UITableView scrolling is not smooth

问题

I have the smooth scrolling issue at my UITableView with UITableViewCell which contains UIImageView. Similar issues could be found all over the StrackOverflow but none of the proposed solutions helped me to completely get rid of the lag.

My case is quite common:

1. images are stored at application storage (in my sample at app bundle)
2. images could have different size (500x500, 1000x1000, 1500x1500)
3. I need to display those images in UITableView where UIImageView size is 120x120 (retina)

I have followed multiple optimization tips and managed to optimize scrolling a lot. Unfortunately it is still not perfect. This is my scenario:

1. first I moved all the image loading/processing/resizing logic to the background thread
2. UITableViewCell reuse is enabled
3. once UITableViewCell is in view I clear old values (settings to null) and start background thread to load the image
4. at this point we are in background thread and I'm adding 500 ms delay to avoid settings new image to often (in case we are scrolling fast) (see below explanation)
5. if UIImage exists at static image cache (regular dictionary with UIImage instances) - fetch that one and go to the step 9.
6. if not - load new image from bundle (imageWithName) using url to app bundle (in real world scenario images will be stored to application storage, not bundle)
7. once image is loaded resize it to 120x120 using graphics context
8. save resized image to the static image cache
9. at this point we have instance to UIImage and process is in the background thread. From here we move back to UI Thread with the given image
10. if data context was cleared (for example UITableViewCell disappeared or was reused to display another image) we skip processing of the currently available image.
11. if data context is the same - assign UIImage to UIImageView with an alpha animation (UIView.Animate)
12. once UITableViewCell is out of view - clear the data context

Originally before starting new background thread to fetch the image here (step 1) was UIImage cache check without background thread. In this case if we have the image in the cache we assign it instantly and this introduces a great lag during fast scrolling (we assign images to often as long as we fetch them instantly). Those lines are commented at my example attached below.

There are still two issues:

1. at some point during scrolling I still have a small lag (at the moment when I'm assign new UIImage to UIImageView.
2. (this one is more noticeable) when you tap on item and go back from details there is a lag right before back navigation animation is finished.

Any suggest how to deal with those two issues or how to optimize my scenario are appreciated

Please take into account that sample written in Xamarin but I don't believe that Xamarin is the cause of the problem as long as I have the same issue for the app written in ObjectiveC as well.

Smooth Scrolling Test App

回答1:

Did you every tried to populate your TableView with only one 120x120 Image which is saved in your Bundle? This way you can check, if the problem occurs of your Image rendering

Instead of resizing all your images to 120x120 and save them in cache, I would recommend creating and using a thumbnail of all your images. You are somehow already doing this, but you are doing this couple of times (everytime you are scrolling or if your cache is full).

In our last project we had a UICollectionView with book covers. Most of the covers were between 400-800kb big and the feeling while scrolling was really bad. So we created a thumbnail for each image (thumbails about 40-50kb) and used the thumbnails instead of real covers. Works like a charm! I attached the thumbnail creation function

- (BOOL) createThumbnailForImageAtFilePath:(NSString *)sourcePath withName:(NSString *)name {

    UIImage* sourceImage = [UIImage imageWithContentsOfFile:sourcePath];
    if (!sourceImage) {
        //...
        return NO;
    }

    CGSize thumbnailSize = CGSizeMake(128,198);

    float imgAspectRatio = sourceImage.size.height / sourceImage.size.width;
    float thumbnailAspectRatio = thumbnailSize.height/thumbnailSize.width;

    CGSize scaledSize = thumbnailSize;

    if(imgAspectRatio >= thumbnailAspectRatio){
         //image is higher than thumbnail
         scaledSize.width = scaledSize.height * thumbnailSize.width / thumbnailSize.height;
    }
    else{
        //image is broader than thumbnail
        scaledSize.height = scaledSize.width * imgAspectRatio;
    }

    UIGraphicsBeginImageContextWithOptions( scaledSize, NO, 0.0 );
    CGRect scaledImageRect = CGRectMake( 0.0, 0.0, scaledSize.width, scaledSize.height );
    [sourceImage drawInRect:scaledImageRect];
    UIImage* destImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    NSString* thumbnailFilePath = [[self SOMEDIRECTORY] stringByAppendingPathComponent:name];

   BOOL success = [UIImageJPEGRepresentation(destImage, 0.9) writeToFile:thumbnailFilePath atomically:NO];

return success;

}

回答2:

Try facebook's Async Display library.

https://github.com/facebook/AsyncDisplayKit

Really easy to use.. from their guide: http://asyncdisplaykit.org/guide/

_imageNode = [[ASImageNode alloc] init];
_imageNode.backgroundColor = [UIColor lightGrayColor];
_imageNode.image = [UIImage imageNamed:@"hello"];
_imageNode.frame = CGRectMake(10.0f, 10.0f, 40.0f, 40.0f);
[self.view addSubview:_imageNode.view];

This decodes the image on a background thread.

I'm not sure if it's easy to use iOS libraries on Xamarin but if it's easy, give this a shot.

回答3:

I sub-class Paul Hegarty's CoreDataTableViewController and employ thumbnails of my photos in the CoreDataTableView.

Look for the examples in Lecture 14 titled FlickrFetcher and Photomania. You will also need to download the CoreDataTableViewController at that same link.

Make a CoreData Entity with an appropriate title and define whatever attributes (data variables) you want. You will need to define two "Transformable" attributes, one for the photo and one for the thumbnail.

Then load your thumbnail in the CoreDataTableView:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

{ NSArray *exceptions = [NSArray arrayWithObjects:@"SCR", @"DNS", @"NT", @"ND", @"NH", nil];

static NSString *CellIdentifier = @"resultsDisplayCell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];

if (cell == nil) {
    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier];
}

cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;

MarksFromMeets *athleteMarks = [self.fetchedResultsController objectAtIndexPath:indexPath];
NSString* date = [ITrackHelperMethods dateToAbbreviatedString:athleteMarks.meetDate];
NSMutableString *title = [NSMutableString stringWithFormat:@"%@", athleteMarks.markInEvent];
NSMutableString *subTitle = [NSMutableString stringWithFormat:@"%@ - %@",date, athleteMarks.meetName];
[title replaceOccurrencesOfString:@"(null)"
                       withString:@""
                          options:0
                            range:NSMakeRange(0, [title length])];

// cell.imageView.image = athleteMarks.photoThumbNail; // Don't like image in front of record.

[cell.textLabel setFont:[UIFont
                         fontWithName:@"Helvetica Neue" size:18]];

[cell.detailTextLabel setFont:[UIFont fontWithName:@"Helvetica Neue" size:16]];
[cell.detailTextLabel setTextColor:[UIColor grayColor]];

// make selected items orange
if ([athleteMarks.eventPR integerValue] != 0
    && (![exceptions containsObject:athleteMarks.markInEvent])) {

    title = [NSMutableString stringWithFormat:@"%@      (PR)",title];
    [cell.textLabel setTextColor:[UIColor redColor]];
}
else if ([athleteMarks.eventSB integerValue] != 0
         && (![exceptions containsObject:athleteMarks.markInEvent])) {
    title = [NSMutableString stringWithFormat:@"%@      (SB)",title];

    [cell.textLabel setTextColor:[UIColor orangeColor]];
} else {
    [cell.textLabel setTextColor:[UIColor grayColor]];
}

cell.textLabel.text = title;
cell.detailTextLabel.text = subTitle;

cell.indentationLevel = indentationLevelOne;
cell.indentationWidth = indentationForCell;

return cell;

}

If you want, I can send you an example of a Category for an Entity's NSManagedObject Sub-Class. This Category loads the photo and the thumbnail into CoreData Entity. The first time will be slow. However, after that the user should be able to scroll through TableView smoothly and then all the updated results will load automatically. Let me know.

One nice thing is that CoreData handles all the memory management.

Good luck!

回答4:

I don't have enough rep to comment, So here's an answer which helped my tableview scrolling performance:

• Make the tableview height larger than the viewable window. Cells will load "off screen" and helps improve scroll smoothness.
• Do your image processing in the following method: -(void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath

Those two tricks got my table flowing really nice. I'm getting my image data from an API service and AFNETWORKING has an awesome image loader, but not necessary for you since images are in the bundle.

回答5:

Maybe you could try SDWebImage instead. It is also a xamarin component which fashions an asynchronous image downloader and asynchronous memory and disk image caching with automatic cache expiration handling. Using it would probably mean throwing away a lot of hard written code, but it might be worth it -plus your code will become a lot simpler. In iOS you can also setup a SDWebImageManager inside the viewDidLoad of a controller:

- (void)viewDidLoad{
...
SDWebImageManager *manager = [SDWebImageManager sharedManager];
manager.delegate = self;
...
}

and set the view controller as the delegate. Then, when the following delegate method is called:

- (UIImage *)imageManager:(SDWebImageManager *)imageManager transformDownloadedImage:(UIImage *)image withURL:(NSURL *)imageURL

you could scale your images to thumbs of the appropriate size before caching them.

Hope that helps.

回答6:

Weel I had a similar problem, my scroll was not smooth. I am inserting in the table a variable UIImageView with inside labelViews. What I did was to change the method HeightforRowAtIndexPath for estimatedHeightforRowAtIndexPath and now scroll is smooth.

来源：https://stackoverflow.com/questions/26310754/uitableview-scrolling-is-not-smooth