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**Professional GEM**
by Tim Oren
THE DIALOG HANDLER
11/7/85
A MEANINGFUL DIALOG
This issue of ST PRO GEM begins an exploration of ST GEM's dialog
handler. I will discuss basic system calls for presenting the
dialog, and then continue with techniques for initializing and
reading on/off button and "radio" button objects. We will also take
some short side-trips into the operation of the GEM Resource
Construction Set to assist you in building these dialogs.
There are a number of short C routines which accompany this column.
These are stored as file GEMCL3.XMO in DL 5 on SIG*ATARI. Before
reading this column, you should visit SIG*ATARI (go pcs-132) and
download this file.
DEFINING TERMS
A dialog box is an "interactive form" in which the user may enter
text and indicate selections by pointing with the mouse. Dialogs in
GEM are "modal", that is, when a dialog is activated other screen
functions such as menus and window controls are suspended until the
dialog is completed.
In most cases, the visual structure of a GEM dialog is specified
within your application's resource file. The GEM Resource
Construction Set (RCS) is used to build a picture of the dialog.
When the RCS writes out a resource, it converts that picture into a
tree of GEM drawing objects and stores this data structure within the
resource. Before your application can display the dialog, it must
load this resource file and find the address of the tree which
defines the dialog.
To load a resource, the AES checks its size and allocates memory for
the load. It then reads in the resource, adjusting internal pointers
to reflect the load address. Finally, the object sizes stored in the
resource are converted from characters to pixels using the system
font size.
(A note for those with Macintosh experience: Although Mac and GEM
resources share a name, there are fundamental differences which can
be misleading. A Mac resource is a fork within a file; a GEM
resource is a TOS file by itself. Mac resources may be paged in and
out of memory; GEM resources are monolithic. GEM resources are
internally tree structured; Mac resources are not. Finally, Mac
resources include font information, while ST GEM does this with font
loading at the VDI level.)
The resource load is done with the GEM AES call:
ok = rsrc_load(ADDR("MYAPP.RSC"));
"MYAPP" should be replaced with the name of your program. Resources
conventionally have the same primary name as their application, with
the RSC extent name instead of PRG. The ok flag returned by
rsrc_load will be FALSE is anything went wrong during the load.
The most common causes of failure are the resource not being in the
application's subdirectory, or lack of sufficient memory for GEM to
allocate space for the resource. If this happens, you must terminate
the program immediately.
Once you have loaded the resource, you find the address of a dialog's
object tree with:
rsrc_gaddr(R_TREE, MYDIALOG, &tree);
Tree is a 32-bit variable which will receive the address of the root
node of the tree.
The mnemonic MYDIALOG should be replaced with the name you gave your
dialog when defining it in the RCS. At the same time that it writes
the resource, RCS generates a corresponding .H file containing tree
and object names. In order to use these mnemonics within your
program, you must include the name file in your compile: #include
"MYAPP.H"
BUG ALERT!
When using the DRI/Alcyon C compiler, .H files must be in the
compiler's home directory or they will not be found. This is
especially annoying using a two floppy drive ST development system.
The only way around this is to explicitly reference an alternate
disk in the #include, for instance: "B:MYAPP.H"
Now that the address of the dialog tree has been found, you are ready
to display it. The standard (and minimal) sequence for doing so is
given in routine hndl_dial() in the download. We will now walk
through each step in this procedure.
The form_center call establishes the location of the dialog on the
screen. Dialog trees generated by the RCS have an undefined origin
(upper-left corner).
Form_center computes the upper-left location necessary to center the
dialog on the screen, and inserts it into the OB_X and OB_Y fields of
the ROOT object of the tree. It also computes the screen rectangle
which the dialog will occupy on screen and writes its pixel
coordinates into variables xdial, ydial, wdial, and hdial.
There is one peculiarity of form_center which occasionally causes
trouble. Normally the rectangle returned in xdial, etc., is exactly
the same size as the basic dialog box.
However, when the OUTLINED enhancement has been specified for the
box, form_center adds a three pixel margin to the rectangle returned.
This causes the screen area under the outline to be correctly redrawn
later (see below). Note that OUTLINED is part of the standard dialog
box in the RCS. Other enhancements, such as SHADOWED or "outside"
borders are NOT handled in this fashion, and you must compensate for
them in your code.
The next part of the sequence is a form_dial call with a zero
parameter. This reserves the screen for the dialog action about to
occur. Note that the C binding given for form_dial in the DRI
documents is in error: there are nine parameters, not five. The
first set of xywh arguments is actually used with form_dial calls 1
and 2 only, but place holders must be supplied in all cases.
The succeeding form_dial call (parameter one) animates a "zoom box"
on the screen which moves and grows from the first screen rectangle
given to the second rectangle, where the dialog will be displayed.
The use of this call is entirely optional. In choosing whether to
use it or not, you should consider whether the origin of the "zoom"
is relevant to the operation. For instance, a zoom from the menu
bar is relatively meaningless, while a zoom from an object about to
be edited in the dialog provides visual feedback to the user, showing
whether the correct object was chosen.
If the origin is not relevant, then the zoom is just a time-waster.
If you decide to include these effects, consider a "preferences"
option in your app which will allow the experienced and jaded user
to turn them off in the interests of speed.
The objc_draw call actually displays the dialog on the screen. Note
that the address of the tree, the beginning drawing object, and the
drawing depth are passed as arguments, as well as the rectangle
allotted for the dialog.
In general, dialogs (and parts of dialogs) are ALWAYS drawn
beginning at the ROOT (object zero). When you want to draw only a
portion of the dialog, adjust the clipping rectangle, but not the
object number. This ensures that the background of the dialog is
always drawn correctly.
The objc_xywh() utility in the download can be used to find the
clipping rectangle for any object within a dialog, though you may
have to allow an extra margin is you have used shadows, outlines, or
outside borders with the object.
Calling form_do transfers control to the AES, which animates the
dialog for user interaction. The address of the dialog tree is
passed as a parameter. The second paramter is the number of the
editable object at which the text cursor will first be positioned.
If you have no text fields, pass a zero. Note that again the DRI
documents are in error: passing a -1 default may crash the system.
Also be careful that the default which you specify is actually a text
field; no error checking is performed.
The form_do call returns the number of the object on which the
clicked to terminate the dialog. Usually this is a button type
object with the EXIT and SELECTABLE attributes set. Setting the
DEFAULT attribute as well will cause an exit on that object is a
carriage return is struck while in the dialog.
If the top bit of the return is set, it indicates that the exit
object had the TOUCHEXIT attribute an