how can I reduce time complexity from picking images from array - ios

Question

That is how I have my data:
(
{
image = "https://www.myurl/abc- a/uploads/1161570652.jpg";
}
{
image = "https://www.myurl/abc- a/uploads/1161570652.jpg";
}
{
image = "https://www.myurl/abc- a/uploads/1161570652.jpg";
}
{
image = "https://www.myurl/abc- a/uploads/11615720652.jpg";
}
{
image = "https://www.myurl/abc- a/uploads/11615730652.jpg";
}
)
I could retrieve it successfully. But the problem is it is very slow.
Here is my code:
seeview=[[UIImageView alloc]init];
for (int k=0; k<[dictofimage count]; k++)
{
img = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[image_array objectAtIndex:k]]]];
NSLog(#"image name is 0 %#",img);
[myimageview setImage:img];
}
It takes time to pick every element and is slow in progress. Is there any faster way to do it?

Answer 1

You can't get away from looping through all of the contents of image_array so you're stuck with O(n) but you can use a faster construct. The for:in loop is much faster than a typical C style loop, so something like:
for (imageString in image_array) {
img = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:imageString]]];
NSLog(#"image name is 0 %#",img);
[myimageview setImage:img];
}
For a breakdown of the benchmarks check out: http://iosdevelopertips.com/objective-c/high-performance-collection-looping-objective-c.html
EDIT: I should also note that dataWithContentsOfURL is a synchronous call that you probably shouldn't be making in this loop. You'd be better off creating an NSOperation that downloads the image, and when it finishes you can dispatch back to the main queue to set the actual image in the imageView.

Answer 2

You are better off handling the array of objects concurrently, in the background. This is precisely what dispatch_apply is for.
dispatch_apply(
image_array.count,
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),
^(size_t index) {
NSURL *url = [NSURL URLWithString:[image_array objectAtIndex:index]];
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];
dispatch_async(dispatch_get_main_queue(), ^{
// Handle the image on the main thread for UI elements
});
});
You only need the dispatch_async if you need to handle the image on the main thread as part of a UI change.
Note, that dispatch_apply is synchronous, in that it kicks off one background task for each iteration. Those tasks run in the background, but dispatch_apply will not return until they are all done. Thus, you may also want to wrap that in an async block.
Or, you can do it yourself, and get notified when all the blocks are complete...
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
for (size_t i = 0; i < image_array.count; ++i) {
dispatch_group_async(group, queue, ^{
NSURL *url = [NSURL URLWithString:[image_array objectAtIndex:index]];
// The rest is the same...
});
}
dispatch_group_notify(group, queue, ^{
// This code will execute on `queue` when all the above blocks have finished
});
Note, however, that dispatch_apply has a nice algorithm to not overload the system. The above approach is not so kind.
However, you can get dispatch_apply nice-ness using an async call.
dispatch_async(queue, ^{
dispatch_apply(image_array.count, queue, ^(size_t index) {
NSURL *url = [NSURL URLWithString:[image_array objectAtIndex:index]];
// The rest is the same...
});
// This code will execute on `queue` when all the above blocks have finished
});