TweetFollow Us on Twitter

When Wild Animals Bite

Volume Number: 18 (2002)
Issue Number: 10
Column Tag: Mac OS X

When Wild Animals Bite

Or How To Workaround Cocoa bugs

by Andrew C. Stone

Of the last 50 articles I have written about Cocoa and Mac OS X, I think 49 have extolled the virtues of object oriented programming and the unparalleled advantages of using the dynamic runtime of Objective C. You might think I'd been fed some KoolAid, and you might not be too far off. But effects and mileage will vary.

The question that might arise in your mind is "Aren't there any serious drawbacks to using such a high level system?". And the answer to that would be the same as the drawbacks to any complex system: "Yes - Beware of Bit Rot", the inevitable complexification of simple architectures over time as more people work on it.

Once upon a time, the entire Cocoa development system was in the hands of a few able engineers, which kept it focused and robust between system releases. However, with Mac 10.2, aka Jaguar, we've seen a new and dangerous trend. Components that have been working almost flawlessly for over ten years have been "re-plumbed" without enough real world testing, and this can cause trouble for applications which push the envelope. Sometimes the pressure to release a new version is greater than the ability to do adequate quality control, so all of this is quite forgivable.

The same features that seem like an advantage can turn into a disadvantage! Since a Cocoa application links against the runtime AppKit and Foundation frameworks, when improvements are made to a subsystem framework, your application, even unrecompiled, will take advantage of these improvements. For example, Mac OS X 10.2's text system introduced 3 new tab types: decimal, right aligned, and centered tabs. Users of our page layout and web authoring program, Create(R), now automatically have access to these features under 10.2 - even with versions compiled under 10.1. By the same token, some changes in the subsystem framework can break existing, unrecompiled applications.

Because of the relatively small number of Cocoa engineers at Apple compared to the number of traditional Carbon engineers, there has been an increasing tendency to "pollute" our Cocoa purity with underlying Carbon implementations, and consequently, unexpected results can ensue! In Object Oriented theory, all objects are so well encapsulated and isolated that one can just swap out implementations of components and everything should just work. Reality just doesn't conform to these simplistic expectations.

When Carbon Met Cocoa

In the last several years of shipping and maintaining OS X applications, I've found that most of the problems have come up where traditional Mac OS 9 technologies have been shoehorned into the Cocoa model. The interstice between them is riddled with the same ambiguities that bedevils any organization that is a hybrid of philosophies and techniques. Cocoa's AppleScript implementation is an example of this - from the outside, it looks neat and clean, but getting it to work just right is a lot tougher than normal Cocoa programming tasks.

One object whose plumbing was changed in 10.2 is the NSPrintInfo - an object which maintains information about page size, orientation, margins and selected printer. The backend Print Manager grew out of the Carbon Tioga effort and is coded in C and C++. In 10.1, you could set the page size to any value (ideal for custom page sizes) with the simple invocation:

   [myPrintInfo setPaperSize:NSMakeSize(someWidthInPoints, someHeightInPoints)];

And the printInfo dutifully obeyed. This is useful for designing large scale posters to be printed offsite or for web sites with long content. However, in Jaguar, new behavior was introduced which attempts to see if that size is "valid" for the given printer. To be fair to the Apple engineers, I can see how this might be useful. But from my point of view, it broke the existing version of Create(R)! Luckily, since our corporate philosophy is free upgrades downloadable via the Internet, some quick coding, testing and uploading would solve the problem.

This new validation behavior occurs whenever "setPaperSize:" is called, and in the case of Create(R), that's when the document is unarchived from a file. So, when you open an old Create document with a custom size, the old size is lost forever, and all of a sudden, size "A4" is chosen! What's worse is that any attempt to even read this paper size, such as with the public NSPrintInfo API call -(NSSize)paperSize or even -objectForKey on the dictionary returned from -(NSMutableDictionary)dictionary will also trigger this new validation behavior.

Redemption Song

So, what's the workaround? Somehow, we'll find Cocoa's redemption! For any flaw that comes in a shipping OS, there are ALWAYS tricky ways in Cocoa to do post-ship fixes! Once again, Objective C Categories save us from getting egg on our digital faces.

By examining the header file NSPrintInfo.h, we see a private instance variable (IVAR), _attributes, which is the actual mutable dictionary which holds all the values of the PrintInfo:

@interface NSPrintInfo : NSObject<NSCopying, NSCoding> {
    @private
    NSMutableDictionary *_attributes;
    void *_moreVars;
}

Because the IVAR is designated private, even a subclass cannot access it directly, so subclassing NSPrintInfo won't work. Only a category of the class containing the private IVAR can directly access the variable. Here's a category that can return any set value for paperSize, without triggering the unwanted validation behavior:

@interface NSPrintInfo (peek)

- (NSSize)grabOriginalSize;
@end
@implementation NSPrintInfo(peek)
- (NSSize)grabOriginalSize {
    id obj = [_attributes objectForKey:NSPrintPaperSize];
    return obj? [obj sizeValue]: NSZeroSize;
}
@end

Documents which had no custom page size will return NSZeroSize, which indicates that no extra work will be required. So, here's how we'll use the new category method:

            NSPrintInfo *printInfo = [NSUnarchiver unarchiveObjectWithData:obj];
       // a freshly unarchived PrintInfo still has its old values in the dictionary....
            if (printInfo) {
                NSSize raw = [printInfo grabOriginalSize];
                if (!NSEqualSizes(raw, NSZeroSize)) {   
         // it could have been written
                    // by a 10.1 version of Create
                   [self setUpPrintInfo:printInfo toPaperSize:raw];
                }             
                [self setPrintInfo:printInfo];
            }

Now, all we need to do is fake NSPrintInfo out so that it thinks the custom paper size is valid. To do this, I had to dig into undocumented API using class-dump as explained in several of my last few articles, and even more dreaded by an Objective C coder, into the Carbon header files! It turns out that there is a C function to make custom paper sizes programmatically, which stands to reason because the new Page Setup... panel in Jaguar has a popup option to set custom paper sizes:

OSStatus PMPaperCreate(PMPrinter printer, CFStringRef id, CFStringRef name, double width, double height, const PMPaperMargins *margins, PMPaper *paperP);

And we'll call it like this:

        OSStatus osStatus = PMPaperCreate([[pi printer] _printer], 
(CFStringRef)[NSString stringWithFormat:@"%ld",self], 
(CFStringRef)[NSString stringWithFormat:@"%2.2f X %2.2f",userW,userH],(double) paperSize.width,
(double) paperSize.height, marginPtr, &myPaper);

If PMPaperCreate() returns an OSStatus of 0, then a new PMPaper * object (which you'll need to later release after use) has been created in myPaper. The first parameter, PMPrinter, can be returned from the NSPrintInfo via undocumented NSPrintInfo API, -(PMPrinter)_printer:

[[pi printer] _printer]

Another sweet feature of Cocoa is "toll-free bridging" between Core Foundation objects, such as a CFStringRef and the equivalent Cocoa object, in this case, the NSString. We will cast the NSString parameters to CFStringRef's just to hush the compiler. The other fancy dancing that we're doing is using the pointer to memory of the document object as our uniquing strategy for the second argument, but it could be any string as long as it is a unique identifier:

(CFStringRef)[NSString stringWithFormat:@"%ld",self]

Finally, we'll name the custom page in the user's units by converting the points to their measurement units - these functions are included below.

// these functions are declared here:
#import <CoreServices/CoreServices.h>
#import <ApplicationServices/ApplicationServices.h>
typedef struct OpaquePMPaper *PMPaper;
typedef struct {
    double top;
    double left;
    double bottom;
    double right;
} PMPaperMargins;

OSStatus PMPaperCreate(PMPrinter printer, CFStringRef id, CFStringRef name, double width, double height, const PMPaperMargins *margins, PMPaper *paperP);

- (void)setUpPrintInfo:(NSPrintInfo *)pi toPaperSize:(NSSize)paperSize {
    if (floor(NSAppKitVersionNumber) <= NSAppKitVersionNumber10_1) {
        /* On a 10.1 - 10.1.x system */
   // nothing special to do...
    } else {
        /* 10.2 or later system */
        float userW = convertToUserUnits(paperSize.width);
        float userH = convertToUserUnits(paperSize.height);
        PMPaperMargins margins;
        PMPaperMargins *marginPtr = &margins;   
                        // does C suck or what?
        PMPaper *myPaper;
        // Create uses WYSIWYG full page model:
        margins.top = margins.left = margins.bottom = margins.right = 0.0;
        
        OSStatus osStatus = PMPaperCreate([[pi printer] _printer], (CFStringRef)[NSString 
        stringWithFormat:@"%ld",self], (CFStringRef)[NSString stringWithFormat:@"%2.2f X %2.2f",
        userW,userH],(double) paperSize.width,(double) paperSize.height, marginPtr, &myPaper);
        
     if (osStatus  != 0) {
      // You may want to do something else here:
      NSLog(@"Trouble creating a custom page size: %f by %f",paperSize.width, paperSize.height);
        }
    }
     // on 10.1 this just works:
    [pi setPaperSize:paperSize];
}
// getting user units from an NSUserDefault: - sometimes C is cool!
float
pointsFromUserUnits()
{
    switch([[NSUserDefaults standardUserDefaults] integerForKey:@"MeasurementUnits"]) {
            case 0: return 72.0;
            case 1: return 28.35;
            case 2: return 1.0;
            case 3: return 12.0;
            default: return 72.0;
    }
}
float
convertToUserUnits(float points)
{
    return points/pointsFromUserUnits();
}

Conclusion

So now, our printInfo will return the correct new size! Well, the dust hasn't settled entirely, so hopefully this will work and continue to work in the future. Ideally, NSPrintInfo would just "do the right thing" in terms of creating these custom page sizes. The proposed solution doesn't address custom page size uniquing and coalescing of similar-sized pages - but then, I haven't finished coding the solution entirely either! Even the mighty Cocoa has its compatibility weaknesses as it grows, but its native power can even pull the tractor out of the mud when it gets really stuck.


Andrew Stone, founder of Stone Design <www.stone.com>, spends too much time coding and not enough time gardening.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

The best video player on mobile
We all know the stock video player on iOS is not particularly convenient, primarily because it asks us to hook a device up to iTunes to sync video in a world that has things like Netflix. [Read more] | Read more »
Four apps to help improve your Super Bow...
Super Bowl Sunday is upon us, and whether you’re a Panthers or a Broncos fan you’re no doubt gearing up for it. [Read more] | Read more »
LooperSonic (Music)
LooperSonic 1.0 Device: iOS Universal Category: Music Price: $4.99, Version: 1.0 (iTunes) Description: LooperSonic is a multi-track audio looper and recorder that will take your loops to the next level. Use it like a loop pedal to... | Read more »
Space Grunts guide - How to survive
Space Grunts is a fast-paced roguelike from popular iOS developer, Orange Pixel. While it taps into many of the typical roguelike sensibilities, you might still find yourself caught out by a few things. We delved further to find you some helpful... | Read more »
Dreii guide - How to play well with othe...
Dreii is a rather stylish and wonderful puzzle game that’s reminiscent of cooperative games like Journey. If that sounds immensely appealing, then you should immediately get cracking and give it a whirl. We can offer you some tips and tricks on... | Read more »
Kill the Plumber World guide - How to ou...
You already know how to hop around like Mario, but do you know how to defeat him? Those are your marching orders in Kill the Plumber, and it's not always as easy as it looks. Here are some tips to get you started. This is not a seasoned platform... | Read more »
Planar Conquest (Games)
Planar Conquest 1.0 Device: iOS Universal Category: Games Price: $12.99, Version: 1.0 (iTunes) Description: IMPORTANT: Planar Conquest is compatible only with iPad 3 & newer devices, iPhone 5 & newer. It’s NOT compatible with... | Read more »
We talk to Cheetah Mobile about its plan...
Piano Tiles 2 is a fast-paced rhythm action high score chaser out now on iOS and Android. You have to tap a series of black tiles that appear on the screen in time to the music, being careful not to accidentally hit anywhere else. Do that and it's... | Read more »
Ultimate Briefcase guide - How to dodge...
Ultimate Briefcase is a simple but tricky game that’s highly dependent on how fast you can react. We can still offer you a few tips and tricks on how to survive though. Guess what? That’s exactly what we’re going to do now. Take it easy [Read more... | Read more »
SoundPrism Link Edition (Music)
SoundPrism Link Edition 1.0 Device: iOS Universal Category: Music Price: $4.99, Version: 1.0 (iTunes) Description: ***Introductory price for a the first few days after launch - if you're reading this, get it while it's fresh out of... | Read more »

Price Scanner via MacPrices.net

iPads on sale at Target: $100 off iPad Air 2,...
Target has WiFi iPad Air 2s and iPad mini 4s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for... Read more
Target offers Apple Watch for $100 off MSRP
Target has Apple Watches on sale for $100 for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - Apple... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and shipping is free... Read more
Macs available for up to $300 off MSRP, $20 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, and... Read more
Watch Super Bowl 50 Live On Your iPad For Fre...
Watch Super Bowl 50 LIVE on the CBS Sports app for iPad and Apple TV. Get the app and then tune in Sunday, February 7, 2016 at 6:30 PM ET to catch every moment of the big game. The CBS Sports app is... Read more
Two-thirds Of All Smart Watches Shipped In 20...
Apple dominated the smart watch market in 2015, accounting for over 12 million units and two-thirds of all shipments according to Canalys market research analysts’ estimates. Samsung returned to... Read more
12-inch 1.2GHz Retina MacBooks on sale for up...
B&H Photo has 12″ 1.2GHz Retina MacBooks on sale for $180 off MSRP. Shipping is free, and B&H charges NY tax only: - 12″ 1.2GHz Gray Retina MacBook: $1499 $100 off MSRP - 12″ 1.2GHz Silver... Read more
12-inch 1.1GHz Gray Retina MacBook on sale fo...
B&H Photo has the 12″ 1.1GHz Gray Retina MacBook on sale for $1199 including free shipping plus NY sales tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model... Read more
Apple now offering full line of Certified Ref...
Apple now has a full line of Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are... Read more
Free GUI Speedometer – The Ultimate Digital D...
Miami, Florida based RMKapps has announced the official release of GUI Speedometer 1.0, their digital dashboard display developed for iOS devices. GUI Speedometer allows users to track their precise... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Subject Matter Expert - Experis (Uni...
This position is for an Apple Subject Matter Expert to assist in developing the architecture, support and services for integration of Apple devices into the domain. Read more
*Apple* Macintosh OSX - Net2Source Inc. (Uni...
…: * Work Authorization : * Contact Number(Best time to reach you) : Skills : Apple Macintosh OSX Location : New York, New York. Duartion : 6+ Months The associate would Read more
Computer Operations Technician ll - *Apple*...
# Web Announcement** Apple Technical Liaison**The George Mason University, Information Technology Services (ITS), Technology Support Services, Desktop Support Read more
Restaurant Manager - Apple Gilroy Inc./Apple...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.