UIActivityIndicator and GCD/Threading - objective-c

Question

i am fetching a list of photos from a server and displaying them. i'm using GCD to thread the server calls. I got it working, but i want to add in a UIActivityIndicator for each UIImageView to show that its doing something and will appear.
i'm not sure what the best method would be.
code:
UIScrollView *myScrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, [[UIScreen mainScreen] bounds].size.width, photoView.frame.size.height)];
myScrollView.backgroundColor = [UIColor whiteColor];
myScrollView.pagingEnabled = TRUE;
myScrollView.scrollEnabled = TRUE;
myScrollView.frame = CGRectMake(myScrollView.frame.origin.x, myScrollView.frame.origin.y, (6 * THUMBNAIL_SIZE) + (PHOTO_VIEWER_OFFSET_COLUM_1 * 2), myScrollView.frame.size.height);
//get list of photos
for (int i = 0; i < 6; i++)
{
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
NSLog(#"async thread %#", [NSThread currentThread]);
//retrieveThumbnailedGeotaggedPhotoWithPhotoID will make a server call
MyUIImageView *myImageView = [[MyUIImageView alloc] initWithImage:[self retrieveThumbnailedGeoTaggedPhotoWithPhotoID:#"test"]];
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"main thread %#", [NSThread currentThread]);
myImageView.frame = CGRectMake(myImageView.frame.origin.x, myImageView.frame.origin.y, THUMBNAIL_SIZE, THUMBNAIL_SIZE);
myImageView.backgroundColor = [UIColor clearColor];
myImageView.position = i;
myImageView.isEnlarged = FALSE;
myImageView.delegate = self;
int group = (i / 6);
myImageView.frame = CGRectMake((i * myImageView.frame.size.width) + (PHOTO_VIEWER_OFFSET_COLUM_1 * ((group * 2) + 1)),
PHOTO_VIEWER_OFFSET_COLUM_1,
myImageView.frame.size.width,
myImageView.frame.size.height);
myScrollView.contentSize = CGSizeMake((myScrollView.frame.size.width * (group + 1)), myScrollView.contentSize.height);
[myScrollView addSubview:myImageView];
});
});
}
i'm pretty sure animating and stopping the activityIndicator needs to be on the main thread, but not sure how to include it. it needs to be animating during the "retrieveThumbnailedGeoTaggedPhotoWithPhotoID" method which is in a thread (but not main thread).

Answer 1

k, i re-ordered it:
for (int i = 0; i < 8; i++)
{
//create the subviews first
MyUIImageView *myImageView = [[MyUIImageView alloc] initWithFrame:CGRectMake((i * THUMBNAIL_SIZE) + (PHOTO_VIEWER_OFFSET_COLUM_1 * (((i / 6) * 2) + 1)),
PHOTO_VIEWER_OFFSET_COLUM_1,
THUMBNAIL_SIZE,
THUMBNAIL_SIZE)];
myImageView.backgroundColor = [UIColor whiteColor];
[myScrollView addSubview:myImageView];
UIActivityIndicatorView *spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
spinner.center = CGPointMake(THUMBNAIL_SIZE / 2, THUMBNAIL_SIZE / 2);
[myImageView addSubview:spinner];
[spinner startAnimating];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
UIImage *myImage = [[UIImage alloc] init];
//only make the server call in the async queue
myImage = [self retrieveThumbnailedGeoTaggedPhotoWithPhotoID:#"test"];
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"main thread %#", [NSThread currentThread]);
//MyUIImageView *myImageView = [[MyUIImageView alloc] initWithImage:myImage];
[myImageView setImage:myImage];
myImageView.frame = CGRectMake(myImageView.frame.origin.x, myImageView.frame.origin.y, THUMBNAIL_SIZE, THUMBNAIL_SIZE);
myImageView.backgroundColor = [UIColor clearColor];
myImageView.position = i;
myImageView.isEnlarged = FALSE;
myImageView.delegate = self;
int group = (i / 6);
myImageView.frame = CGRectMake((i * myImageView.frame.size.width) + (PHOTO_VIEWER_OFFSET_COLUM_1 * ((group * 2) + 1)),
PHOTO_VIEWER_OFFSET_COLUM_1,
myImageView.frame.size.width,
myImageView.frame.size.height);
myScrollView.contentSize = CGSizeMake((myScrollView.frame.size.width * (group + 1)), myScrollView.contentSize.height);
[spinner stopAnimating];
});
});
}