Question

如果libsqlite不是线程安全的那样的代码

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    
});

会引发BAD_EXEC。

那么如何使这段代码线程安全？

我的解决方案是使用主队列

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

但我对此并不满意。如何让它更好？

Answer 1

而不是使用dispatch_get_main_queue()获取主队列，您可能希望在非主线程上创建单独的private dispatch queue，如下所示：

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
});

或者你可以使用来自Gus Mueller（@ccgus）精彩FMDB SQLite包装器框架的FMDatabaseQueue（这就是我要做的）：

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:aPath];

[queue inDatabase:^(FMDatabase *db) {

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

}];

...它会将您的查询块发送到串行调度队列，同步包装它的执行。

还不确定吗？

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

现在怎么样？

另外，custom-defined block-based SQLite functions。

Answer 2

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

});

libsqlite和dispatch_async

2 个答案: