Gwynne's Blog

In the course of experimentation, I just discovered a neat trick. You can use key-value observing on a class object! Consider this simple example:

#import <Cocoa/Cocoa.h>

static int      Test_x = 0;

@interface Test : NSObject
{
}
+ (int)x;
+ (void)setX:(int)x_;
@end

@implementation Test
+ (int)x
{
        return Test_x;
}

+ (void)setX:(int)x_
{
        [self willChangeValueForKey:@"x"];
        Test_x = x_;
        [self didChangeValueForKey:@"x"];
}
@end
 
@interface Observer : NSObject
{
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context;
@end

@implementation Observer
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context;
{
        printf("observed for %s, value = %d\n", [keyPath UTF8String], [Test x]);
}
@end

int     main(int argc, char **argv)
{
        NSAutoreleasePool               *pool = [[NSAutoreleasePool alloc] init];
        Observer                        *o = [[[Observer alloc] init] autorelease];

        [[Test class] addObserver:o forKeyPath:@"x" options:NSKeyValueObservingOptionInitial context:NULL];
        [Test setX:99];
        [[Test class] removeObserver:o forKeyPath:@"x"];

        [pool drain];
        return 0;
}

And it works!

$ gcc kvotest.m -o ./kvotest
$ ./kvotest
observed for x, value = 0
observed for x, value = 99
$ 

It has some caveats, of course:

• Automatic notification doesn’t work, probably for some reason having to do with the way the runtime implements class objects and the method swizzling KVO does to make automatic notifications work. Your setters have to explicitly call -willChangeValueForKey: and -didChangeValueForKey:
• This is a hack. It’s not documented to work at all and thus Apple is under no obligation to see that it keeps working.
• You’re not supposed to do things like this with classes. You’re supposed to define a singleton object, e.g. [NSNotificationCenter defaultCenter] and use that.
• I’ve only tried this on Snow Leopard (though it was compiled against the Leopard SDK). I don’t have anything else handy to test with.
• I haven’t tried this with any of the more advanced KVO features (to-many relationships, dependent keys, NSKeyValueObservingOptionPrior).
• Obviously, as shown this isn’t thread-safe, though that’s more for the use of the static variable to hold the value.

The only use for it I can think of offhand is when you have a class which keeps a list of its objects and provides a method for accessing that list as an array, and you want some other object to be notified when that list changes (for example, one could in theory want to observe changes in NSValueTransformer’s +valueTransformerNames). The documentation explicitly states, “Instead of observing an array, observe the ordered to-many relationship for which the array is the collection of related objects.”, so to see changes in the list by KVO, one has to observe the property of the class itself. Classes can’t have properites (or static instance variables, come on Apple), so something like this is one way of doing the trick.

Use at your own risk!

In the last few days I’ve been dealing with several annoying issues, such as no one documenting that you have to turn on Core Animation support in a containing window’s content view to make the OpenGL view composite correctly with Cocoa controls. Four hours wasted on one checkbox. Sigh.

Still, there’s some progress to be had.

1. The loading bar now displays and loads all the various data needed.
2. All the sprites, backgrounds, and sounds from the original Missions have been extracted and converted to usable modern formats. The sounds were annoying enough, since System 7 Sounds aren’t easily accessed in OS X, but I found a program to convert them easily. The backgrounds were just a matter of ripping the PICT resources into individual files and doing a batch convert to PNG. The sprites… those were a problem. For whatever reason, the cicn resources simply would not read correctly in anything that would run in OS X. Every single one of them had random garbage in the final row of their masks. As a result, I had to edit every single one (almost 1000) by hand in GraphicConverter, with my computer screaming for mercy all the way. Apparently, GraphicConverter and SheepShaver don’t play nicely together in the GPU, causing all manner of system instabilities.
3. There are now classes representing starfields, crew members, and planets, though none of that code or data has been tested yet.
4. I’m now building with PLBlocks GCC instead of Clang. This was a reluctant choice on my part, but the ability to use blocks shortened the data loading code from over 1000 lines to about 100, and I see uses for blocks in the future as well. Pity the Clang that comes with 10.6 refuses to work correctly with files using blocks and the 10.5 SDK.
5. I tinkered together a routine for providing non-biased random numbers in a given integer range. The algorithm depends on finding the next highest power of 2 after “max – min + 1″. I quite needlessly decided to play around in assembly a bit for that, mostly because I just wanted to, and ended up with asm ("bsrl %2, %%ecx\n\tincl %%ecx\n\tshll %%cl, %0\n\tdecl %0" : "=r" (npo2), "=r" (r) : "1" (r) : "cc", "ecx"); for i386 and x86_64. I fall back on a pure-C approach for PPC compilation. I haven’t benchmarked this in any way, and I know for a fact that doing so would be meaningless (as the arc4random() call is inevitably far slower than either approach). It was mostly an exercise in knowing assembly language.
6. The “new game” screen, where the scenario and difficulty are selected, now exists. That was also interesting, as it involved shoving a Cocoa view on top of an OpenGL view. I can use that experience for all the other dialogs in the game.

As always, more updates will be posted as they become available.

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While in the process of working on the WordPress plugin mentioned in my last post, I found myself having to do a edit/save/upload cycle annoyingly often, as this WordPress install doesn’t run on my local machine and I didn’t feel like dealing with getting httpd up and running here again. More than once I caught myself trying to figure out why my changes weren’t having an effect until I realized I was forgetting the upload step. And entering a passphrase for scp every time I hit Save is tedious at best. But this blog is hosted on DreamHost, and TextMate doesn’t have SFTP support, so I couldn’t just mount a network drive in the Finder.

But I’d heard of this magical, wonderful thing called sshfs that would let me mount an sftp server as a volume on my machine. That would solve a lot of problems! At first I did the obvious: I went to Terminal and entered sudo port install macfuse sshfs. It didn’t work. Small wonder; the last version of MacFUSE in MacPorts is 2.0.3, which is a considerable distance behind the most recent release, 2.1.5). So I used the MacFUSE prefpane to update to 2.1.5, but then the sshfs install wanted nothing to do with me because it was built against the older version. I went to download a prebuilt binary, since that was just easier, but there wasn’t one for Snow Leopard. I tried to build sshfs from source, but the prefpane hadn’t installed sufficient libraries to do that with. So I went to build MacFUSE itself from source.

We all make mistakes when coding, and some of them are quite deeply foolish. But this particular snafu I just made takes the cake, in my opinion (dumbed-down code):

// Written as a static inline C function for max
// speed; this is called in a very tight loop, usually
// at something like 40,000 times per second.
static inline uint8_t _readByte(NSUIntger *cursor,
                                const char *bytes,
                                NSUInteger maxlen)
{ return (*cursor < maxlen) ? bytes[*cursor++] : 0x66; }
// Macro for readability, known to be called only from the one function
#define readByte() _readByte(&cursor, buffer, bufferlen)
// Macro to skip so many bytes
#define skipBytes(n) do { \
    __typeof__(n) nn = (n); \
    for (uint8_t i = 0; i < nn; ++i) \
        readByte(); \
    } while (0)
- (void)processBuffer
{
    // assume the declarations of n, cursor, buffer, bufferlen
    // ...
    n = readByte();
    // ...
    else if (n > 0x29)
        skipBytes((uint8_t[16]){0,0,0,1,1,2,0,0,0,0,2,2,3,3,4,4})(n & 0xF0) >> 4);
}

See it? (Hint: Look at the implementation of skipBytes().)