ios - what is the best way to download large amount of images from server to the ipad? -

we developing enterprise app our company.the app supposed work offline, means need download more 10k product images ipad.each image can several kb 4m.for doing @ first used following code download images:

-(bool)downloadfileinbatch :(nsarray*) filearr {     nsarray *paths = nssearchpathfordirectoriesindomains(nsdocumentdirectory, nsuserdomainmask, yes);      nsstring *documentdirectory = [paths objectatindex:0];      nsoperationqueue *queue = [[nsoperationqueue alloc] init];     queue.maxconcurrentoperationcount = 4;      nsblockoperation *completionoperation = [nsblockoperation blockoperationwithblock:^{         [[nsoperationqueue mainqueue] addoperationwithblock:^{             [[nsnotificationcenter defaultcenter] postnotificationname:@"taskdone" object:self];         }];     }];      @autoreleasepool     {         nsblockoperation *operation;          for(int = 0; < filearr.count; i++)         {              operation = [nsblockoperation blockoperationwithblock:^{                  nsurl *url = [[nsurl alloc] initwithstring:[[filearr objectatindex:i] objectforkey:@"image_url"]];                  nsdata *data = [nsdata datawithcontentsofurl:url];                  nsstring *filename = [documentdirectory stringbyappendingstring:[nsstring stringwithformat:@"/%@", [url lastpathcomponent]]];                  nslog(@"downloading file : %@ : no : %d", [documentdirectory stringbyappendingstring:[nsstring stringwithformat:@"/%@", [url lastpathcomponent]]], i);                  bool success = [data writetofile:filename atomically:yes];                  if(!success)                 {                    // failed download                 }             }];              [completionoperation adddependency:operation];         }     }      [queue addoperations:completionoperation.dependencies waituntilfinished:no];     [queue addoperation:completionoperation];      return yes; } 

but when using code download, app crash due memory problem.(even putting @autoreleasepool did not solve crashing problem). seems downloading one-by-one (or 4 images parallel)is not suitable option amount of data since cause memory problem.i want know best way download amount of images server ipad?

thanks in advance...