FMDB Blocking UI. But why? Any suggestions for alternative implementation? - ios

Question

I have an application that uses FMDB and performs an update as soon as the application starts (only once). The update is quite heavy and takes between 12-20 seconds to process . I am using FMDatabaseQueue with transactions based on a singleton class.
======== DB.h ====================
#interface DB : NSObject{
FMDatabaseQueue *dbqueue;
NSArray *queriesCreateSchema;
NSString *dbFullPath;
}
#property (nonatomic, strong) FMDatabaseQueue *dbqueue;
==================================
======== DB.m ====================
- (id)initWithFullPath: (NSString*)fullPath {
if (self = [super init]) {
dbFullPath = fullPath;
//Opening/Creating the serial queue
dbqueue = [FMDatabaseQueue databaseQueueWithPath:dbFullPath];
if (dbqueue == nil){
return nil;
}
queriesCreateSchema = [NSArray arrayWithObjects:DBQUERY_ENABLE_FOREIGN_KEYS,
DBQUERY_CREATE_DB,
DBQUERY_CREATE_USERS,
DBQUERY_CREATE_BOOKS,
DBQUERY_INDEX_BOOKS,
DBQUERY_CREATE_BOOKUSER,
DBQUERY_CREATE_PAGES,
DBQUERY_CREATE_PAGEUSER,
DBQUERY_CREATE_TAGS,
DBQUERY_CREATE_TAGBOOK,
DBQUERY_CREATE_CATEGORIES,
DBQUERY_CREATE_CATBOOK,
nil];
}
return self;
}
==================================
======== DBManager.h ====================
#interface DBManager : NSObject <CommsManagerDelegate> {
__weak id <DBMDelegate> delegate;
DB *database;
NSString *dbFullPath;
}
#property (nonatomic, weak) id <DBMDelegate> delegate;
#property (nonatomic, strong) DB *database;
#property (nonatomic, strong) NSString *dbFullPath;
=========================================
======== DBManager.m ====================
-(BOOL) parseMetaDataDict: (NSDictionary*) serverDict {
/// Getting the lists of books from the Server's JSON dictionary
NSDictionary *dictAllBooks = [serverDict objectForKey:#"Books"];
int bookNum=0;
int totalBooks = [[dictAllBooks valueForKey:#"Book"] count];
// Updates the UI
[delegate dbmNumberOfBooksProcessedByDB:totalBooks];
/// Browsing it book by book
for (id serverDictBook in [dictAllBooks valueForKey:#"Book"]){
bookNum++;
/// Trimming book from the server and placing it into the local book dictionary
BookDict *bookDict = [[BookDict alloc]initWithServerDict:serverDictBook];
__block BOOL isError = NO;
/// Sending the queries into the serial queue
[database.dbqueue inTransaction:^(FMDatabase *db, BOOL *rollback) {
/// Inserting book into the BOOKS table
if(![db executeUpdate:DBUPDATE_INSERT_BOOK withParameterDictionary:bookDict.dictionary])
{
isError = YES;
DDLogWarn(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
}];
if (isError){
return NO;
}
__block NSString *bookID;
/// Getting the bookID automatically generated by the DB
NSString *query = [NSString stringWithFormat:#"SELECT bookID FROM BOOKS where isbn = '%#'", [bookDict.dictionary valueForKey:#"isbn"]];
[database.dbqueue inTransaction:^(FMDatabase *db, BOOL *rollback) {
FMResultSet *result = [db executeQuery:query];
if([result next])
{
int num = [result intForColumnIndex:0];
bookID = [NSString stringWithFormat:#"%d", num];
}
else{
isError = YES;
DDLogWarn(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
}];
if (isError){
return NO;
}
int numPages = [[serverDictBook objectForKey:#"numberOfPages"] intValue];
/// Browsing the book page by page
///VCC Today probably replace by 0
for (int i=1; i<=numPages; i++)
{
PageDict *pageDict = [[PageDict alloc]initWithPage:i andBookID:bookID ofServerDict:serverDictBook];
__block BOOL isError = NO;
/// Sending the queries into the serial queue
[database.dbqueue inTransaction:^(FMDatabase *db, BOOL *rollback) {
/// Inserting page into the PAGES table
if(![db executeUpdate:DBUPDATE_INSERT_PAGE withParameterDictionary:pageDict.dictionary])
{
isError = YES;
DDLogWarn(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
}];
if (isError)
return NO;
}
__block NSString *catID;
/// Browsing the book categories one by one
for (id serverCatDict in [serverDictBook valueForKey:#"categories"]){
__block BOOL isError = NO;
/// Sending the queries into the serial queue
[database.dbqueue inTransaction:^(FMDatabase *db, BOOL *rollback) {
/// Inserting row into the CATEGORY table
if(![db executeUpdate:DBUPDATE_INSERT_CATEGORY withParameterDictionary:serverCatDict])
{
isError = YES;
DDLogWarn(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
/// Getting the catID automatically generated by the DB
NSString *query = [NSString stringWithFormat:#"SELECT catID FROM CATEGORIES where name = '%#'", [serverCatDict valueForKey:#"name"]];
FMResultSet *result = [db executeQuery:query];
if([result next])
{
catID = [result stringForColumnIndex:0];
}
else{
isError = YES;
DDLogError(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
CatBookDict *catBookDict = [[CatBookDict alloc] initWithCatID:catID andBookID:bookID];
/// Inserting row into the CATBOOK table
if(![db executeUpdate:DBUPDATE_INSERT_CATBOOK withParameterDictionary:catBookDict.dictionary])
{
isError = YES;
DDLogError(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
}];
if (isError)
return NO;
}
// /// Browsing the book categories one by one
// for (id serverCatDict in [serverDictBook valueForKey:#"name"]){
//
// __block BOOL isError = NO;
//
// CatBookDict *catBookDict = [[CatBookDict alloc] initWithCatID:[serverCatDict valueForKey:#"catID"]];
//
// /// Sending the queries into the serial queue
// [database.dbqueue inTransaction:^(FMDatabase *db, BOOL *rollback) {
// andBookID:bookID];
// /// Inserting row into the CATBOOK table
// if(![db executeUpdate:DBUPDATE_INSERT_CATBOOK withParameterDictionary:catBookDict.dictionary])
// {
// isError = YES;
// DDLogVerbose(#"%#", [db lastErrorMessage]);
// *rollback = YES;
// return; // Carefull - It returns from the transaction, not the function
// }
// }];
//
// if (isError)
// return NO;
//
// }
[database.dbqueue inTransaction:^(FMDatabase *db, BOOL *rollback) {
FMResultSet *result = [db executeQuery:query];
if([result next])
{
int num = [result intForColumnIndex:0];
bookID = [NSString stringWithFormat:#"%d", num];
}
else{
isError = YES;
DDLogError(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
}];
if (isError){
return NO;
}
/// Browsing the book tags one by one
for (id serverTagDict in [serverDictBook valueForKey:#"tags"]){
// TagDict *tagDict = [[TagDict alloc] initWithServerDict:serverTagDict[0]];
// TagBookDict *tagBookDict = [[TagBookDict alloc] initWithTagID:[serverTagDict valueForKey:#"tagID"]
// andBookID:bookID];
__block BOOL isError = NO;
/// Sending the queries into the serial queue
[database.dbqueue inTransaction:^(FMDatabase *db, BOOL *rollback) {
/// Inserting tag into the TAGS table
if(![db executeUpdate:DBUPDATE_INSERT_TAG withParameterDictionary:serverTagDict])
{
isError = YES;
DDLogError(#"%#", [db lastErrorMessage]);
*rollback = YES;
return; // Carefull - It returns from the transaction, not the function
}
// /// Inserting the row into the TAGBOOK table
// if(![db executeUpdate:DBUPDATE_INSERT_TAGBOOK withParameterDictionary:tagBookDict.dictionary])
// {
// isError = YES;
// DDLogVerbose(#"%#", [db lastErrorMessage]);
// *rollback = YES;
// return; // Carefull - It returns from the transaction, not the function
// }
}];
if (isError)
return NO;
}
// Updates the UI
[delegate dbmBookProcessedByDB:bookNum];
}
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
if (![defaults objectForKey:#"firstSynced"]){
[defaults setObject:[NSDate date] forKey:#"firstSynced"];
[[NSUserDefaults standardUserDefaults] synchronize];
}
return TRUE;
}
I have a view controller that uses the DBManager Class by calling the previous method "parseMetaDataDict". For every iteration of the for a book is processed and inserts take place in the DB. I am using delegation and updating the UI within the loop.
============= SplashViewController.h ============================
-(void)dbmBookProcessedByDB:(int)bookNum{
dispatch_async(dispatch_get_main_queue(), ^(void){
//Run UI Updates
NSString *strMsg = [NSString stringWithFormat:#"DB Processing Book %d / %d", bookNum, _totalBooksToBeDownloaded];
[self.progressBar setText:strMsg];;
if (bookNum == _totalBooksToBeDownloaded){
[self.progressBar setText:#"Books library has successfully been updated"];
[self.progressBar setNeedsDisplay];
[self performSegueWithIdentifier:#"splashToHome" sender:self];
}
});
}
=========================================
The updates to the progress Bar are not taking place. I believe that the dispatch_async is unnecessary. Debugging shows that it passes over the dispatch_async and never enters inside.
Is the FMDB queue blocking the entire Main thread. How can I make periodical updates to a label every time a book is processed?

Answer 1

In FMDB the dispatch_sync function is used to put your transaction blocks into the serial queue.
Documentation for dispatch_sync says:
As an optimization, this function invokes the block on the current
thread when possible.
I think that's why calls to -inTransaction: may block the main thread.
Try to make the -inTransaction: to be called from the background thread. To do this you can put the body of your for cycle into the background queue via CGD like this:
-(BOOL) parseMetaDataDict: (NSDictionary*) serverDict {
...
/// Browsing it book by book
for (id serverDictBook in [dictAllBooks valueForKey:#"Book"]){
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void){
... all -inTransaction: calls are here
dispatch_async(dispatch_get_main_queue(), ^(void){
// Updates the UI
[delegate dbmBookProcessedByDB:bookNum];
});
});
}
Note: It's also better to jump between threads in one scope, to make code look clear, so you can also move dispatch_async(dispatch_get_main_queue(), ...) from -dbmBookProcessedByDB inside for body as shown in code above.