I ran this about 10 times and it did not print in order once:
NSInteger iterations = 10;
dispatch_apply(iterations, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long) NULL), ^(size_t index) {
NSLog(@"%zu", index);
});
My suggestion, and what I have done in the past, is to just run a for loop inside the block of a background thread:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long) NULL);
dispatch_async(queue, ^{
int iterations = 0;
for (int x = 0; x < iterations; ++x) {
// Do stuff here
}
dispatch_async(dispatch_get_main_queue(), ^{
// Set content generated on background thread to main thread
});
});
When using background threads, it is important to make sure that either the the objects you initialized on the main thread are thread-safe or that you initialize objects in the background and then set the main thread's objects with the background-thread-created objects like in the above example. This is especially true with Core Data.
EDIT:
Seems like the iterations using dispatch_apply return immediately so they will probably execute out of order when doing anything meaningful. If you run these two, you will see that printf
always runs in order but NSLog
does not:
NSInteger iterations = 10;
dispatch_apply(iterations, the_queue, ^(size_t idx) {
printf("%zu\n", idx);
});
dispatch_apply(iterations, the_queue, ^(size_t idx) {
NSLog(@"%zu\n", idx);
});
In my opinion, it would be best to run a standard for statement in a background thread rather than dispatch_apply if the order is important.
EDIT 2:
This would be your implementation:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long) NULL);
dispatch_async(queue, ^{
int iterations = 0;
NSMutableArray *backgroundThreadImages = [NSMutableArray array];
for (int i = 0; i < iterations; ++i) {
NSString *strURL = [NSString stringWithFormat:@"%@%i%@", @"http://theURL.com/popular/", i, @".jpg"];
NSURL *imageURL = [NSURL URLWithString:strURL];
NSData *imageData = [NSData dataWithContentsOfURL: imageURL];
UIImage *oneImage =[UIImage imageWithData:imageData];
if(oneImage!=nil){
[backgroundThreadImages addObject:oneImage];
}
}
dispatch_async(dispatch_get_main_queue(), ^{
self.imageArray = backgroundThreadImages;
});
});