How to run multiple task at the same time safely? - objective-c

Question

I'm developing a iPhone app and I'm relatively new to objective-c so I hope some one can give a clue.
What im doing is reading a file in chunks and encoding the chuncks into base64 and everything is working fine, the problem is that in this line NSString *str = [data base64EncodedString]; it takes a little bit of time because im encodeing chunks of 256KB, there is no problem with one file the problem is that i'm encoding image files so imagine that I encode 10 images it will be alot of chunks per image so the process can be slow.
this is the process:
*Get the file.
*Read chunck of 256KB of the file.
*Encode chunck to base64.
*Save the encoded chunck and repeat until there is no more bytes to read from the file.
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
[library assetForURL:referenceURL resultBlock:^(ALAsset *asset)
{
NSUInteger chunkSize =262144;
uint8_t *buffer = calloc(chunkSize, sizeof(*buffer));
ALAssetRepresentation *rep = [asset defaultRepresentation];
NSUInteger length = [rep size];
self.requestsToServer=[[NSMutableArray alloc]init];
NSUInteger offset = 0;
do {
NSUInteger bytesCopied = [rep getBytes:buffer fromOffset:offset length:chunkSize error:nil];
offset += bytesCopied;
NSData *data = [[NSData alloc] initWithBytes:buffer length:bytesCopied];
NSString *str = [data base64EncodedString];
//After this I add the str in a NSMutableURLRequest and I store the request
//in a NSMutableArray for later use.
} while (offset < length);
free(buffer);
buffer = NULL;
}
failureBlock:^(NSError *error)
{
}];
I want to start another thread so I can be encoding the chuncks in paralel and know when the process finish, this way while encoding one chunck I can be encodign another 3 or 4 chunks at the same time.
How I can implement this in a safely way or is this a good idea?
Thanks for your time.

Answer 1

Look at NSOperation and NSOperationQueue.
http://developer.apple.com/library/ios/DOCUMENTATION/Cocoa/Reference/NSOperationQueue_class/Reference/Reference.html
Simply create one NSOperation per chunk and pass them the chunk they need to encode and queue them up.
You can tell the queue how many operations can run simultaneously.

Answer 2

There are lots of good options for doing chunks of work in parallel for iOS.
Take a look at the Apple's Concurrency Programming Guide to get you going.