Access already created view controllers in a UIPageViewController - ios

Question

I'm using a UIPageViewController for displaying images, and I download the images asynchronously and throw them into the gallery when they're ready (but you're still able to go to the page in the gallery where the image will be).
My issue is that, when the image does load, I need to supply it for the view controller it corresponds to. Right now, I add it to an NSCache instance, and when UIPageViewController's data source method viewControllerAfterViewController: is called, I check if the image for the view controller being requested has already been downloaded (is in the cache) and if it is, I call initWithImage: on the specific view controller, passing the downloaded image.
My issue is with when viewControllerAfterViewController: is called and the image hasn't been downloaded yet. Right now I just load the view controller without the image, and when the imageDidFinishDownloading: delegate method is called, I try to supply the view controller with the image.
However, it seems that even though UIPageViewController asks for the next view controller the previous gets displayed (i.e.: when I get to the 3rd page, it requests the 4th page's view controller) I'm not able to access this requested view controller that I hand off.
If I access pageViewController.viewControllers, the count of the NSArray is never more than 1. So even though it seems like it should have 2 (the currently showing view controller and the next one that it requested), it only ever has one, the currently visible view controller.
The problem is that I need the other one. The image finishes downloading, and I really only have two options.
Give it to my NSCache, so when the init method is called for the view controller, it is handed off.
In the event that it has already been inited, hand it off to the view controller that's already been inited.
But 1 doesn't always work, as sometimes the init method is called when there's no image yet (it hasn't finished downloading), so we'd sometimes depend on 2, but pageViewController.viewControllers only holds the current visible view controller.
So there's basically this third situation where the view controller has already been inited without an image, and the image finishes downloading and I want to assign it to that view controller, but I'm currently on the one before it, and my only reference to the view controllers in the UIPageViewController is on the current visible one.
How do I handle 3? I need to assign something to a view controller but I can't find any way to access that view controller.

Answer 1

I'd use something like a future from PMConcurrency to do this. A future is just an object that promises to return your image as soon as it's available. So by giving the future to your view controller instead of the image, you don't need to worry about when the image arrives.
Your view controller's initializer would look something like this:
- initWithImageFuture:(PMFuture *)future
{
if (self = [super init]) {
[[future onMainThread] onComplete:^(id result, NSError *error) {
if (!error) {
_image = result;
}
}];
}
return self;
}
By adding an onComplete block to the future, the view controller will receive the image as soon as it's available (immediately if the image is already downloaded). Futures run in the background by default, so you'd use onMainThread to have the result returned on the main thread.
So instead of populating your cache with images, you'd populate it with image futures. There are several ways to create futures, but the block-based one is pretty simple:
PMFuture *imageFuture = [PMFuture futureWithBlock:^id{
UIImage *image = ...; // Your image fetching code here
return image;
}];
[myCache setObject:imageFuture forKey:myImageKey];

Answer 2

I have a similar sort of function but I load the image in the view controller that is displaying the image (the VC handed to the UIPageViewController). In your context I guess I'd had the image ID info and the NSCache object to the VC. That isn't what you want to do of course, but just something to think about.
Originally I tried to handle image loading in the parent like you are. As I recall, I kept a weak pointer to the target view controller. When the image arrived if the VC was still alive I could set the UIImageView property.