Ensuring Run Order of FOR Loop with a GCD Thread ? iOS - ios

Question

I have a serial queue which contains two methods which load and image and then, once completed, add the image to the subview. The images are in a NSMutableArray so I am iterating over a For loop to load them in as follows :
dispatch_queue_t queue = dispatch_queue_create("com.example.MyQueue", NULL);
for (int i =0; i<=[pictureThumbnailArray count]-1; i++) {
dispatch_async(queue, ^{
NSLog(#"Thumbnail count is %d", [pictureThumbnailArray count]);
finishedImage = [self setImage:[pictureThumbnailArray objectAtIndex:i]:i];
if (finishedImage !=nil) {
dispatch_async(dispatch_get_main_queue(), ^ {
[self.view addSubview:finishedImage];
});
}
});
}
The problem is that the images seem to be loaded randomly. What I want to achieve is that each iteration of the For loop runs and completes before the next iteration starts - that way the images should load in the same way each time.
Can anyone suggest the best way to achieve this - I am thinking I may need to synchronise the setImage method (first method in queue) ?
Changed To :
for (int i =0; i<=[pictureThumbnailArray count]-1; i++) {
NSLog(#"Thumbnail count is %d", [pictureThumbnailArray count]);
finishedImage = [self setImage:[pictureThumbnailArray objectAtIndex:i]:i];
if (finishedImage !=nil) {
dispatch_async(dispatch_get_main_queue(), ^ {
[self.view addSubview:finishedImage];
});
}
}
});

Answer 1

You have some other problem - perhaps your image array is not in the order you think it is. Both queue and mainQueue are serial queues. To verify this I just did a quick test and got the log messages in the expected order. I suggest you try adding log messages or so to figure out why the order is not as you expect:
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
static dispatch_queue_t queue;
queue = dispatch_queue_create("com.example.MyQueue", NULL);
for (int i =0; i<=20; i++) {
dispatch_async(queue, ^{
dispatch_async(dispatch_get_main_queue(), ^ {
NSLog(#"Image %d", i);
});
} );
}
}

Answer 2

What if we do the things simpler without GCD? I suggest get rid of it and use NSURLConnectionDelegate methods.
This method downloads next image:
-(void)startDownload
{
if (index < URLs.count)
{
NSURL *URL = [NSURL URLWithString:URLs[index]];
_connection = [[NSURLConnection alloc] initWithRequest:[NSURLRequest requestWithURL:URL] delegate:self];
}
}
The connectionDidFinishLoading: delegate method places the image to the view and starts next download.
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
UIImage *image = [UIImage imageWithData:_data];
_data = nil;
_connection = nil;
UIImageView *imageView = (UIImageView *)[self.view viewWithTag:100+index];
imageView.image = image;
index++;
[self startDownload];
}
Here is the complete example: https://github.com/obrizan/TestImageDownload The images rather big so give some time to load them.