Smalltalk and Object Orientation: an Introduction - Free

21.5.5 Running the editor
You have now done enough to open a BoxDrawEditor. Do this by evaluating:
BoxDrawEditor open.
in a Workspace window.
The BoxDrawFigure is the last icon in the ToolPalette. However, all you can do is place a
BoxDrawFigure in a drawing and change the label. Note to change the label, select the Text tool (the
big ‘T’) and hold down the shift button while selecting the label with the left mouse button.
21.5.6 Adding handles
To be able to link a BoxDrawFigure to another figure we need to add another handle. Handles
provide a way of telling a figure that so me operation should be performed. The most common handles
are for shrinking or growing a figure. For example, handles on the four corners of a figure. We want to
add another handle which will allow us to link one BoxDrawFigure to another BoxDrawFigure.
By default we have inherited the handles defined in the figure. These handles are on the four corners
of our figure and allow the user to resize the figure. The method handles in the protocol “accessing”
on the class Figure provides for this. We want to add an additional handle which allows links to be
created. We shall select the center point. However we do not want to lose the ability to resize our
figures, we shall therefore define a new handles method in the “accessing” protocol of the
BoxDrawFigure which uses super to obtain the original handles and then add our handle to this
collection of handles:
handles
"Add a handle at my center that draws connections to other figures"
^(super handles) add: (CommandHandle
connectionFor: self
at: #center:
class: LineFigure); yourself
We now have a fully functional BoxDraw application. Open the BoxDrawEditor again and place
some BoxDrawFigures on the drawing area. Now select the “arrow” icon on the icon menu. This is
the top icon. Next select one of the BoxDrawFigures. You should see a small square (a handle) in
the center of the figure. Click on this and drag the resulting line to another BoxDrawFigure. This should
connect the two figures together. Now move the figures around. You should find that the lin k between
the two figures moves with you.
21.5.7 Changing a figure menu
To complete this application we shall change the menu available when clicking on a BoxDrawFigure
using the middle mouse button. At present the user has the option to show visible area, hide v isible area
and reset visible area as well as the standard cut, copy and paste options. We only want the user to have
access to the standard cut, copy and paste options. To do this we redefine the menu method inherited
from CompositeFigure. This method is defined within the accessing protocol:
menu
"Define a pop up Menu for use with BoxDrawFigure"
^Menu labels: 'cut\copy\paste' withCRs
values: #(#cut #copy #paste )
21.5.8 The (pattern) solution statement
The ability to specify handles which perform speci fic functions allows operations to be performed
through operation of the mouse. The use of a CommandHandle with a LineFigure can be used to link
two figures together.
179