UICollection View Scroll backlog with SDWebImage

Question

Background

I searched around SO and the apple forum. Quite a lot of people talked about the performance of a collection view cell with an image. Most of them said that it has lagged behind the scroll since loading the image into the main stream.

Using SDWebImage , images must be uploaded to a separate stream. However, it lags only in landscape mode in the iPad simulator.

Description of the problem

In portrait mode, the collection collects 3 cells for each row. And he has no lag or slight delay. In landscape mode, viewing a collection loads 4 cells for each row. And it has an obvious lag and drop in frame rate.

I checked toolbars with active animations. When a new cell appears, the frame rate decreases to about 8 frames per second. I'm not sure what action brings me such low performance for the collection.

Hope someone knows some of the tricks.

Here is the reference code

In the view controller

 - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath { ProductCollectionViewCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"ProductViewCell" forIndexPath:indexPath]; Product *tmpProduct = (Product*)_ploader.loadedProduct[indexPath.row]; cell.product = tmpProduct; if (cellShouldAnimate) { cell.alpha = 0.0; [UIView animateWithDuration:0.2 delay:0 options:(UIViewAnimationOptionCurveLinear | UIViewAnimationOptionAllowUserInteraction) animations:^{ cell.alpha = 1.0; } completion:nil]; } if(indexPath.row >= _ploader.loadedProduct.count - ceil((LIMIT_COUNT * 0.3))) { [_ploader loadProductsWithCompleteBlock:^(NSError *error){ if (nil == error) { cellShouldAnimate = NO; [_collectionView reloadData]; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{ cellShouldAnimate = YES; }); } else if (error.code != 1){ #ifdef DEBUG_MODE ULog(@"Error.des : %@", error.description); #else CustomAlertView *alertView = [[CustomAlertView alloc] initWithTitle:@"Connection Error" message:@"Please retry." buttonTitles:@[@"OK"]]; [alertView show]; #endif } }]; } return cell; } 

PrepareForReuse in the ViewCell Collection

 - (void)prepareForReuse { [super prepareForReuse]; CGRect bounds = self.bounds; [_thumbnailImgView sd_cancelCurrentImageLoad]; CGFloat labelsTotalHeight = bounds.size.height - _thumbnailImgView.frame.size.height; CGFloat brandToImageOffset = 2.0; if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) { brandToImageOffset = 53.0; } CGFloat labelStartY = _thumbnailImgView.frame.size.height + _thumbnailImgView.frame.origin.y + brandToImageOffset; CGFloat nameLblHeight = labelsTotalHeight * 0.46; CGFloat priceLblHeight = labelsTotalHeight * 0.18; _brandLbl.frame = (CGRect){{15, labelStartY}, {bounds.size.width - 30, nameLblHeight}}; CGFloat priceToNameOffset = 8.0; if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) { priceToNameOffset = 18.0; } _priceLbl.frame = (CGRect){{5, labelStartY + nameLblHeight - priceToNameOffset}, {bounds.size.width-10, priceLblHeight}}; [_spinner stopAnimating]; [_spinner removeFromSuperview]; _spinner = nil; } 

Override the setProduct method

 - (void)setProduct:(Product *)product { _product = product; _spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray]; _spinner.center = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds)); [self addSubview:_spinner]; [_spinner startAnimating]; _spinner.hidesWhenStopped = YES; // Add a spinner __block UIActivityIndicatorView *tmpSpinner = _spinner; __block UIImageView *tmpImgView = _thumbnailImgView; ProductImage *thumbnailImage = _product.images[0]; [_thumbnailImgView sd_setImageWithURL:[NSURL URLWithString:thumbnailImage.mediumURL] completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) { // dismiss the spinner [tmpSpinner stopAnimating]; [tmpSpinner removeFromSuperview]; tmpSpinner = nil; if (nil == error) { // Resize the incoming images dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ CGFloat imageHeight = image.size.height; CGFloat imageWidth = image.size.width; CGSize newSize = tmpImgView.bounds.size; CGFloat scaleFactor = newSize.width / imageWidth; newSize.height = imageHeight * scaleFactor; UIGraphicsBeginImageContextWithOptions(newSize, NO, 0.0); [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)]; UIImage *small = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); dispatch_async(dispatch_get_main_queue(),^{ tmpImgView.image = small; }); }); if (cacheType == SDImageCacheTypeNone) { tmpImgView.alpha = 0.0; [UIView animateWithDuration:0.2 delay:0 options:(UIViewAnimationOptionCurveLinear | UIViewAnimationOptionAllowUserInteraction) animations:^{ tmpImgView.alpha = 1.0; } completion:nil]; } } else { // loading error [tmpImgView setImage:[UIImage imageNamed:@"broken_image_small"]]; } }]; _brandLbl.text = [_product.brand.name uppercaseString]; _nameLbl.text = _product.name; [_nameLbl sizeToFit]; // Format the price NSNumberFormatter * floatFormatter = [[NSNumberFormatter alloc] init]; [floatFormatter setNumberStyle:NSNumberFormatterDecimalStyle]; [floatFormatter setDecimalSeparator:@"."]; [floatFormatter setMaximumFractionDigits:2]; [floatFormatter setMinimumFractionDigits:0]; [floatFormatter setGroupingSeparator:@","]; _priceLbl.text = [NSString stringWithFormat:@"$%@ USD", [floatFormatter stringFromNumber:_product.price]]; if (_product.salePrice.intValue > 0) { NSString *rawStr = [NSString stringWithFormat:@"$%@ $%@ USD", [floatFormatter stringFromNumber:_product.price], [floatFormatter stringFromNumber:_product.salePrice]]; NSMutableAttributedString * string = [[NSMutableAttributedString alloc] initWithString:rawStr]; // Change all the text to red first [string addAttribute:NSForegroundColorAttributeName value:[UIColor colorWithRed:157/255.0 green:38/255.0 blue:29/255.0 alpha:1.0] range:NSMakeRange(0,rawStr.length)]; // find the first space NSRange firstSpace = [rawStr rangeOfString:@" "]; // Change from zero to space to gray color [string addAttribute:NSForegroundColorAttributeName value:_priceLbl.textColor range:NSMakeRange(0, firstSpace.location)]; [string addAttribute:NSStrikethroughStyleAttributeName value:@2 range:NSMakeRange(0, firstSpace.location)]; _priceLbl.attributedText = string; } } 

Answer 1

SDWebImage is very amazing, but DLImageLoader is absolutely incredible, and a key part of many large production applications

stack overflow

It is surprisingly easy to use.

To avoid skimming issues, just enter a delay before starting the image upload. So essentially this ... this is simple

 dispatch_after_secs_on_main(0.4, ^ { if ( ! [urlWasThen isEqualToString:self.currentImage] ) { // so in other words, in fact, after a short period of time, // the user has indeed scrolled away from that item. // (ie, the user is skimming) // this item is now some "new" item so of course we don't // bother loading "that old" item // ie, we now know the user was simply skimming over that item. // (just TBC in the preliminary clause above, // since the image is already in cache, // we'd just instantly load the image - even if the user is skimming) // NSLog(@" --- --- --- --- --- --- too quick!"); return; } // a short time has passed, and indeed this cell is still "that" item // the user is NOT skimming, SO we start loading the image. //NSLog(@" --- not too quick "); [DLImageLoader loadImageFromURL:urlWasThen completed:^(NSError *error, NSData *imgData) { if (self == nil) return; // some time has passed while the image was loading from the internet... if ( ! [urlWasThen isEqualToString:self.currentImage] ) { // note that this is the "normal" situation where the user has // moved on from the image, so no need toload. // // in other words: in this case, not due to skimming, // but because SO much time has passed, // the user has moved on to some other part of the table. // we pointlessly loaded the image from the internet! doh! //NSLog(@" === === 'too late!' image load!"); return; } UIImage *image = [UIImage imageWithData:imgData]; self.someImage.image = image; }]; }); 

This is an “incredibly simple” solution.

IMO, after extensive experimentation, actually works significantly better than a more sophisticated tracking solution when scrolling through scrolling.

once again, DLImageLoader makes all this extremely simple

---

Note that the section of the above code is just a “normal” way to load an image inside a cell.

Here is a typical code that will do this:

 -(void)imageIsNow:(NSString *)imUrl { // call this routine o "set the image" on this cell. // note that "image is now" is a better name than "set the image" // Don't forget that cells very rapidly change contents, due to // the cell reuse paradigm on iOS. // this cell is being told that, the image to be displayed is now this image // being aware of scrolling/skimming issues, cache issues, etc, // utilise this information to apprporiately load/whatever the image. self.someImage.image = nil; // that UIImageView self.currentImage = imUrl; // you need that string property [self loadImageInASecIfItsTheSameAs:imUrl]; } -(void)loadImageInASecIfItsTheSameAs:(NSString *)urlWasThen { // (note - at this point here the image may already be available // in cache. if so, just display it. I have omitted that // code for simplicity here.) // so, right here, "possibly load with delay" the image // exactly as shown in the code above ..... dispatch_after_secs_on_main(0.4, ^ ...etc.... ...etc.... } 

Again all this is easily possible thanks to the DLImageLoader, which is awesome. This is a stunningly robust library.