Progress Bar updates out of synch with image downloads - ios

Question

I have an iPad app where I download a list of images and a progress bar updates as the images are downloaded. I can't seem to get the progress bar and image downloads to match up. The progress bar always finishes before the image downloads are completed. I have a method UpdateProgressBar that should increment the progress bar every time an image is downloaded.
-(void)DownloadPhoto{
NSMutableArray *failedDownloads = [[NSMutableArray alloc]init];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
listPhoto = [CoreDataRead GetPhotoList:[self selectedAccountGuid]];
dispatch_group_t downloadGroup = dispatch_group_create();
for (Photo *item in listPhoto) {
NSString *imageName = item.photoName;
NSString *myURL = [NSString stringWithFormat:#"%#%#", #"http://acimagedownload.com/photos/", imageName];
NSURL *url = [NSURL URLWithString:myURL];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
dispatch_group_enter(downloadGroup);
[NSURLConnection sendAsynchronousRequest:request queue:[[NSOperationQueue alloc] init] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
if (connectionError == nil && data != nil)
{
if (data != nil)
{
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString* path = [documentsDirectory stringByAppendingPathComponent: [NSString stringWithFormat:#"%#%#",item.guid, #".png"]];
[data writeToFile:path atomically:YES];
NSLog(#"Photo Downloaded %#!", #"");
}
else
{
NSLog(#"image is not downloaded");
}
}
else if (connectionError != nil)
{
[failedDownloads addObject:myURL];
NSLog(#"Error %#",[connectionError description]);
}
else
{
[failedDownloads addObject:myURL];
NSLog(#"Image Download Failed %#!", #"");
}
dispatch_group_leave(downloadGroup);
dispatch_async(dispatch_get_main_queue(), ^{
[self UpdateProgressBar];
});
}];
dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER);
}
dispatch_async(dispatch_get_main_queue(), ^{
[self DownloadVideo];
});
});
}
-(void)UpdateProgressBar{
currentTask = currentTask + 1;
NSLog(#"Current Task %#!", [#(currentTask) stringValue]);
float progressPercentage = (float)currentTask/(float)taskCount;
[self.progressBar setProgress:progressPercentage animated:YES];
if(currentTask == taskCount){
[self ShowDoneAlert];
}
}

Answer 1

It's hard to tell without seeing the updateProgressBar method why you get that behaviour but one thing that I noticed is that your dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER); is inside your loop.
So what happens is, you go into the first iteration of the loop and at the end of it, the thread execution stops and waits for the first dispatch_group_leave. After that is continues to the next iteration and so on.
If this is the desired behaviour (but I doubt it, I think you actually want your downloads to run in parallel) you should probably use a dispatch_semaphore_t or even a serial queue.