UITableView layout messing up on push segue and return. (iOS 8, Xcode beta 5, Swift)

Question

tldr; Auto constrains appear to break on push segue and return to view for custom cells

Edit: I have provided a github example project that shows off the error that occurs https://github.com/Matthew-Kempson/TableViewExample.git

I am creating an app which requires the title label of the custom UITableCell to allow for varying lines dependent on the length of the post title. The cells load into the view correctly but if I press on a cell to load the post in a push segue to a view containing a WKWebView you can see, as shown in the screen shot, the cells move immediately to incorrect positions. This is also viewed when loading the view back through the back button of the UINavigationController.

In this particular example I pressed on the very end cell, with the title "Two buddies I took a picture of in Paris", and everything is loaded correctly. Then as shown in the next screenshot the cells all move upwards for unknown reasons in the background of loading the second view. Then when I load the view back you can see the screen has shifted upwards slightly and I cannot actually scroll any lower than is shown. This appears to be random as with other tests when the view loads back there is white space under the bottom cell that does not disappear.

I have also included a picture containing the constraints that the cells has.

Images (I need more reputation to provide images in this question apparently so they are in this imgur album): http://imgur.com/a/gY87E

My code:

Method in custom cell to allow the cell to resize the view correctly when rotating:

override func layoutSubviews() {
    super.layoutSubviews()

    self.contentView.layoutIfNeeded()

    // Update the label constaints
    self.titleLabel.preferredMaxLayoutWidth = self.titleLabel.frame.width
    self.detailsLabel.preferredMaxLayoutWidth = self.detailsLabel.frame.width
}

Code in tableview

override func viewDidLoad() {
    super.viewDidLoad()

    // Create and register the custom cell
    self.tableView.estimatedRowHeight = 56
    self.tableView.rowHeight = UITableViewAutomaticDimension
}

Code to create the cell

    override func tableView(tableView: UITableView!, cellForRowAtIndexPath indexPath: NSIndexPath!) -> UITableViewCell! {
    if let cell = tableView.dequeueReusableCellWithIdentifier("LinkCell", forIndexPath: indexPath) as? LinkTableViewCell {

        // Retrieve the post and set details
        let link: Link = self.linksArray.objectAtIndex(indexPath.row) as Link

        cell.titleLabel.text = link.title
        cell.scoreLabel.text = "(link.score)"
        cell.detailsLabel.text = link.stringCreatedTimeIntervalSinceNow() + " ago by " + link.author + " to /r/" + link.subreddit

        return cell
    }

    return nil
}

If you require any more code or information please ask and I shall provide what is necessary

Thanks for your help!

question from:https://stackoverflow.com/questions/25221031/uitableview-layout-messing-up-on-push-segue-and-return-ios-8-xcode-beta-5-sw

Answer 1

The problem of this behavior is when you push a segue the tableView will call the estimatedHeightForRowAtIndexPath for the visible cells and reset the cell height to a default value. This happens after the viewWillDisappear call. If you come back to TableView all the visible cells are messed up..

I solved this problem with a estimatedCellHeightCache. I simply add this code snipped to the cellForRowAtIndexPath method:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    ...
    // put estimated cell height in cache if needed
    if (![self isEstimatedRowHeightInCache:indexPath]) {
        CGSize cellSize = [cell systemLayoutSizeFittingSize:CGSizeMake(self.view.frame.size.width, 0) withHorizontalFittingPriority:1000.0 verticalFittingPriority:50.0];
        [self putEstimatedCellHeightToCache:indexPath height:cellSize.height];
    }
    ...
}

Now you have to implement the estimatedHeightForRowAtIndexPath as following:

-(CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath {
    return [self getEstimatedCellHeightFromCache:indexPath defaultHeight:41.5];
}

Configure the Cache

Add this property to your .h file:

@property NSMutableDictionary *estimatedRowHeightCache;

Implement methods to put/get/reset.. the cache:

#pragma mark - estimated height cache methods

// put height to cache
- (void) putEstimatedCellHeightToCache:(NSIndexPath *) indexPath height:(CGFloat) height {
    [self initEstimatedRowHeightCacheIfNeeded];
    [self.estimatedRowHeightCache setValue:[[NSNumber alloc] initWithFloat:height] forKey:[NSString stringWithFormat:@"%d", indexPath.row]];
}

// get height from cache
- (CGFloat) getEstimatedCellHeightFromCache:(NSIndexPath *) indexPath defaultHeight:(CGFloat) defaultHeight {
    [self initEstimatedRowHeightCacheIfNeeded];
    NSNumber *estimatedHeight = [self.estimatedRowHeightCache valueForKey:[NSString stringWithFormat:@"%d", indexPath.row]];
    if (estimatedHeight != nil) {
        //NSLog(@"cached: %f", [estimatedHeight floatValue]);
        return [estimatedHeight floatValue];
    }
    //NSLog(@"not cached: %f", defaultHeight);
    return defaultHeight;
}

// check if height is on cache
- (BOOL) isEstimatedRowHeightInCache:(NSIndexPath *) indexPath {
    if ([self getEstimatedCellHeightFromCache:indexPath defaultHeight:0] > 0) {
        return YES;
    }
    return NO;
}

// init cache
-(void) initEstimatedRowHeightCacheIfNeeded {
    if (self.estimatedRowHeightCache == nil) {
        self.estimatedRowHeightCache = [[NSMutableDictionary alloc] init];
    }
}

// custom [self.tableView reloadData]
-(void) tableViewReloadData {
    // clear cache on reload
    self.estimatedRowHeightCache = [[NSMutableDictionary alloc] init];
    [self.tableView reloadData];
}