that represents a graphical drawing ed itor.
It is the model of the MVC triad that includes a
DrawingView and a
DrawingController. It shares the DrawingView and the DrawingController with a
Drawing.
DrawingView A DrawingView is the view component of a MVC triad. The Drawing and the
DrawingController are the other components. A DrawingEditor also uses the
DrawingView in a MVC triad.
DrawingController A DrawingController is the controller component of a pair of MVC triads.
A Drawing and A DrawingView is one pair and a DrawingEditor and DrawingView
are the other pair. A DrawingController’s primary task is to delegate mouse and keyboard
activity to the current tool of the Drawing.
Drawing A drawing is a directed graph of figures (or complex figures). As indicated above it is a model
in a MVC triad with the DrawingView and DrawingController.
Figures A Figure is a kind of drawing element or widget which describes how it should be drawn
within a DrawingEditor. A CompositeFigure is a figure which can contain other
figures as components, which all have an influence on what the composite figure looks like.
CompositeFigure also defines a bounding box which is independent of its sub -components.
Only components which are wholly within the bounding box are actually displayed. Unlike
VisualComponents in VisualWorks, figures keep track of which objects which depend on them.
Handles All figures can potentially have handles. Handles are a way to modify a figure’s attributes, for
example by resizing the figure, or by linking a figure to another figure and thereby crea ting a
dependency. The Figure class defines a method handles which returns a basic set of handles for
resizing a figure. It is therefore common for subclasses of figure to redefine the handles method,
either to overwrite this, or to add to it (in which cas e the method would “call super” handles at
some point).
Constraints HotDraw provides Constraint objects which are used to express dependencies between
objects. A MultiheadedConstraint, which is a subclass of constraint, makes the state of
one object be the function of the states of many other objects. The one object is the “sink” and
the many objects are the “sources”. For example, if the value of one cell in a spreadsheet is the
sum of five other cells, then the five cells are the sources and the cell with the sum is the sink.
CreationTools These are used to create a new instance of a figure. They are parameterized by the class
of the figure to display, the icon to display in the tool palette and the cursor to use while that tool
is selected.
21.3.2 Obtaining HotDraw
HotDraw is available by anonymous FTP from the Smalltalk repository at st.cs.uiuc. There are versions
available for most of the recent releases of ParcPlace Smalltalk (e.g. VisualWorks version 1.0, 2.0 and
2.5). You should follow t he installation instructions and then immediately save the image onto your
local file store and exit VisualWorks.
21.3.3 Examples with HotDraw
The examples provided with HotDraw include a Pert Chart tool, a HyperCard clone, HotPaint and
Network Editor which allows you to play with nodes in a network which are either attracted or repulsed
by each other and a diagramming inspector. If you have access to HotDraw (e.g. via ftp) then explore
them! Explore the patterns at the end of this chapter which describes the patt erns based on one of these
example applications.
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