Subclass UICollectionViewFlowLayout to build a tag layout, and make UICollectionView scrolling smonthly while it holds dozens of cells with round corner

Readme - TagFlowLayout.md

TagCollectionViewCell.h

//
//  TagCollectionViewCell.h
//  
//  Created by DingWenchao on 7/14/15.
//  Copyright (c) 2015 demo. All rights reserved.
//  

#import <UIKit/UIKit.h>

@interface TagCollectionViewCell : UICollectionViewCell

@property (weak, nonatomic) IBOutlet UILabel *textLabel;

@end

TagCollectionViewCell.m

#import "TagCollectionViewCell.h"

@implementation TagCollectionViewCell

// Trick about round corner. If you set _textLabel.layer.cornerRadius = [], 
// the scrolling speed of collectionView would suck.
- (void)awakeFromNib
{
    _textLabel.backgroundColor = [UIColor clearColor];
    _textLabel.layer.cornerRadius = 5;
    // Huge change in performance by explicitly setting the below (even though default is supposedly NO)
    _textLabel.layer.masksToBounds = NO;
    // Performance improvement here depends on the size of your view
    _textLabel.layer.shouldRasterize = YES;
    _textLabel.layer.backgroundColor = _invert ? kRedTagColor.CGColor : kGreyTagColor.CGColor;
    _textLabel.layer.rasterizationScale = [UIScreen mainScreen].scale;
}

@end

TagFlowLayout.h

#import <UIKist/UIKit.h>

@interface TagFlowLayout : UICollectionViewFlowLayout

@end

TagFlowLayout.m

#import "TagFlowLayout.h"

const NSInteger kMaxCellSpacing = 10;

@implementation TagFlowLayout

- (instancetype)init
{
    self = [super init];
    if (self) {
        self.scrollDirection = UICollectionViewScrollDirectionVertical;
        self.minimumInteritemSpacing = kMaxCellSpacing;
        self.minimumLineSpacing = kMaxCellSpacing;
    }
    return self;
}

- (instancetype)initWithCoder:(NSCoder *)coder
{
    self = [super initWithCoder:coder];
    if (self) {
        self.scrollDirection = UICollectionViewScrollDirectionVertical;
        self.minimumInteritemSpacing = kMaxCellSpacing;
        self.minimumLineSpacing = kMaxCellSpacing;
    }
    return self;
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray* attributesToReturn = [super layoutAttributesForElementsInRect:rect];
    for (UICollectionViewLayoutAttributes* attributes in attributesToReturn) {
        if (nil == attributes.representedElementKind) {
            NSIndexPath* indexPath = attributes.indexPath;
            attributes.frame = [self layoutAttributesForItemAtIndexPath:indexPath].frame;
        }
    }
    return attributesToReturn;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewLayoutAttributes* currentItemAttributes =
    [super layoutAttributesForItemAtIndexPath:indexPath];
    
    UIEdgeInsets sectionInset = [(UICollectionViewFlowLayout *)self.collectionView.collectionViewLayout sectionInset];
    
    if (indexPath.item == 0) { // first item of section
        CGRect frame = currentItemAttributes.frame;
        frame.origin.x = sectionInset.left; // first item of the section should always be left aligned
        currentItemAttributes.frame = frame;
        
        return currentItemAttributes;
    }
    
    NSIndexPath* previousIndexPath = [NSIndexPath indexPathForItem:indexPath.item-1 inSection:indexPath.section];
    CGRect previousFrame = [self layoutAttributesForItemAtIndexPath:previousIndexPath].frame;
    CGFloat previousFrameRightPoint = previousFrame.origin.x + previousFrame.size.width + kMaxCellSpacing;
    
    CGRect currentFrame = currentItemAttributes.frame;
    CGRect strecthedCurrentFrame = CGRectMake(0,
                                              currentFrame.origin.y,
                                              self.collectionView.frame.size.width,
                                              currentFrame.size.height);
    
    if (!CGRectIntersectsRect(previousFrame, strecthedCurrentFrame)) { // if current item is the first item on the line
        // the approach here is to take the current frame, left align it to the edge of the view
        // then stretch it the width of the collection view, if it intersects with the previous frame then that means it
        // is on the same line, otherwise it is on it's own new line
        CGRect frame = currentItemAttributes.frame;
        frame.origin.x = sectionInset.left; // first item on the line should always be left aligned
        currentItemAttributes.frame = frame;
        return currentItemAttributes;
    }
    
    CGRect frame = currentItemAttributes.frame;
    frame.origin.x = previousFrameRightPoint;
    currentItemAttributes.frame = frame;
    return currentItemAttributes;
}

@end