dispatch_get_main_queue() not running perfect well - ios

Question

I am Newbie in iOS Development. I have no knowledge about dispatch_get_main_queue() so I want to get image size from my Server image url like as
First I parse my JSON Data and Get image Size like as
[self.feedArray addObjectsFromArray:[pNotification.userInfo valueForKey:#"items"]];
[self fillHeightArray];
Here I set parse data in my self.feedArray and after that I get Height like as
-(void)fillHeightArray
{
NSMutableArray *requestArray=[[NSMutableArray alloc]init];
NSMutableArray *dataArray=[[NSMutableArray alloc]init];
for (int i=0; i<[self.feedArray count];i++)
{
NSString *urlString = [[self.feedArray objectAtIndex:i]valueForKey:#"photo"];
NSURL *imageFileURL = [NSURL URLWithString:urlString];
NSURLRequest *urlRequest = [NSURLRequest requestWithURL:imageFileURL];
[requestArray addObject:urlRequest];
}
dispatch_queue_t callerQueue = dispatch_get_main_queue();
dispatch_queue_t downloadQueue = dispatch_queue_create("Lots of requests", NULL);
dispatch_async(downloadQueue, ^{
for (NSURLRequest *request in requestArray) {
[dataArray addObject:[NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil]];
}
dispatch_async(callerQueue, ^{
for (int i=0; i<[dataArray count]; i++)
{
UIImage *imagemain=[UIImage imageWithData:[dataArray objectAtIndex:i]];
UIImage *compimage =[self resizeImage:imagemain resizeSize:CGSizeMake(screenWidth/2-16,180)];
CGSize size = CGSizeMake(screenWidth/2-16,compimage.size.height);
[self.cellHeights addObject:[NSValue valueWithCGSize:size]];
}
[GlobalClass StopSpinner:self.view];
self.cltItem.hidden=FALSE;
[self.cltItem reloadData];
[self.cltItem.collectionViewLayout invalidateLayout];
[[NSUserDefaults standardUserDefaults]setValue:#"1" forKey:Loading];
});
});
}
And resize my image like as
-(UIImage *)resizeImage:(UIImage *)orginalImage resizeSize:(CGSize)size
{
float oldWidth = orginalImage.size.width;
float scaleFactor = size.width / oldWidth;
float newHeight = orginalImage.size.height * scaleFactor;
float newWidth = oldWidth * scaleFactor;
UIGraphicsBeginImageContext(CGSizeMake(newWidth, newHeight));
[orginalImage drawInRect:CGRectMake(0,0,newWidth,newHeight)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}
So, from this code i got fine result at first time but when i want to load more data so this code are Running second time then i got not valid image size
I don't understand what the issue is there, but I think my
dispatch_queue_t callerQueue = dispatch_get_main_queue();
dispatch_queue_t downloadQueue = dispatch_queue_create("Lots of requests", NULL);
Have issue when I load more data.
Please help me for that.

Answer 1

You are always adding objects in this line:
[self.cellHeights addObject:[NSValue valueWithCGSize:size]];
When you run the code a second time, the array gets bigger, and the old values are still present in its beginning. This is probably giving you bad results when running the code a second time.
EDIT:
It might work slower, because you've made some retain cycles / have memory leaks. In this scenario, it will work ok the first time, and slower for each extra run. I do not really see anything wrong with your code, besides the self.cellHeights table growing. Check the rest of the procedure for elements that are getting bigger every time, and ensure that the objects that are not going to be used anymore are getting released.
Also, try using the 'build & analyze' [ALT + CMD + B]. This might point you to some memory leaks, or other issues.
Profiling tools are also very efficient in localizing leaks, and you can access them with [CMD + I] on the keyboard.
Another thing you can try is calling the main_queue directly, like:
dispatch_async(dispatch_get_main_queue(), ^(void) {
//do sth
});
You will avoid creating another object, and you only use the main_queue once in the whole snippet anyway.
Try doing this and let me know if you got anything.