Question

I’m working on a desktop application that watch folders using the fileevent api, so basically this is my code :

#import "PNAppDelegate.h"

void callback(
              ConstFSEventStreamRef streamRef,
              void *clientCallBackInfo,
              size_t numEvents,
              void *eventPaths,
              const FSEventStreamEventFlags eventFlags[],
              const FSEventStreamEventId eventIds[]) 
{
    [(__bridge PNAppDelegate *)clientCallBackInfo reloadStatus];

};

@implementation PNAppDelegate

@synthesize window = _window;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    NSArray *pathsToWatch = [NSArray arrayWithObject: @"/Users/romainpouclet/Projects/foo"];

    void *appPointer = (__bridge void *)self;
    FSEventStreamContext context = {0, appPointer, NULL, NULL, NULL};

    FSEventStreamRef stream;
    CFAbsoluteTime latency = 3.0;

    stream = FSEventStreamCreate(NULL, 
                                 &callback, 
                                 &context, 
                                 (__bridge CFArrayRef) pathsToWatch, 
                                 kFSEventStreamEventIdSinceNow, 
                                 latency, 
                                 kFSEventStreamCreateFlagNone);

    NSLog(@"Schedule with run loop");
    FSEventStreamScheduleWithRunLoop(stream, CFRunLoopGetMain(), kCFRunLoopDefaultMode);
    FSEventStreamStart(stream);
    [self reloadStatus];
}

-(void)reloadStatus
{

}

@end

No problem, it works pretty well for a POC as simple as this one, BUT it feels kinda ugly (and it probably is, I’m not really used to mix Objective-C and C). So here are my questions :

• where should I declare my callback? It feels weird having it at the top of my file, just because it worked there.
• is it possible to have some kind of @selector-based approach instead of callbacks? (I find them reassuring :D)

Thanks for your time !

Answer 1

You are correct, that code IS ugly. However, bridging C and Obj-C is no small task, so you really only have a few options:

1. Create an Objective-C wrapper around the C-based API. This would be my recommended approach, especially if the API is not too complex. It gives you the advantage of using either delegates or blocks, instead of functions.

2. Use blocks for callbacks, by getting their internal function pointer:

   // internal structure of a block
   struct blockPtr {
       void *__isa;
       int __flags;
       int __reserved;
       void *__FuncPtr;
       void *__descriptor;
   };
   
   int main()
   {
       @autoreleasepool {
           __block int b = 0;        
   
           void (^blockReference)(void *) = ^(void *arg) {
               NSLog(@"<%s>: %i", arg, b++);
           };
   
   
           void *blockFunc = ((__bridge struct blockPtr *) blockReference)->__FuncPtr;
           void (*castedFunction)(void *, void *) = blockFunc;
   
           // the first argument to any block funciton is the block 
           // reference itself, similar to how the first argument to
           // any objc function is 'self', however, in most cases you
           // don't need the block reference (unless reading __block variables), it's just difficult to
           // get that first argument from inside the block
           castedFunction((__bridge void *) blockReference, "one");
           castedFunction((__bridge void *) blockReference, "two");
       }
   }
   

   I really don’t think this is practical in most situations, but if you can find a way to make it work, more power to you.

3. Stick with how you are currently doing it. It sucks, but that is how C works.