Uiscrollview lazy loading

I've been working on this lately and have checked many code examples on the internet. None of this made lazy loading (despite the demand) or was much more difficult (useless calls and whistles) than I was willing to put up with. Apple WWDC videos for PhotoPicker seem to show lazy loading, but seem to be more focused on splitting pre-cut images, so not much help.

What I ended up with was to load everything at aspectThumbnails

once, since they are small and don't take up as much memory, and then load the view fullScreenImage

on demand from scrollViewDidEndDecelerating

and reload aspectThumbnail

for the off-screen image. The effect is very fluid and visually, the image gets a little rough and then quickly (via loading the background) is replaced with a higher resolution.

Perhaps it could use more refinements - perhaps loading full resolution images for the current page +1 and -1. I didn’t know it yet. Also, I'm not entirely sure how optimal I am using blocks, but I get no errors.

I am calling this from Segue and setting assetsArray

(NSArray from ALAssets) from a method prepareForSegue:

on my root viewController.

//  ScrollViewController.h
#import <UIKit/UIKit.h>
#import <AssetsLibrary/AssetsLibrary.h>

@interface ScrollViewController : UIViewController <UIScrollViewDelegate>

@property (weak, nonatomic) IBOutlet UIScrollView *scrollView;
@property (strong, atomic) ALAssetsLibrary* assetLibrary;
@property (nonatomic,strong)NSMutableArray* assetsArray;
@end

      

        
        
        
      

    

//ScrollViewController.m

#import "ScrollViewController.h"

@interface ScrollViewController ()

@property (nonatomic,assign) CGSize currentImageSize;
@property (nonatomic, assign)int currentPages;
@property (nonatomic, assign)int currentPageNum;
@property (nonatomic, strong)UIImage* placeHolder;
@end



@implementation ScrollViewController


- (void)viewDidLoad
{
    [super viewDidLoad];

    //self.placeHolder = [UIImage imageNamed:@"loader.jpg"];
    self.scrollView.delegate = self;
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    if (self.assetsArray != nil) {
        self.currentPages = [self.assetsArray count];

        CGSize size = self.scrollView.frame.size;
        int num = self.currentPages;

        self.scrollView.contentSize=CGSizeMake(size.width*num, size.height);

        [self loadThumbnails];

        self.currentPageNum = 0;
        [self loadFullScreenImageByIndex:self.currentPageNum];
    }
}

-(void)loadThumbnails
{
    int pageCount = self.currentPages;
    CGSize size = self.scrollView.frame.size;
    self.scrollView.contentSize=CGSizeMake(size.width*pageCount, size.height);

    for (int i = 0; i < pageCount; i++) {

        ALAsset *asset = [self.assetsArray objectAtIndex:i];//
        CGRect imageViewFrame;

        // x offset is determined by arrayIndex
        imageViewFrame.origin.x = self.scrollView.frame.size.width * i;
        imageViewFrame.origin.y = 0;
        imageViewFrame.size = self.scrollView.frame.size;

        self.currentImageSize = imageViewFrame.size; // THIS IS WRONG

        UIImage *image = [[UIImage alloc] initWithCGImage:asset.aspectRatioThumbnail];
        UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
        imageView.clipsToBounds = YES;
        imageView.contentMode = UIViewContentModeScaleAspectFill;
        imageView.frame = imageViewFrame;
        imageView.tag = i+1;// start tags at 1

        [self.scrollView addSubview:imageView];
    }
}


- (void)viewDidUnload {
    [self setScrollView:nil];
    [super viewDidUnload];
}

- (void)loadFullScreenImageByIndex:(int)index
{
    int arrayIndex = index;
    int tagNumber = index+1;
    ALAsset *asset = [self.assetsArray objectAtIndex:arrayIndex];

    __weak typeof(self) weakSelf = self;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        UIImage *tmpImage = [[UIImage alloc] initWithCGImage:asset.defaultRepresentation.fullScreenImage];

        __strong __typeof__(weakSelf) strongSelf = weakSelf;
        if ([strongSelf.scrollView viewWithTag:tagNumber] != nil){

            dispatch_async(dispatch_get_main_queue(), ^{
                __strong __typeof__(weakSelf) strongSelf = weakSelf;
                if ([strongSelf.scrollView viewWithTag:tagNumber]!= nil){
                    UIImageView * tmpImageView = (UIImageView*)[strongSelf.scrollView viewWithTag:tagNumber];
                    tmpImageView.image = tmpImage;
                }
            });
        }
    });
}

- (void)loadThumbnailImageByIndex:(int)index
{
    int arrayIndex = index;
    int tagNumber = index+1;

    ALAsset *asset = [self.assetsArray objectAtIndex:arrayIndex];//

    __weak typeof(self) weakSelf = self;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        UIImage *tmpImage = [[UIImage alloc] initWithCGImage:asset.aspectRatioThumbnail];        
        __strong __typeof__(weakSelf) strongSelf = weakSelf;
        if ([strongSelf.scrollView viewWithTag:tagNumber] != nil){

            dispatch_async(dispatch_get_main_queue(), ^{
                __strong __typeof__(weakSelf) strongSelf = weakSelf;
                if ([strongSelf.scrollView viewWithTag:tagNumber]!= nil){
                    UIImageView * tmpImageView = (UIImageView*)[strongSelf.scrollView viewWithTag:tagNumber];
                    tmpImageView.image = tmpImage;
                }
            });
        }
    });
}

- (void)manageImages
{
    int currentPage = (self.scrollView.contentOffset.x / self.currentImageSize.width);

    if (currentPage != self.currentPageNum){
        [self loadThumbnailImageByIndex:self.currentPageNum]; //pg no longer visible so load thumb
        [self loadFullScreenImageByIndex:currentPage]; // load full
        self.currentPageNum = currentPage;// store
    }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    [self manageImages];
}