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From: belle@skipper.dfrf.nasa.gov (Steve Belle)
Newsgroups: comp.lang.eiffel
Subject: Graphics cluster:  instances of DEVICE
Message-ID: <772@skipper.dfrf.nasa.gov>
Date: 8 Feb 91 00:18:11 GMT
Organization: NASA Ames-Dryden FRF, Edwards, CA
Lines: 33

I have recently been poking around in the Eiffel graphics classes from ISE.
I have discovered that for any graphical application, there are two
instances of "device" created.  One of these comes from the window cluster,
in class DEV_CONST, and the other comes from the figure cluster, in 
class GRAPH_CONST. 

To the limited extent that I have dug into this, it seems
that this is not necessary.  All of the figure classes (POINT, POLYLINE,
POLYGON, etc.) have respective *_IMP classes that in turn inherit from 
class FIGURE_IMP, which inherits from DEV_CONST.  The various *_IMP classes
redefine the attributes device, black_color, and white_color inherited from
FIGURE_IMP to avoid conflict with attributes of the same names coming
from GRAPH_CONST.  This implementation results in 2 instances of DEVICE
being created, the first when the application is started up, and the 
second when any type of figure is Created.  

It seems that only 1 instance of a device (and a black_color & white_color)
is required (or desired). I believe the attributes device, black_color, and 
white_color should be removed from class GRAPH_CONST, as well as the renaming 
clauses in the various figure_IMP classes.  This also requires the addition
of DEV_CONST to the inherit clause of class GEN_FIGURE, because one of 
the routines in that class has a pre-condition that requires an instance of
device.

In my opinion, the implementation I suggest seems to make more sense because
it means that, per application, there is only one instance each of device,
black_color, and white_color.  Anyone else have thoughts or experience with
this?


Steve Belle
PRC
belle@skipper.dfrf.nasa.gov