Read from file using dispatch_source

gistfile1.m

int rc = 0;
void *opaque = myfile_alloc();
    
const char *incomplete_movie_filename = [[self.incompleteMovieAsset.movieURL path] cStringUsingEncoding:NSASCIIStringEncoding];
rc = myfile_open(opaque, incomplete_movie_filename);
if (rc < 0) {
    if (error) *error = [NSError errorWithFFStatus:rc];
    return NO;
}

//
    
int fd = myfile_fd(opaque);
fcntl(fd, F_SETFL, O_NONBLOCK);  // Avoid blocking the read operation
    
dispatch_queue_t queue = dispatch_queue_create("myfile-io", DISPATCH_QUEUE_SERIAL);
dispatch_source_t readSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_READ,
                                                        fd, 0, queue);
if (readSource == nil) {
    return NO;
}
    
dispatch_source_set_cancel_handler(readSource, ^{
    myfile_close(opaque);
    myfile_dealloc(&opaque);
});
    
dispatch_source_set_event_handler(readSource, ^{
    unsigned long bytes_available = dispatch_source_get_data(readSource);

    // Now we can start reading...
});