UICollectionView insert cells above maintaining position (like Messages.app)

Question

By default Collection View maintains content offset while inserting cells. On the other hand I\'d like to insert cells above the currently displaying ones so that they appea

Answer 1

Love James Martin’s solution. But for me it started to breakdown when inserting/deleting above/below a specific content window. I took a stab at subclassing UICollectionViewFlowLayout to get the behavior I wanted. Hope this helps someone. Any feedback appreciated :)

@interface FixedScrollCollectionViewFlowLayout () {

    __block float bottomMostVisibleCell;
    __block float topMostVisibleCell;
}

@property (nonatomic, assign) BOOL isInsertingCellsToTop;
@property (nonatomic, strong) NSArray *visableAttributes;
@property (nonatomic, assign) float offset;;

@end

@implementation FixedScrollCollectionViewFlowLayout


- (id)initWithCoder:(NSCoder *)aDecoder {

    self = [super initWithCoder:aDecoder];

    if (self) {
        _isInsertingCellsToTop = NO;
    }
    return self;
}

- (id)init {

    self = [super init];

    if (self) {
        _isInsertingCellsToTop = NO;
    }
    return self;
}

- (void)prepareLayout {

    NSLog(@"prepareLayout");
    [super prepareLayout];
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {

    NSLog(@"layoutAttributesForElementsInRect");
    self.visableAttributes = [super layoutAttributesForElementsInRect:rect];
    self.offset = 0;
    self.isInsertingCellsToTop = NO;
    return self.visableAttributes;
}

- (void)prepareForCollectionViewUpdates:(NSArray *)updateItems {

    bottomMostVisibleCell = -MAXFLOAT;
    topMostVisibleCell = MAXFLOAT;
    CGRect container = CGRectMake(self.collectionView.contentOffset.x, self.collectionView.contentOffset.y, self.collectionView.frame.size.width, self.collectionView.frame.size.height);

    [self.visableAttributes  enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes *attributes, NSUInteger idx, BOOL *stop) {

        CGRect currentCellFrame =  attributes.frame;
        CGRect containerFrame = container;

        if(CGRectIntersectsRect(containerFrame, currentCellFrame)) {
            float x = attributes.indexPath.row;
            if (x < topMostVisibleCell) topMostVisibleCell = x;
            if (x > bottomMostVisibleCell) bottomMostVisibleCell = x;
        }
    }];

    NSLog(@"prepareForCollectionViewUpdates");
    [super prepareForCollectionViewUpdates:updateItems];
    for (UICollectionViewUpdateItem *updateItem in updateItems) {
        switch (updateItem.updateAction) {
            case UICollectionUpdateActionInsert:{
                NSLog(@"UICollectionUpdateActionInsert %ld",updateItem.indexPathAfterUpdate.row);
                if (topMostVisibleCell>updateItem.indexPathAfterUpdate.row) {
                    UICollectionViewLayoutAttributes * newAttributes = [self layoutAttributesForItemAtIndexPath:updateItem.indexPathAfterUpdate];
                    self.offset += (newAttributes.size.height + self.minimumLineSpacing);
                    self.isInsertingCellsToTop = YES;
                }
                break;
            }
            case UICollectionUpdateActionDelete: {
                NSLog(@"UICollectionUpdateActionDelete %ld",updateItem.indexPathBeforeUpdate.row);
                if (topMostVisibleCell>updateItem.indexPathBeforeUpdate.row) {
                    UICollectionViewLayoutAttributes * newAttributes = [self layoutAttributesForItemAtIndexPath:updateItem.indexPathBeforeUpdate];
                    self.offset -= (newAttributes.size.height + self.minimumLineSpacing);
                    self.isInsertingCellsToTop = YES;
                }
                break;
            }
            case UICollectionUpdateActionMove:
                NSLog(@"UICollectionUpdateActionMoveB %ld", updateItem.indexPathBeforeUpdate.row);
                break;
            default:
                NSLog(@"unhandled case: %ld", updateItem.indexPathBeforeUpdate.row);
                break;
        }
    }

    if (self.isInsertingCellsToTop) {
        if (self.collectionView) {
            [CATransaction begin];
            [CATransaction setDisableActions:YES];
        }
    }
}

- (void)finalizeCollectionViewUpdates {

    CGPoint newOffset = CGPointMake(self.collectionView.contentOffset.x, self.collectionView.contentOffset.y + self.offset);

    if (self.isInsertingCellsToTop) {
        if (self.collectionView) {
            self.collectionView.contentOffset = newOffset;
            [CATransaction commit];
        }
    }
}