The old contents of a reused UICollectionViewCell are briefly displayed when reusing cells in a UICollectionView

Question

I am using a UICollectionView to display a grid of pictures that are asynchronously loaded from URLs. My collection view uses reusable cells to display the UICollectionViewCells. Everything is displayed properly when cells are not reused, but when I scroll around a bit, the reused cells briefly flash the old contents before they begin their normal behavior.

Here is the implementation of the custom UICollectionViewController:

#import "MyCollectionViewViewController.h"
#import "MyCollectionViewCell.h"

@interface MyCollectionViewViewController ()    
@property (strong, nonatomic) NSArray *data;    
@end

@implementation MyCollectionViewViewController

@synthesize data = _data;

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.data = @[
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr01/2013/1/18/12/enhanced-buzz-wide-851-1358529670-4.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr03/2013/1/18/11/enhanced-buzz-wide-26311-1358526816-5.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr03/2013/1/18/11/enhanced-buzz-wide-26311-1358527190-11.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr02/2013/1/18/11/enhanced-buzz-wide-7517-1358526694-11.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr03/2013/1/18/11/enhanced-buzz-wide-1965-1358527802-7.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr03/2013/1/18/11/enhanced-buzz-wide-2165-1358527708-14.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr03/2013/1/18/11/enhanced-buzz-wide-1965-1358527894-12.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr02/2013/1/18/12/enhanced-buzz-wide-15957-1358529198-18.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr02/2013/1/18/12/enhanced-buzz-wide-16520-1358528981-9.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr02/2013/1/18/12/enhanced-buzz-wide-16517-1358529292-5.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr01/2013/1/18/12/enhanced-buzz-wide-20349-1358529323-20.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr01/2013/1/18/12/enhanced-buzz-wide-32444-1358529959-9.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr01/2013/1/18/12/enhanced-buzz-wide-32343-1358530043-7.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr02/2013/1/18/12/enhanced-buzz-wide-23646-1358530321-2.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr01/2013/1/18/12/enhanced-buzz-wide-28223-1358530801-15.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr01/2013/1/18/12/enhanced-buzz-wide-32273-1358530695-16.jpg",
        @"http://s3-ec.buzzfed.com/static/enhanced/webdr01/2013/1/18/12/enhanced-buzz-wide-31288-1358531103-16.jpg"
    ];

    [self.collectionView registerNib:[UINib nibWithNibName:@"MyCollectionViewCell" bundle:[NSBundle mainBundle]] forCellWithReuseIdentifier:@"myView"];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    NSString *urlString = [self.data objectAtIndex:indexPath.item];

    MyCollectionViewCell *myView = [collectionView dequeueReusableCellWithReuseIdentifier:@"myView" forIndexPath:indexPath];
    myView.urlString = urlString;

    [myView setNeedsDisplay];

    return myView;
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    return CGSizeMake(150, 150);
}

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
    return 1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    return self.data.count;
}

@end

Here is the implementation of the custom UICollectionViewCell.

#import "MyCollectionViewCell.h"
#import <QuartzCore/QuartzCore.h>
#import <AVFoundation/AVFoundation.h>

@interface MyCollectionViewCell ()

@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@end

@implementation MyCollectionViewCell

@synthesize urlString = _urlString;

+ (void)loadAsyncImageFromURL:(NSString *)urlString withCallback:(void (^)(UIImage *))callback {
    NSURL *url = [NSURL URLWithString:urlString];

    dispatch_queue_t downloadQueue = dispatch_queue_create("com.example.downloadqueue", NULL);
    dispatch_async(downloadQueue, ^{
        NSData * imageData = [NSData dataWithContentsOfURL:url];
        UIImage *image = [UIImage imageWithData:imageData];

        dispatch_async(dispatch_get_main_queue(), ^{
            callback(image);
        });
    });
}

-(void) fadeInView:(UIView *)view WithSecondDuration:(double)duration {
    CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"opacity"];
    animation.beginTime = 0;
    animation.duration = duration;
    animation.fromValue = [NSNumber numberWithFloat:0.0f];
    animation.toValue = [NSNumber numberWithFloat:1.0f];
    animation.removedOnCompletion = NO;
    animation.fillMode = kCAFillModeBoth;
    animation.additive = NO;
    [view.layer addAnimation:animation forKey:@"opacityIN"];
}

- (void) setUrlString:(NSString *)urlString {
    _urlString = urlString;

    self.imageView.image = nil;
    self.imageView.layer.opacity = 0;

    if (urlString) {
        [MyCollectionViewCell loadAsyncImageFromURL:urlString withCallback:^(UIImage *image) {
            self.imageView.image = image;
            [self fadeInView:self.imageView WithSecondDuration:.25];
        }];
    }
}


@end

Answer 1

You could implement -(void)prepareForReuse in your UICollectionViewCell subclass. and do your resetting there.

Answer 2

You could try setting the URL for the cell to nil when the cell is finished displaying, it worked on my project.

-(void)collectionView:(UICollectionView *)cv didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath
{
    [(MyCollectionViewCell *)cell setUrlString:nil];
}

I hope this helps!