DynamicHeightLayout.m

//
//  DynamicHeightLayout.m
//  DynamicHeightLayout
//
//  Created by Vivek Molkar on 26/11/17.
//  Copyright © 2017 Vivek Molkar. All rights reserved.
//

#import "DynamicHeightLayout.h"

@interface DynamicHeightLayout()
@property int numberOfColumns;
@property CGFloat cellPadding;
@property NSMutableArray *cache;
@property CGFloat contentHeight;
@property CGFloat contentWidth;
@end

@implementation DynamicHeightLayout

- (id) init {
    NSLog(@"init");
    if((self = [super init]))
        [self initialize];

    return self;
}

- (id) initWithCoder:(NSCoder *)aDecoder {
    NSLog(@"initWithCoder:");
    if((self = [super initWithCoder:aDecoder]))
        [self initialize];

    return self;
}

- (void) initialize {
    NSLog(@"initialize");
    self.numberOfColumns = 2;
    self.contentHeight = 0;
    self.cellPadding = 10;
    self.cache = [NSMutableArray new];
}

- (void) prepareLayout {
    if (self.cache.count == 0) {
        UIEdgeInsets insets = self.collectionView.contentInset;
        self.contentWidth = self.collectionView.bounds.size.width - (insets.left + insets.right);
        
        CGFloat columnWidth = self.contentWidth / self.numberOfColumns;
        NSMutableArray *xOffsets = [NSMutableArray new];
        NSMutableArray *yOffsets = [NSMutableArray new];
        for (int i = 0; i < self.numberOfColumns; i++) {
            [xOffsets addObject: @(i * columnWidth)];
            [yOffsets addObject: @(0)];
        }

        int column = 0;
        for (int i = 0; i < [self.collectionView numberOfItemsInSection:0]; i++) {
            NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];

            CGFloat cellHeight = arc4random_uniform(200);
            CGFloat height = self.cellPadding * 2 + cellHeight;
            CGRect frame = CGRectMake((CGFloat)[[xOffsets objectAtIndex:column] floatValue],
                                      (CGFloat)[[yOffsets objectAtIndex:column] floatValue],
                                      columnWidth,
                                      height);
            CGRect insetFrame = CGRectInset(frame, self.cellPadding, self.cellPadding);

            UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
            attributes.frame = insetFrame;
            [self.cache addObject:attributes];

            self.contentHeight = MAX(self.contentHeight, CGRectGetMaxY(frame));
            [yOffsets replaceObjectAtIndex:column withObject:@([[yOffsets objectAtIndex:column]floatValue] + height)];

            column = column < (self.numberOfColumns - 1) ? (column + 1) : 0;

        }
    }
}

- (CGSize) collectionViewContentSize {
    return CGSizeMake(self.contentWidth, self.contentHeight);
}

- (UICollectionViewLayoutAttributes *) layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    return [self.cache objectAtIndex:indexPath.item];
}

- (NSArray<UICollectionViewLayoutAttributes *> *) layoutAttributesForElementsInRect:(CGRect)rect {
    NSMutableArray *visibleAttributes = [[NSMutableArray alloc] init];
    for (UICollectionViewLayoutAttributes *attributes in self.cache) {
        if (CGRectIntersectsRect(attributes.frame, rect)) {
            [visibleAttributes addObject:attributes];
        }
    }
    return visibleAttributes;
}
@end