Question

Related to this SO question, I want to put the data loading in the background.

However, I get ‘library routine called out of sequence’ errors.

In this SO thread say that the way is using NSOperation, but looking on the samples on the web I not know how that could solve the issue.

I share a single sqlite connection with the singleton pattern:

@interface Db : NSObject {
    NSString *path;
    FMDatabase* theDb;
    BOOL isOpen;
}

@property (retain, nonatomic) FMDatabase *theDb;
@property (retain, nonatomic) NSString *path;
@property (nonatomic) BOOL isOpen;
--------
static Db *currentDbSingleton = nil;
#pragma mark Global access

+(id)currentDb {
    @synchronized(self) {
        if (!currentDbSingleton) {
            NSString *reason = NSLocalizedString(@"The database is not set globally",
                                                 @"Error Db: database is not set");
            NSException *e = [NSException exceptionWithName:@"DBError"                        
                                                     reason:reason;
                                                   userInfo:nil];
            @throw e;
        }
    }
    return currentDbSingleton;  
}

So is harder open twice the same db….

Any ideas?

EDIT:

I confirmed the error is in calling sqlite. I use FDBM as thin wrapper for calling it.

I’m running 2 threads: the main and a background task for loading of the data. I run it this way:

- (void) fillCache:(NSString *)theTable {
    [NSThread detachNewThreadSelector:@selector(fillCacheBackground:)
                             toTarget:self
                           withObject:theTable];
}

- (void)loadComplete {
    [self.table reloadData];
}

- (void) fillCacheBackground:(NSString *)theTable {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    Db *db= [Db currentDb];
    [db beginTransaction];
        ..... STUFF HERE
    [db commitTransaction];
    //Tell our callback what we've done
    [self performSelectorOnMainThread:@selector(loadComplete) 
                           withObject:nil 
                        waitUntilDone:YES];
    [pool drain];
}

The code for the db interface is at http://code.google.com/p/chibiorm/source/browse/#svn/trunk/src — specifically the Db.h/m that are the only units that interface with fdbm/sqlite.

The error happend when try to call sqlite functions from FDBM.

For example happend here:

-(void) checkError {
    if ([self.theDb hadError]) { // <====ERROR HERE
        NSLog(@"Err %d: %@", [self.theDb lastErrorCode], [self.theDb]);
    }
}

This call the FDBM code:

- (BOOL) hadError {
    int lastErrCode = sqlite3_errcode(db);
    return (lastErrCode > SQLITE_OK && lastErrCode < SQLITE_ROW);
}

Answer 1

I finally found a working solution.

The idea is build a database pool of connection and open twice the db. Use one connection for the main thread and the another for the background.

Everything is now in http://code.google.com/p/chibiorm/