Not receiving notification dispatch_group_notify - ios

Question

I am currently using a dispatch_group to get notify when all concurrent tasks are done. I am offloading some heavy tasks on one concurrent queue within the [TWReaderDocument documentFileURL:url withCompletionBlock:] class method.
I have implemented the following code but never received any notification. I don't see what i am potentially doing wrong in the below code:
dispatch_group_t readingGroup = dispatch_group_create();
NSFileManager* manager = [NSFileManager defaultManager];
NSString *docsDir = [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:#"Data"];
NSDirectoryEnumerator *dirEnumerator = [manager enumeratorAtURL:[NSURL fileURLWithPath:docsDir]
includingPropertiesForKeys:[NSArray arrayWithObjects:NSURLNameKey,
NSURLIsDirectoryKey,nil]
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:nil];
// An array to store the all the enumerated file names in
NSMutableArray *arrayFiles;
// Enumerate the dirEnumerator results, each value is stored in allURLs
for (NSURL *url in dirEnumerator) {
// Retrieve the file name. From NSURLNameKey, cached during the enumeration.
NSString *fileName;
[url getResourceValue:&fileName forKey:NSURLNameKey error:NULL];
// Retrieve whether a directory. From NSURLIsDirectoryKey, also cached during the enumeration.
NSNumber *isDirectory;
[url getResourceValue:&isDirectory forKey:NSURLIsDirectoryKey error:NULL];
if (![isDirectory boolValue]) {
dispatch_group_enter(readingGroup);
TWReaderDocument* doc = [TWReaderDocument documentFileURL:url withCompletionBlock:^(BOOL success) {
dispatch_group_leave(readingGroup);
}];
[arrayFiles addObject:doc];
}
else if ([[[fileName componentsSeparatedByString:#"_" ] objectAtIndex:0] isEqualToString:#"XXXXXX"]) {
TreeItem* treeItem = [[TreeItem alloc] init];
arrayFiles = [NSMutableArray arrayWithCapacity:10];
treeItem.child = arrayFiles;
treeItem.nodeName = [[fileName componentsSeparatedByString:#"_" ] lastObject];
[self addItem:treeItem];
}
}
dispatch_group_notify(readingGroup, dispatch_get_main_queue(), ^{ // 4
NSLog(#"All concurrent tasks completed");
});
Does the dispatch_group_enter and dispatch_group_leave have to be executed on the same thread?
EDIT
The code snippet of my factory method might help aswell:
+ (TWReaderDocument *)documentFileURL:(NSURL *)url withCompletionBlock:(readingCompletionBlock)completionBlock{
TWReaderDocument * twDoc = [[TWReaderDocument alloc] init];
twDoc.status = ReaderDocCreated;
twDoc.doc = [ReaderDocument withDocumentFilePath:[url path] withURL:url withLoadingCompletionBLock:^(BOOL completed) {
twDoc.status = completed ? ReaderDocReady : ReaderDocFailed;
completionBlock(completed);
}];
return twDoc;
}
TWReaderDocument is a wrapper class that call internally the following methods of a third-party library (it is a PDF reader)
+ (ReaderDocument *)withDocumentFilePath:(NSString *)filePath withURL:(NSURL*)url withLoadingCompletionBLock:(readingCompletionBlock)completionBlock{
ReaderDocument *document = [[ReaderDocument alloc] initWithFilePath:filePath withURL:url withLoadingCompletionBLock:[completionBlock copy]];
return document;
}
- (id)initWithFilePath:(NSString *)fullFilePath withURL:(NSURL*)url withLoadingCompletionBLock:(readingCompletionBlock)completionBlock {
id object = nil; // ReaderDocument object;
if ([ReaderDocument isPDF:fullFilePath] == YES) // File must exist
{
if ((self = [super init])) // Initialize superclass object first
{
_fileName = [ReaderDocument relativeApplicationFilePath:fullFilePath]; // File name
dispatch_async([ReaderDocument concurrentLoadingQueue], ^{
self.guid = [ReaderDocument GUID]; // Create a document GUID
self.password = nil; // Keep copy of any document password
self.bookmarks = [NSMutableIndexSet indexSet]; // Bookmarked pages index set
self.pageNumber = [NSNumber numberWithInteger:1]; // Start on page 1
CFURLRef docURLRef = (__bridge CFURLRef)url;// CFURLRef from NSURL
self.fileURL = url;
CGPDFDocumentRef thePDFDocRef = CGPDFDocumentCreateX(docURLRef, self.password);
BOOL success;
if (thePDFDocRef != NULL) // Get the number of pages in the document
{
NSInteger pageCount = CGPDFDocumentGetNumberOfPages(thePDFDocRef);
self.pageCount = [NSNumber numberWithInteger:pageCount];
CGPDFDocumentRelease(thePDFDocRef); // Cleanup
success = YES;
}
else // Cupertino, we have a problem with the document
{
// NSAssert(NO, #"CGPDFDocumentRef == NULL");
success = NO;
}
NSFileManager *fileManager = [NSFileManager new]; // File manager instance
self.lastOpen = [NSDate dateWithTimeIntervalSinceReferenceDate:0.0]; // Last opened
NSDictionary *fileAttributes = [fileManager attributesOfItemAtPath:fullFilePath error:NULL];
self.fileDate = [fileAttributes objectForKey:NSFileModificationDate]; // File date
self.fileSize = [fileAttributes objectForKey:NSFileSize]; // File size (bytes)
completionBlock(success);
});
//[self saveReaderDocument]; // Save the ReaderDocument object
object = self; // Return initialized ReaderDocument object
}
}
return object;
}

Answer 1

It's hard to say what's going on here without knowing more about TWReaderDocument, but I have a suspicion...
First off, no, dispatch_group_enter and dispatch_group_leave do not have to be executed on the same thread. Definitely not.
My best guess based on the info here would be that for some input, [TWReaderDocument documentFileURL:withCompletionBlock:] is returning nil. You might try this instead:
if (![isDirectory boolValue]) {
dispatch_group_enter(readingGroup);
TWReaderDocument* doc = [TWReaderDocument documentFileURL:url withCompletionBlock:^(BOOL success) {
dispatch_group_leave(readingGroup);
}];
// If the doc wasn't created, leave might never be called.
if (nil == doc) {
dispatch_group_leave(readingGroup);
}
[arrayFiles addObject:doc];
}
Give that a try.
EDIT:
It's exactly as I expected. There are cases in which this factory method will not call the completion. For instance:
if ([ReaderDocument isPDF:fullFilePath] == YES) // File must exist
If -isPDF: returns NO the completionBlock will never be called, and the returned value will be nil.
Incidentally, you should never compare something == YES. (anything non-zero is equivalent to YES, but YES is defined as 1. Just do if ([ReaderDocument isPDF:fullFilePath]). It's equivalent, and safer.