objective C what should I do for making these function threadsafe? - objective-c

Question

Here is the case.
I got a singleton class call DBManager with sqlite3. And insert,delete,select are done via this class.
suppose now ClassA need to insert a record to table a and ClassB need to select a record from table a. So the presudo code is like this.
Class A
[[DBManager getInstance] insertRecord:#"12345"];
Class B
-(void)processData{
Record r = [[DBManager getInstance] getRecord];
// working with record r. after that doing some layout update
[self.lbTitle setText:r.name];
}
cause in my project, there are 4-5 class will access the db. So sometime a Database is locked error is throw and my app crashed.
I know one of the solution is using GCD dispatch_async(CUSTOM_DB_QUEUE,block)
using these block for all the codes related to the db.
But if so, i need to also modify Class B like this. Am I right?
Class B
-(void)processData{
dispatch_async(CUSTOM_DB_QUEUE,{
[[DBManager getInstance] getRecord onRecordGot:^(Record* r) {
// working with record r. after that doing some layout update
dispatch_async(MAIN_QUEUE,{
[self.lbTitle setText:r.name];
)};
}];
)};
}
Is this approach correct?? so i need to change all my code in DBManager to return the result using block coding??
Is there any simple way for queue up the hold DBManager class??

Answer 1

There's a better approach. Do not make any changes to your code that calls DBManager. All changes should be made inside your DBManager class. And there is no reason to use dispatch_async. Use dispatch_sync.
This code stays the same:
Class A
[[DBManager getInstance] insertRecord:#"12345"];
Class B
-(void)processData{
Record r = [[DBManager getInstance] getRecord];
// working with record r. after that doing some layout update
[self.lbTitle setText:r.name];
}
In your insertRecord: method you change it to something like:
- (void)insertRecord:(NSString *)record {
dispatch__barrier_sync(CUSTOM_DB_QUEUE, ^{
// original code to insert record
};
}
And your getRecord method becomes:
- (Record *)getRecord {
__block Record *result = nil;
dispatch_sync(CUSTOM_DB_QUEUE, ^{
// original code that sets result
};
return result;
}
By using dispatch_sync and dispatch_barrier_sync, this code allows for any number of concurrent calls to getRecord but only one call to insertRecord. It also ensures calls to getRecord are blocked while insertRecord: is being run.

Answer 2

To make it thread safe, you can use pthread_mutex_t as a global instance. Lock/unlock this mutex according to its usage
e.g.
pthread_mutex_t *dbLock;
Class A
pthread_mutex_lock(dbLock);
[[DBManager getInstance] insertRecord:#"12345"];
pthread_mutex_unlock(dbLock);
Class B
-(void)processData{
pthread_mutex_lock(dbLock);
Record r = [[DBManager getInstance] getRecord];
// working with record r. after that doing some layout update
[self.lbTitle setText:r.name];
pthread_mutex_unlock(dbLock);
}