iTunes File Sharing app: realtime monitoring for incoming datas

Question

I\'m working on iOS project that supports iTunes file sharing feature. The goal is realtime tracking incoming/changed data\'s.

I\'m

Answer 1

I found two rather reliable (i.e. not 100% reliable but reliable enough for my needs) approaches, which only work in conjunction with polling the contents of the directory:

1. Check NSURLContentModificationDateKey. While the file is being transferred, this value is set to the current date. After transfer has finished, it is set to the value the original file had: BOOL busy = (-1.0 * [modDate timeintervalSinceNow]) < pollInterval;
2. Check NSURLThumbnailDictionaryKey. While the file is being transferred, this value is nil, afterwards it cointains a thumbnail, but probably only for file types from which the system can produce a thumbnail. Not a problem for me cause I only care about images and videos, but maybe for you. While this is more reliable than solution 1, it hammers the CPU quite a bit and may even cause your app to get killed if you have a lot of files in the import directory.

Dispatch sources and polling can be combined, i.e. when a dispatch source detects a change, start polling until no busy files are left.

Answer 2

You can use GCD's dispatch sources mechanism - using it you can observe particular system events (in your case, this is vnode type events, since you're working with file system). To setup observer for particular directory, i used code like this:

- (dispatch_source_t) fileSystemDispatchSourceAtPath:(NSString*) path
{
    int fileDescr = open([path fileSystemRepresentation], O_EVTONLY);// observe file system events for particular path - you can pass here Documents directory path
    //observer queue is my private dispatch_queue_t object
    dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, fileDescr, DISPATCH_VNODE_ATTRIB| DISPATCH_VNODE_WRITE|DISPATCH_VNODE_LINK|DISPATCH_VNODE_EXTEND, observerQueue);// create dispatch_source object to observe vnode events
    dispatch_source_set_registration_handler(source, ^{
        NSLog(@"registered for observation");
        //event handler is called each time file system event of selected type (DISPATCH_VNODE_*) has occurred
        dispatch_source_set_event_handler(source, ^{

            dispatch_source_vnode_flags_t flags = dispatch_source_get_data(source);//obtain flags
            NSLog(@"%lu",flags);

            if(flags & DISPATCH_VNODE_WRITE)//flag is set to DISPATCH_VNODE_WRITE every time data is appended to file
            {
                NSLog(@"DISPATCH_VNODE_WRITE");
                NSDictionary* dict = [[NSFileManager defaultManager] attributesOfItemAtPath:path error:nil];
                float size = [[dict valueForKey:NSFileSize] floatValue];
                NSLog(@"%f",size);
            }
            if(flags & DISPATCH_VNODE_ATTRIB)//this flag is passed when file is completely written.
            {
                NSLog(@"DISPATCH_VNODE_ATTRIB");
                dispatch_source_cancel(source);
            }
            if(flags & DISPATCH_VNODE_LINK)
            {
                NSLog(@"DISPATCH_VNODE_LINK");
            }
            if(flags & DISPATCH_VNODE_EXTEND)
            {
                NSLog(@"DISPATCH_VNODE_EXTEND");
            }
            NSLog(@"file = %@",path);
            NSLog(@"\n\n");
        });

        dispatch_source_set_cancel_handler(source, ^{
            close(fileDescr);
        });
    });

    //we have to resume dispatch_objects
    dispatch_resume(source);

    return source;
}