Smalltalk and Object Orientation: an Introduction - Free

Finally, it is interesting to note a comment made in Ralph Johnson’s paper “The implementation [of
HotDraw for ObjectWorks Rel 4.0] was simpler than previous ones because the design of the user
interface framework for release 4.0 was influenced by HotDraw”.
21.3.4 Facilities provided by HotDraw
So what does HotDraw provide in this framework. (I will use the term framework to describe the basic
HotDraw editor). You get the following facilities:
• A ToolPaletteView. This is the tool bar down the left hand side of the display.
• A DrawingView. This is the area a user would actually draw in.
• A DrawingEditor. This is the model for the DrawingView and Drawing Controller.
• A Drawing. This is also a model used by the DrawingView and Drawing Controller.
• A DrawingController. which allows you to interact with the application.
All these are already connected together in the appropriate manner, unlike many class -only packages, in
which you as the developer must determine how to bring these elements together.
21.4 Where do you start?
Figure 21.1: The BoxDraw application to be built
The remainder of this document is in effect a sample pattern which illustrates how you create a simple
extension to the drawing editor. The problem to be addressed by this pattern is:
The basic facilities provided by HotDraw make it very easy to construct a simple graphic
editor for drawing diagrams. One useful feature of such diagrams is the ability to link figu res
together and to get those links to move when the figures are moved. Thus there needs to be a
way to link two figures together.
21.4.1 The DrawingEditor
You will almost always start by subclassing the DrawingEditor. This is the basic element of any
HotDraw tool. In this case we shall subclass DrawingEditor to create BoxDrawEditor, which will be the
basis of our application BoxDraw. The definition of the BoxDrawEditor is presented below:
DrawingEditor subclass: #BoxDrawEditor
176