TweetFollow Us on Twitter

Jun 97 - Macintosh Q and A

Volume Number: 13 (1997)
Issue Number: 6
Column Tag: develop

Macintosh Q&A

by Apple Developer Support Center

Q: When I used CopyBits to do a screen capture, the cursor isn't included. Why not?

A: CopyBits calls ShieldCursor to hide the cursor before it does its work. That's why you never see the cursor in your screen capture. The following code hides the cursor and decrements the cursor count in the low-memory global to fool CopyBits not to call ShieldCursor. Standard warning: Using low-memory globals is dangerous and is subject to future OS changes.

unsigned char   oldCrsrVis;
short           oldCrsrState;
      
// Hide the cursor.
oldCrsrVis = GetCrsrVis();
oldCrsrState = GetCrsrState();
SetCrsrVis((unsigned char) 0);
SetCrsrState(oldCrsrState - 1);   
// Do the screen capture.
CopyBits(...)
      
// Restore the old cursor.
SetCrsrState(oldCrsrState);
SetCrsrVis(oldCrsrVis);

You should define the cursor functions as follows:

enum {
   CrsrVis = 0x8CC,      // Cursor visible? [byte]
   CrsrState = 0x8D0      // Cursor nesting level [word]
};
   
void SetCrsrVis(unsigned char cursorVisible)   
{
   *(unsigned char*)CrsrVis = cursorVisible;
}

unsigned char GetCrsrVis(void)
{
   return ( *(unsigned char*)CrsrVis );
}   

short GetCrsrState(void)
{
   return ( *(unsigned char*)CrsrState );
}   

void SetCrsrState(short val)
{
   *(unsigned char*)CrsrState = val;
}

Q: How can I determine whether the cursor is hidden?

A: Currently the best way to determine this is to use a low-memory global that has been removed from LowMem.h. The reason it has been removed is that a future operating system may handle it differently.

Using low-memory globals has never really been sanctioned, though they were documented in older interface files. The Universal Interfaces were designed to make way for future operating systems; access to some low-memory globals was provided through accessor functions for safety, while access to others was removed.

The global you want, CrsrVis, is a byte-sized flag that tells whether the cursor is hidden. It was defined in an older interface file like this:

enum {
   CrsrVis = 0x8CC,   // Cursor visible? [byte]
};

You can simply check this global to see if the cursor is hidden. As always, the direct use of low-memory globals is not recommended and will probably cause your code to break in the future.


Q: I want to support dragging from my application to the Trash without having the Finder create a clipping file inside the Trash. I tried several options but they all failed. Is this possible?

A: There is no good way to prevent a clipping file from being created in the Trash, or to prevent the "zoomback" animation that signals a failed drag. The problem is that the Finder treats the Trash like a folder; the drag succeeds or fails according to the user's supposed intent with respect to a folder.

In our opinion, however, this is good, since it's consistent with the Trash metaphor. When the user drags an icon from a Finder window into the Trash, she expects the Trash to get fat so that she can later change her mind, open the Trash, and drag the icon back out. Creating a clipping file in the Trash merely extends the metaphor. The only problem this might cause is that the user ought to be able to reverse her decision and drag the clipping back into the application. If your application doesn't already support this action, you may have some work ahead.

We realize that other applications manage to avoid making the Trash fat. This is, in our opinion, a Bad Thing. A Hack. Wrong. Nevertheless, if you still feel you need to avoid making the Trash fat, it might be possible to do so. It would probably involve creating a file via flavorTypePromiseHFS, then deleting it before the Finder has a chance to make the Trash fat. Unfortunately, since this involves "fooling" the Finder, it isn't likely to work for future versions of the Finder even if it does work today.

In short, this is not recommended.


Q: When my application calls TrackDrag, it crashes in low memory at an illegal instruction. The MacsBug stack crawl doesn't produce any useful information. I've stared and stared at my application's calls to the Drag Manager, and all the parameters appear to be valid. My drag tracking handler is never called, incidentally. If I take all calls to the Drag Manager out of my application, it runs just fine. I've been investigating this crash for two months. Why is life so cruel?

A: You've unearthed a really ugly problem. The Drag Manager thinks the drag originated in the Finder, and makes some fatal assumptions based on that.

Early versions of the Drag Manager did not enjoy the benefits of a drag-enabled Finder, so the Drag Manager plays a little fast and loose with the Finder's jump table. Yes, that means what it sounds like: the Drag Manager calls the Finder routines through its jump table.

The even more interesting story concerns the method by which the Drag Manager decides that the current application is the Finder. When TrackDrag is called, the Drag Manager determines whether the drag originates in any of the windows in the window list of the current process. If not, the Drag Manager determines whether the drag originates in any Finder window. Since the desktop is a "window" for these purposes, there's a large area that qualifies.

Once the Drag Manager has decided the drag originated in a Finder window, it assumes that the Finder is the current process. (This is the fatal mistake.) Once this assumption is made, the next thing for the Drag Manager to do to coax the Finder into exhibiting the correct drag behavior is call an entry in whatever jump table can be found by offsetting the current value of register A5. This is valid if the Finder is the current process, but of course if the current process is your application instead, things go terribly astray: The Drag Manager calls a jump table entry in your application, thinking it's the Finder, and your application's routine doesn't do the same thing that the Finder routine would. Any number of spectacular effects can result.

"Now wait a minute," you're thinking, "the drag originated in one of my application's windows; how is this stuff about the Finder relevant?" The answer is in the event record you're passing to TrackDrag. An event record is supposed to contain a where field expressed in global coordinates. However, the where field your application is passing is in local coordinates. How did this happen? Well, that depends on your application, but often it can result from an application frameworks (like PowerPlant, MacApp, or THINK Class Library) modifying the event record before passing it to your code. There's no language-level way to specify that the record has been modified, so the compiler doesn't warn you. (Honestly, this is Not Your Fault.)

Your code blithely calls TrackDrag with what it assumes is a valid event record but is not. TrackDrag interprets the where field, which is actually in local coordinates, as global coordinates. This point is somewhere up and to the left of where your application expects, and quite often it's in the desktop, which as we said above is considered a Finder window for these purposes. The Drag Manager reacts by going through its ritual for drags originating in Finder windows and eventually crashes after calling some odd routine in your application, as described above.

To solve this problem, simply call LocalToGlobal on the where field of the event record before calling TrackDrag.


Q: When I drag from my application into a Finder window, the system crashes. I notice the Finder uses a windowKind value of 20 for its windows, and so does my application. When my application avoids windowKind 20, everything's hunky-dory. What's going on?

A: Through the Drag Manager, the Finder has gotten access to the windows in your application's window list (specifically, by using undocumented calls to obtain the source window of a drag). If your window's windowKind field is 20, the Finder assumes the window is one of its own (as opposed to a driver window, whose windowKind would be negative, or a dialog window, whose windowKind would be 2, and so on). The Finder grabs the value in the window's refCon field and type-casts it to a pointer to a C++ object in the Finder's heap. I think you can see where this is going: when the Finder attempts to dereference the pointer, many fascinating crashing behaviors can result.

The upshot of this Finder bug is that your application should not use windowKind values of 20.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Bookends 12.7.8 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
VirtualBox 5.1.14 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
FileZilla 3.24.0 - Fast and reliable FTP...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.24.0: New The context menu for remote file search... Read more
BusyContacts 1.1.6 - Fast, efficient con...
BusyContacts is a contact manager for OS X that makes creating, finding, and managing contacts faster and more efficient. It brings to contact management the same power, flexibility, and sharing... Read more
BusyCal 3.1.4 - Powerful calendar app wi...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. Its unique features... Read more
Duplicate Annihilator 5.8.3 - Find and d...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
MarsEdit 3.7.10 - Quick and convenient b...
MarsEdit is a blog editor for OS X that makes editing your blog like writing email, with spell-checking, drafts, multiple windows, and even AppleScript support. It works with with most blog services... Read more
WALTR 2 2.0.9 - $39.95
WALTR 2 helps you wirelessly drag-and-drop any music, ringtones, videos, PDF, and ePub files onto your iPhone, iPad, or iPod without iTunes. It is the second major version of Softorino's critically-... Read more
Paperless 2.3.9 - $49.95
Paperless is a digital documents manager. Remember when everyone talked about how we would soon be a paperless society? Now it seems like we use paper more than ever. Let's face it - we need and we... Read more
Adobe After Effects CC 2017 14.1 - Creat...
After Effects CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). The new, more connected After Effects CC... Read more

Super Mario Run dashes onto Android in M...
Super Mario Run was one of the biggest mobile launches in 2016 before it was met with a lukewarm response by many. While the game itself plays a treat, it's pretty hard to swallow the steep price for the full game. With that said, Android users... | Read more »
WarFriends Beginner's Guide: How to...
Chillingo's new game, WarFriends, is finally available world wide, and so far it's a refreshing change from common mobile game trends. The game's a mix of tower defense, third person shooter, and collectible card game. There's a lot to unpack here... | Read more »
Super Gridland (Entertainment)
Super Gridland 1.0 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 1.0 (iTunes) Description: Match. Build. Survive. "exquisitely tuned" - Rock Paper Shotgun No in-app purches, and no ads! | Read more »
Red's Kingdom (Games)
Red's Kingdom 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Mad King Mac has kidnapped your father and stolen your golden nut! Solve puzzles and battle goons as you explore and battle your... | Read more »
Turbo League Guide: How to tame the cont...
| Read more »
Fire Emblem: Heroes coming to Google Pla...
Nintendo gave us our first look at Fire Emblem: Heroes, the upcoming mobile Fire Emblem game the company hinted at last year. Revealed at the Fire Emblem Direct event held today, the game will condense the series' tactical RPG combat into bite-... | Read more »
ReSlice (Music)
ReSlice 1.0 Device: iOS Universal Category: Music Price: $9.99, Version: 1.0 (iTunes) Description: Audio Slice Machine Slice your audio samples with ReSlice and create flexible musical atoms which can be triggered by MIDI notes or... | Read more »
Stickman Surfer rides in with the tide t...
Stickson is back and this time he's taken up yet another extreme sport - surfing. Stickman Surfer is out this Thursday on both iOS and Android, so if you've been following the other Stickman adventures, you might be interested in picking this one... | Read more »
Z-Exemplar (Games)
Z-Exemplar 1.4 Device: iOS Universal Category: Games Price: $3.99, Version: 1.4 (iTunes) Description: | Read more »
5 dastardly difficult roguelikes like th...
Edmund McMillen's popular roguelike creation The Binding of Isaac: Rebirth has finally crawled onto mobile devices. It's a grotesque dual-stick shooter that tosses you into an endless, procedurally generated basement as you, the pitiable Isaac,... | Read more »

Price Scanner via MacPrices.net

Twelve South Releases RelaxedLeather Cases fo...
Inspired by the laid-back luxury of burnished leather boots and crafted in rich tones of taupe, herb and marsala, RelaxedLeather cases deliver smart, easy protection for the iPhone 7. Each genuine... Read more
Week’s Best Deal: New 2016 13-inch 2.0GHz Mac...
Amazon has the new 2016 13″ 2.0GHz non-Touch Bar MacBook Pros on sale for a limited time for $225 off MSRP including free shipping: - 13″ 2.0GHz MacBook Pro, Space Gray (MLL42LL/A): $1274.99 $225 off... Read more
Back in stock: Apple refurbished Mac minis fr...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off MSRP - 2.6GHz Mac... Read more
Apple Ranked ‘Most Intimate Brand’
The top ranked ‘”intimate” brands continued to outperform the S&P and Fortune 500 indices in revenue and profit over the past 10 years, according to MBLM’s Brand Intimacy 2017 Report, the largest... Read more
B-Eng introduces SSD Health Check for Mac OS
Fehraltorf, Switzerland based independant Swiss company- B-Eng has announced the release and immediate availability of SSD Health Check 1.0, the company’s new hard drive utility for Mac OS X. As the... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
4-core 3.7GHz Mac Pro on sale for $2290, save...
Guitar Center has the 3.7GHz 4-core Mac Pro (MD253LL/A) on sale for $2289.97 including free shipping or free local store pickup (if available). Their price is a $710 savings over standard MSRP for... Read more
128GB Apple iPad Air 2, refurbished, availabl...
Apple has Certified Refurbished 128GB iPad Air 2s WiFis available for $419 including free shipping. That’s an $80 savings over standard MSRP for this model. A standard Apple one-year warranty is... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 13″ 2.7GHz/128GB Retina Apple MacBook Pro on sale for $100 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A): $... Read more
Laptop Market – Flight To Quality? – The ‘Boo...
Preliminary quarterly PC shipments data released by Gartner Inc. last week reveal an interesting disparity between sales performance of major name PC vendors as opposed to that of less well-known... Read more

Jobs Board

*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Manhattan, NY Introduction: We have immediate job openings for several Desktop Support Technicians with one of our most Read more
Senior Workstation Administrator - *Apple*...
…with extraordinary HR. QualificationsJOB SUMMARY/OVERVIEWThe Senior Workstation Administrator - Apple supports the mission of TriNet by providing advanced level Read more
Intermediate *Apple* macOS Systems Integrat...
**Position Summary:** SC3 is actively seeking an Intermediate Apple macOS systems integration administrator that will be responsible for providing Apple Mac Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Los Angeles, CA Introduction: We have immediate job openings for several Desktop Support Technicians with one of our Read more
*Apple* Retail - Multiple Positions - Apple,...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.