Apple II Technical Notes _____________________________________________________________________________ Developer Technical Support Apple IIgs #71: DA Tips and Techniques Revised by: Dave "Mr. Tangent" Lyons July 1991 Written by: Dave Lyons November 1989 This Technical Note presents tips and techniques for writing Desk Accessories. Changes since September 1990: Added information about opening an NDA that is already open. Added a section on interaction between the Memory Manager, Event Manager, CDAs, and interrupts. _____________________________________________________________________________ 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 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. 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, Preventing Memory Compacting and Purging o Apple IIgs Technical Note #69, The Ins and Outs of Slot Arbitration