FMDB与线程安全

简介

FMDB是对苹果SQLite的封装

多线程操作

1. 如果出现多个线程同时操作数据库，怎么办，用他，用他，FMDatabaseQueue

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:path];

[queue inDatabase:^(FMDatabase *db) {
        
        [db executeUpdate:@"INSERT INTO myTable (name) VALUES (?)", @"will"];
        [db executeUpdate:@"INSERT INTO myTable (name) VALUES (?)", @"carter"];
        
        FMResultSet *rs = [db executeQuery:@"select name from myTable"];
        while([rs next]) {
            NSString *obj = [rs stringForColumn:@"name"];
            NSLog(@"%@", obj);
        }
        
    }];

2. 大量查询数据库的情况

比如循环1000次执行插入查询的操作

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:self.filePath];
    for (NSInteger i = 0; i < 1000; i ++) {
        [queue inDatabase:^(FMDatabase *db) {
            
            [db executeUpdate:@"INSERT INTO myTable (name) VALUES (?)", @"will"];
            [db executeUpdate:@"INSERT INTO myTable (name) VALUES (?)", @"carter"];
            
            FMResultSet *rs = [db executeQuery:@"select name from myTable"];
            while([rs next]) {
                NSString *obj = [rs stringForColumn:@"name"];
                NSLog(@"%@", obj);
            }
            
        }];
    }

image.png

结果发现按钮一直处于被点击状态，说明已经阻塞了主线程，看了inDatabase的源码

FMDatabaseQueue *currentSyncQueue = (__bridge id)dispatch_get_specific(kDispatchQueueSpecificKey);
    assert(currentSyncQueue != self && "inDatabase: was called reentrantly on the same queue, which would lead to a deadlock");
    
    FMDBRetain(self);
    
    dispatch_sync(_queue, ^() {
        
        FMDatabase *db = [self database];
        block(db);
        
        if ([db hasOpenResultSets]) {
            NSLog(@"Warning: there is at least one open result set around after performing [FMDatabaseQueue inDatabase:]");
            
#if defined(DEBUG) && DEBUG
            NSSet *openSetCopy = FMDBReturnAutoreleased([[db valueForKey:@"_openResultSets"] copy]);
            for (NSValue *rsInWrappedInATastyValueMeal in openSetCopy) {
                FMResultSet *rs = (FMResultSet *)[rsInWrappedInATastyValueMeal pointerValue];
                NSLog(@"query: '%@'", [rs query]);
            }
#endif
        }
    });
    
    FMDBRelease(self);

发现他是用同步的方法进行的操作，这样就避免了多线程锁死的情况，但是确定呢就是会阻塞主线程，所以，有如下解决方法：

• 在inDatabase的外面套了个异步，就不会阻塞主线程了

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:self.filePath];
    for (NSInteger i = 0; i < 1000; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            [queue inDatabase:^(FMDatabase *db) {
                
                [db executeUpdate:@"INSERT INTO myTable (name) VALUES (?)", @"will"];
                [db executeUpdate:@"INSERT INTO myTable (name) VALUES (?)", @"carter"];
                
                FMResultSet *rs = [db executeQuery:@"select name from myTable"];
                while([rs next]) {
                    NSString *obj = [rs stringForColumn:@"name"];
                    NSLog(@"%@", obj);
                }
                
            }];
        });
    }

image.png

image.png

哈哈，就是线程有点多

• 用inTransaction事务方法查询，会比inDatabase速度快些

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:self.filePath];
    [queue inTransaction:^(FMDatabase *db, BOOL *rollback) {
        for (NSInteger i = 0; i < 1000; i ++) {
            BOOL isSuccess = [db executeUpdate:@"INSERT INTO myTable (name) VALUES (?)", @"will"];
            
            if (!isSuccess) {
                *rollback = YES;
                break;
            }else{
                NSLog(@"success");
            }
            
        }
    }];

• 或者拆解查询的数量，慢慢来

3. 前面的FMDatabaseQueue都是用同步的方法来实现，我们自定义一个FMDatabaseAsynQueue，用异步加线程锁的方法尝试看看