How to perform all other tasks once the asynchronous dispatch queue finishes execution - objective-c

Question

I have a PDFDocument where it has some actions on pages like delete,crop,rotate etc.
So when I click on the delete button and click on save(current thread : main thread)
-(void)save
{
// -- process deleted pages first
for( NSString* deletedPageId in self.deletedPageIdList )
{
[self.pdfCoordinator removePageWithId:deletedPageId];
}
// -- wait for all pages to delete before continuing
[self.pdfCoordinator waitUntilAllOperationsAreFinished];
// few lines of code after this should run only when the above code finishes its execution
// code to save the changes in the pdf to managedObject context
}
The code for removePageWithId:
- (void) removePageWithId:(NSString*)pageId
{
NRMPDFOperation *op = [[NRMPDFOperation alloc] initWithNRMPDF:self
pageId:pageId
selector:#selector(removePageOpImpl:)
args:nil];
[self addOperation:op];
[op release];
}
above code creates an operation and adds it to the operation queue for each deletion of the page
code for removePageOpImpl:
- (NSError *)removePageOpImpl:(NRMPDFOperation *)op
{
NSError* error = [self loadDocument];
if( !error )
{
NSUInteger index = [self pageIndexForId:[op pageId]];
if( index < [self pageCount] )
{
[[self pdfDocument] removePageAtIndex:index];
[[self mutablePageIdList] removeObjectAtIndex:index];
[self updatePageLabelsFromIndex:index];
[self updateChangeCount:NSChangeDone];
self.contentsChanged = YES;
}
else
{
// TODO: error
}
}
return error;
}
In the removePageOpImpl: method the line of code
[[self pdfDocument] removePageAtIndex:index]; internally executing some tasks on main thread(but we are making the main thread to wait until this operation finishes).which causes the deadlock.
I tried to execute the code inside removePageOpImpl: in an asynchronous dispatch queue to avoid the deadlock.below is the code for that
- (NSError *)removePageOpImpl:(NRMPDFOperation *)op
{
NSError* error = [self loadDocument];
if( !error )
{
NSUInteger index = [self pageIndexForId:[op pageId]];
if( index < [self pageCount] )
{
dispatch_async(dispatch_get_main_queue(), ^{
[[self pdfDocument] removePageAtIndex:index];
[[self mutablePageIdList] removeObjectAtIndex:index];
[self updatePageLabelsFromIndex:index];
[self updateChangeCount:NSChangeDone];
self.contentsChanged = YES;
});
}
else
{
// TODO: error
}
}
return error;
}
Now I am out from the deadlock. But another issue is by putting the above code into asynchronous block the code which should run after this tasks is executing before this, because of that my app is not behaving as expected.
Code inside waitUntilAllOperationsAreFinished method
- (void) addOperation:(NRMPDFOperation*)operation
{
[_operationSet addObject:operation];
[[self operationQueue] addOperation:operation];
}
- (void) waitUntilAllOperationsAreFinished
{
[[self operationQueue] waitUntilAllOperationsAreFinished];
}
- (NSOperationQueue *)operationQueue
{
if( !_operationQueue )
{
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[queue setMaxConcurrentOperationCount:1];
_operationQueue = queue;
}
return _operationQueue;
}
This is how the original Save method looks like :
- (void)saveDocumentWithDelegate:(id)delegate didSaveSelector:(SEL)didSaveSelector contextInfo:(void *)contextInfo
{
// TODO: don't just ignore saveOperation
__block BOOL success = YES;
__block NSError *error = nil;
/* write back field changes */
if ([item hasChangesInEditFieldsetFor:#"values"] )
{
//Some code
}
if( self.isPDFEdited )
{
// -- process deleted pages first
for( NSString* deletedPageId in self.deletedPageIdList )
{
[self.itemPDFCoordinator removePageWithId:deletedPageId];
}
// -- wait for all pages to delete before continuing
[self.itemPDFCoordinator waitUntilAllOperationsAreFinished];
// -- replace the search text any pages we deleted
if( [self.deletedPageIdList count] )
{
[self.item setValue:[self.editPDF string] forKeyPath:#"dict.values._searchText"];
}
NSMutableDictionary* originalRotations = [NSMutableDictionary dictionaryWithCapacity:
[self.itemPDFCoordinator pageCount]];
for( NSString* pageId in self.itemPDFCoordinator.pageIdList )
{
NSInteger rotation = [[self.itemPDFCoordinator pageForId:pageId] rotation];
[originalRotations setObject:[NSNumber numberWithInteger:rotation] forKey:pageId];
}
// -- now process actions on remaining pages (crop, rotate, and convert to b&w)
BOOL didCropAnyPages = NO;
NSMutableArray* convertToBwJobs = [NSMutableArray array];
for( NSString* pageId in [self.pageActionDict allKeys] )
{
NSArray* actions = [self.pageActionDict objectForKey:pageId];
for( NSDictionary* action in actions )
{
NSNumber* rotationNumber = [action objectForKey:#"rotation"];
NSValue* cropRectVal = [action objectForKey:#"cropRect"];
NSNumber* convertToBlackAndWhite = [action objectForKey:#"convertToBlackAndWhite"];
if( rotationNumber )
{
[self.itemPDFCoordinator rotateByDegrees:[rotationNumber integerValue]
forPageWithID:pageId];
}
else if( cropRectVal )
{
[self.itemPDFCoordinator setNormalizedBounds:[cropRectVal rectValue]
forBox:kPDFDisplayBoxCropBox
forPageWithID:pageId];
// -- set a flag so we know to recrop the entire document
didCropAnyPages = YES;
}
else if( [convertToBlackAndWhite boolValue] )
{
NSUInteger pageIndex = [self.itemPDFCoordinator pageIndexForId:pageId];
NRMJob* job = [NRMAppJobFactory convertToBlackAndWhiteJobForItem:self.item
pageIndex:pageIndex];
[convertToBwJobs addObject:job];
}
}
}
// -- reapply crop box to any cropped pages
if( didCropAnyPages )
{
[self.itemPDFCoordinator applyCropBoxToAllPages];
}
[self.itemPDFCoordinator waitUntilAllOperationsAreFinished];
for( NRMJob* job in convertToBwJobs )
{
if( ![[self.masterDocument docjob] addJob:job forItem:self.item error:&error] )
[NSApp presentError:error];
else
[job waitUntilDone];
}
// -- make sure document attributes are updated
NSDictionary *docDict = [self.itemPDFCoordinator documentAttributes];
NSDictionary *newDict = [(NRMItem *)item updateDocumentAttributes:docDict];
if (![newDict isEqualToDictionary:docDict])
[self.itemPDFCoordinator setDocumentAttributes:newDict];
[self.itemPDFCoordinator waitUntilAllOperationsAreFinished];
// -- check if we need to reprocess any pages
for( NSString* pageId in self.itemPDFCoordinator.pageIdList )
{
NSInteger oldRotation = [[originalRotations objectForKey:pageId] integerValue];
NSInteger newRotation = [[self.itemPDFCoordinator pageForId:pageId] rotation];
if( oldRotation != newRotation )
{
// -- if it's an image page and we already have OCR data for it, we should reanalyze
NSUInteger pageIndex = [self.itemPDFCoordinator pageIndexForId:pageId];
BOOL isPageImage = [self.itemPDFCoordinator isImagePageAtIndex:pageIndex DPI:NULL];
if( isPageImage && [item OCRDataForPageIndex:pageIndex] )
{
NRMJob* job = [NRMAppJobFactory reprocessPageJobForItem:self.item
pageIndex:pageIndex];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(reanalyzeJobFinished:)
name:kNRMJobFinishedNotification
object:job];
success = [[self.masterDocument docjob] addJob:job forItem:self.item error:&error];
if( !success )
{
if( error )
[self presentError:error];
}
//goto bail;
}
}
}
//Force Save of PDF to Disk
[self.itemPDFCoordinator setManagedObjectContext:[item managedObjectContext]];
[self.itemPDFCoordinator saveChanges];
}
if( success )
{
[self updateChangeCount:NSChangeCleared];
if( self.isPDFEdited )
{
// !CLEARSPLIT! please do not remove this comment
[self.item setValue:nil forKeyPath:#"dict.values._splitId"];
if( ![self loadPDFForItem:item error:&error] )
goto bail;
}
}
bail:
if( error )
[self presentError:error];
if( delegate )
{
/* signature:
- (void)document:(NSDocument *)document didSave:(BOOL)didSaveSuccessfully contextInfo:(void *)contextInfo;
*/
objc_msgSend( delegate, didSaveSelector, self, (error ? NO : YES), contextInfo );
}
}
can anyone suggest me how can I get out of this issue.

Answer 1

Rather than calling waitUntilAllOperationsAreFinished (which blocks a thread), I'd suggest specifying an additional completion operation, which you can make dependent upon the other operations finishing:
-(void)save {
NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
// whatever you want to do when it's done
}];
for (NSString* deletedPageId in self.deletedPageIdList) {
NSOperation *operation = [self.pdfCoordinator removePageWithId:deletedPageId];
[completionOperation addDependency:operation];
}
// add this completionOperation to a queue, but it won't actually start until the other operations finish
[queue addOperation:completionOperation];
}
Clearly this assumes that you change removePageWithId to return the operation it added to the queue:
- (NSOperation *)removePageWithId:(NSString*)pageId {
NRMPDFOperation *operation = [[NRMPDFOperation alloc] initWithNRMPDF:self
pageId:pageId
selector:#selector(removePageOpImpl:)
args:nil];
[self addOperation:operation];
[operation release];
return operation;
}
This way, you're not blocking any thread waiting for the operations to finish, but simply specify what to do when they do finish.