Developer CD Series 1992 June: ROMin Holiday/ADC Developer CD (1992-06) (''ROMin Holiday'')_iso/Developer Connection - 06-1992.iso/Development Platforms/Apple II/Essentials/Technical.Notes/IIGS/TN.IIGS.071

Text File | 1991-11-15 | 15.5 KB | 358 lines | [TEXT/pdos]

Apple II
Technical Notes
_____________________________________________________________________________
                                                  Developer Technical Support


Apple IIgs
#71:         DA Tips and Techniques

Revised by:  Dave "Mr. Tangent" Lyons                           December 1991
Written by:  Dave Lyons                                         November 1989

This Technical Note presents tips and techniques for writing Desk
Accessories.

Changes since July 1991:  Added information on NDAs finding their own menu
item IDs.
_____________________________________________________________________________


Classic Desk Accessory Tips and Techniques

Reading the Keyboard

For a CDA that runs only under GS/OS, the Console Driver is the best choice
for reading from the keyboard.  Other CDAs have two cases to deal with:  the
Event Manager may or may not be started.  The Text Tools can read the
keyboard in either case, but you should avoid using the Text Tools whenever
possible (see Apple IIgs Technical Note #69, The Ins and Outs of Slot
Arbitration).

You can call EMStatus to determine whether the Event Manager is started.
When it is, you can read keypresses by calling GetNextEvent.  When the Event
Manager is not started, you can read keys directly from the keyboard hardware
by waiting for bit 7 of location $E0C000 to turn on.  When it does, the lower
seven bits represent the key pressed.  Once you've detected a keypress, you
need to write to location $E0C010 to remove the keypress from the buffer.

Alternately, you can use IntSource (in the Miscellaneous Tools) to
temporarily disable keyboard interrupts and then read the keyboard hardware
directly.  Be sure to reactivate keyboard interrupts if, and only if, they
were previously enabled.

Just One Page of Stack Space

CDAs normally have only a single page of stack space available to them (256
bytes at $00/01xx).  Your CDA may or may not be able to allocate additional
stack space from bank 0 during execution.  The following code (written for
the MPW IIgs cross-assembler) shows a safe way to try to allocate more stack
space and to switch between stacks when the space is available.

If ProDOS 8 is active, your CDA cannot allocate additional space (and there
is no completely safe way to "borrow" bank 0 space from the ProDOS 8
application).


HowMuchStack   gequ    $1000          ;try for 4K of stack space

start          phd
               phb
               phk
               plb
               pha                    ;Space for result
               pha
               PushLong #HowMuchStack
               pha
               _MMStartUp
               pla
               ora    #$0f00          ;OR in an arbitrary auxiliary ID
               pha
               PushWord #$C001        ;fixed, locked, use specified bank
               PushLong #0            ;(specify bank 0)
               _NewHandle
               tsc
               sta    theOldStack
               bcs    NoStackSpace    ;still set from _NewHandle
               tcd
               lda    [1]
               tcd
;               clc                    ;carry is already clear
               adc    #HowMuchStack-1
NoStackSpace   pha
               ldx    #$fe
keepStack      lda    >$000100,x
               sta    stackImage,x
               dex
               dex
               bpl    keepStack
               pla
               tcs
               jsl    RealCDAentry     ;carry is clear if large stack
available
               php
               php
               pla
               sta    pRegister
               sei
               ldx    #$fe
restoreStack   lda    stackImage,x
               sta    >$000100,x
               dex
               dex
               bpl    restoreStack
               lda    theOldStack
               tcs
               lda    pRegister
               pha
               plp
               plp
               lda    1,s
               ora    3,s
               beq    noDispose
               _DisposeHandle
               bra    Exit
noDispose      pla
               pla
Exit           plb
               pld
               rtl
pRegister     ds 2
theOldStack   ds 2
stackImage    ds.b 256

When this routine calls RealCDAentry, the carry flag is set if no extra stack
space is available.  If the carry is clear, the additional stack space was
available and the direct-page register points to the bottom of that space.

RealCDAentry   bcs    smallStack            ;if c set, only 1 page of stack
                                            ;is available
               ...                          ; put something interesting here
               rtl

smallStack     _SysBeep
               rtl

Note that interrupts are disabled while the page-one stack is being restored;
they are reenabled (if they were originally enabled) only after the stack
pointer is safely back in page one.

Interrupts, Event Manager, Memory, and CDAs

Whether the Event Manager is active or not, the user hits Apple-Ctrl-Esc and
usually gets to the CDA menu.  It looks the same, but what happens internally
is different affects what happens when your CDA allocates memory.

When the Event Manager is active (as it normally is while the user is running
a Desktop application), hitting Apple-Ctrl-Esc posts a deskAcc event to the
event queue.  The CDA menu appears only when the application calls
GetNextEvent or EventAvail with the deskAcc bit enabled in the event mask.

So with the Event Manager active, the CDA menu and individual CDAs are
running in the "foreground"--no processor interrupt is being serviced, and
the foreground application is stuck inside the GetNextEvent or EventAvail
call.  The Memory Manager knows that no interrupt is in progress, so it will
happily compact and purge memory if necessary to carry out a memory
allocation request from your CDA.  This is just fine, since the foreground
application made a toolbox call--unlocked memory blocks are not guaranteed to
stay put.

When the Event Manager is not active, hitting Apple-Ctrl-Esc either enters
the CDA menu immediately (if the system Busy Flag is zero) or calls
SchAddTask so that the CDA menu appears during a the next DECBUSYFLG call
that brings the system Busy Flag down to zero.  If the CDA menu appears
during a DECBUSYFLG , normal memory compaction and purging are possible, just
like when the Event Manager is active.

But if the Busy Flag was zero when the user hit Apple-Ctrl-Esc, then the CDA
menu appears inside of the interrupt, and the foreground application is at an
unknown point where it may justifiably expect that unlocked memory blocks
will not move or be purged (see Apple IIgs Toolbox Reference, Volume 1, page
12-5).  (Note that the Desk Manager does a tricky dance to allow additional
interrupts to occur, even though the Apple-Ctrl-Esc interrupt will not return
until the user chooses Quit from the CDA menu.  Normally interrupts cannot be
nested; the Desk Manager and AppleTalk are exceptions.)

The Memory Manager knows an interrupt is in progress, so CompactMem takes no
action and memory allocation requests do not cause unlocked memory blocks to
move and do not attempt to purge purgeable blocks to make room.  Memory
allocation requests will still normally succeed, but you will not be able to
allocate a block larger than the value returned by MaxBlock.

New Desk Accessory Tips and Techniques

An NDA Can Find its Menu Item ID

After the application has called FixAppleMenu, an NDA can look at its
menu item template (after the "\H" in the NDA header) to determine the
menu ID corresponding to the NDA's name in the Apple menu.  This is
sometimes useful to pass to OpenNDA (if the NDA has some way to open
itself), or to pass to a Menu Manager call.

Finding the menu item ID in the NDA's header is easy if the NDA is
written in assembly.  In a high-level language it may be harder (if you
don't have direct access to your NDA's header, you need to find it on
the fly and scan for the "\H").

NDAs Continue to Get Command- Keystrokes

Previous versions of this Note said that NDAs are not guaranteed to receive
key-down events with the Command key down, because System Software might be
revised to pass such keys directly to the current application instead.

In light of the reaction of several interested parties, that warning is being
withdrawn.  When an NDA is in front, it has first crack at key-down events,
even when the Command key is down.

There is currently no way for an NDA to accept some keystrokes and pass
others along to applications, but if your NDA does not want any keystroke
events, turn off the corresponding eventMask bits in the NDA header (this
allows the application to receive keystrokes while your NDA window is in
front).

You may want to support the standard Close key equivalent, Command-W.  When
your NDA's action routine receives an Command-W (or Command-w) keyDown or
autoKey event, you can call CloseNDAByWinPtr with your own window pointer.
This causes a call to your NDA's Close routine within the call to your Action
routine--if your code has no problem with this, neither does the Desk
Manager.

Note:    System 6.0 handles Command-W automatically when a system window is
         in front.  It calls CloseNDAbyWinPtr without letting the NDA or the
         application see the Command-W.



Calling InstallNDA From Within an NDA

It is possible to write an NDA that installs other NDAs.  However, with
System Software 5.0 and later, InstallNDA returns an error when called from
an NDA.  When your NDA has control because the Desk Manager called one of
your NDA's entry points, the Desk Manager's data structures are already in
use, so InstallNDA is unable to modify them.

The solution is to use SchAddTask in the Scheduler to postpone the InstallNDA
call until the system is not busy.  Remember that the Bank and Direct Page
registers are not defined when your scheduled task is executed.

Processing mouseUp Events

When an NDA's action routine receives a mouseUp event, it is not always safe
for the NDA to draw in its window.

For example, when the user drags an NDA window, the NDA receives the mouseUp
before the window is actually moved, and before DragWindow erases the outline
of the new window position, which may overlap the window's content.  In
addition, when the user chooses a menu item, the front NDA receives the
mouseUp before the menu's image is removed, and the image may overlap the
NDA's window.  In either case, drawing in the window makes a mess.

The solution is to avoid drawing in direct response to a mouseUp.  Instead,
invalidate part of the window to force an update event to happen later.

NDAs Can Have Resource Forks

Following is the recommended way for a New Desk Accessory to use its file's
resource fork.

In the NDA's Open routine, do the following:

    1.    Call GetCurResourceApp and keep the result.
    2.    If the NDA does not already know its Memory Manager user ID, call
          MMStartUp to get it.
    3.    Call ResourceStartUp using the NDA's user ID.
    4.    Call the Loader function LGetPathname2 with the NDA's user ID (and
          a fileNumber of $0001) to get a pointer to the NDA's pathname.
          (The result is a pointer to a class-one GS/OS string.)
    5.    Use GetLevel to get the current file level, then use SetLevel to
          set it to zero.  This helps protect your resource fork from being
          closed accidentally.
    6.    Use GetSysPrefs to get the current OS preferences, then use
          SetSysPrefs to ensure that the user is prompted, if necessary, to
          insert the disk containing your resource fork.  (To compute the new
          preferences word, take the current one, AND it with $1FFF, and ORA
          it with $8000.  This tells GS/OS to deal with volume-not-found
          conditions by putting up a please-insert-disk dialog with an OK
          button and a Cancel button.)
    7.    Call OpenResourceFile using the result from LGetPathname2.  Save
          the returned fileID--you need it when closing the file.  (Be
          prepared to deal with an error, such as $0045, Volume Not Found.)
    8.    Use SetSysPrefs to restore the OS preferences saved in step six.
    9.    Use SetLevel to restore the file level to its old value (saved in
          step five).
    10.   Call SetCurResourceApp with the old value saved in step one.

In the NDA's action routine, no special calls are necessary--the Desk Manager
calls SetCurResourceApp automatically before calling your action routine, so
your NDA's own resource search path is already in effect.

Run queue routines and NDA installs with AddToRunQ are treated the same
way--the NDA's resource search path is automatically in effect when the run
queue routine is called.

In the NDA's Close routine, do the following:

1.    Call CloseResourceFile with the fileID that was returned when you
      opened it.
2.    Call ResourceShutDown with no parameters (the last version of this Note
      incorrectly said to pass the NDA's User ID--oops).


NDAs Must Be Careful Handling Modal Windows

If your NDA uses its resource fork and calls TaskMaster with a restricted
wmTaskMask to produce a modal window, you must be careful not to allow
TaskMaster to update the contents of any application windows that happen to
need updating.

The problem is that an application window's wContDraw routine can reasonably
assume that the current Resource Manager search path is the application's,
but TaskMaster does not take any special steps to set it.  When the
content-draw routine draws controls which were created from resources which
are not presently in the resource search path, the system may crash.

If your NDA does not start up the Resource Manager, the Desk Manager is
unable to SetCurResourceApp to your NDA, so the application's search path is
still in effect--no problem.  But if your NDA does start the Resource
Manager, you have to be careful not to cause application routines to be
called.

Avoid Hard-Coding Your Pathname

If your NDA needs to know its own pathname or the pathname of the directory
it's in, call LGetPathname or LGetPathname2 using your User ID.  This is a
better method than hard-coding "*:System:Desk.Accs:MyDAName" because the user
may change your DA's file name or use a utility to install it from some
non-standard directory.

Avoid Extra GetNewID calls

Normally there is no reason for a Desk Accessory to call GetNewID.  When you
can, just call MMStartUp to find your own User ID, and use that.  You can
freely use all the auxiliary IDs derived from your main ID (MMStartUp+$0100,
MMStartUp+$0200, ..., MMStartUp+$0F00).

By not calling GetNewID, you conserve the limited supply of IDs (255 of in
the $50xx range for Desk Accessories), and you make life easier for people
trying to debug their systems, since all your allocated memory can be readily
identified.

Open is Not Called if NDA is Already Open

Your NDA's Open routine does not get called if the user chooses the NDA from
the Apple menu while the NDA is already open.  In this case, the Desk Manager
simply calls SelectWindow on your existing window.

There is no need to include code in your Open routine to check if your window
is already open, and to call SelectWindow if it is.


Further Reference
_____________________________________________________________________________
  o   Apple IIgs Toolbox Reference, Volumes 1-3
  o   GS/OS Reference
  o   Apple IIgs Hardware Reference
  o   Apple IIgs Technical Note #53, Desk Accessories and Tools
  o   Apple IIGS Technical Note #57, The Memory Manager and Interrupts
  o   Apple IIgs Technical Note #69, The Ins and Outs of Slot Arbitration