CODEWITHSUNDEEP
objective-csqlitethread-safetygrand-central-dispatch
loretoparisi
loretoparisi

Reputation: 16301

libsqlite and dispatch_async

If libsqlite is not thread safe a code like that

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

__block NSArray *__albumsCollection = albumCollections;

dispatch_apply(count, queue, ^(size_t i) 
{ 
   MPMediaItem *albumObj = [[__albumsCollection objectAtIndex:i] representativeItem];

   ///// making some sqlite queries    
});

would raise a BAD_EXEC.

So how to make this code thread safe?

My solution was using the main queue

dispatch_apply(count, dispatch_get_main_queue(), ^(size_t i) 
{
   /// my sqllite queries
});

but I'm not satisfied with that. How to make it better?

Upvotes: 0

Views: 2159

Answers (2)

Dheeraj Kumar
Dheeraj Kumar

Reputation: 441

dispatch_queue_t q=dispatch_queue_create("name", NULL);
dispatch_sync(q, ^{
//your code is here
});

dispatch_async(dispatch_get_main_queue(), ^(void) {
//as soon as above block is completed, this block executed and you will be notified that     
//work is completed

});

Upvotes: 0

Regexident
Regexident

Reputation: 29552

Instead of using dispatch_get_main_queue() to get the main queue you might want to create a separate private dispatch queue on a non-main thread like so:

dispatch_queue_t queue = dispatch_queue_create("com.example.MyQueue", DISPATCH_QUEUE_SERIAL); // or NULL as last parameter if prior to OS X 10.7/iOS 5.0
dispatch_apply(count, queue, ^(size_t i) {
 /// your SQLite queries
});

Alternatively you could use a FMDatabaseQueue from Gus Mueller's (@ccgus) brilliant FMDB SQLite wrapper framework (which is what I'd do):

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:aPath];

[queue inDatabase:^(FMDatabase *db) {

    // Your SQLite queries:
    [db executeQuery:@"...", ...];
    ...

}];

…which will send your query block to a serial dispatch queue, wrapping its execution synchronously.

Not convinced yet?

[queue inTransaction:^(FMDatabase *db, BOOL *rollback) {
    // Your SQLite queries:
    [db executeQuery:@"...", ...];
    ...
}];

How about now?

Also, custom-defined block-based SQLite functions.

Upvotes: 5

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