A UICollectionViewFlowLayout that supports flexible-width cells, fixed-width margins between cells, and centering the cells.

CHTCenteredCellLayout.m

//
//  CHTCenteredCellLayout.m
//
//  Created by Steve Streza on 10/25/12.
//  Use however you like.
//

#import "CHTCenteredCellLayout.h"

@implementation CHTCenteredCellLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect{
	NSArray *attributes = [super layoutAttributesForElementsInRect:rect];

	// Break the attributes up into rows based on their frame.origin.y values
	NSMutableDictionary *rows = [NSMutableDictionary dictionary];
	[attributes enumerateObjectsUsingBlock:^(id object, NSUInteger idx, BOOL *stop) {
		UICollectionViewLayoutAttributes *attribute = (UICollectionViewLayoutAttributes *)object;
		
		NSNumber *originY = @(attribute.frame.origin.y);
		NSMutableArray *cells = rows[originY];
		if(!cells){
			cells = rows[originY] = [NSMutableArray array];
		}
		[cells addObject:attribute];
	}];
	
	double rowWidth = self.collectionView.frame.size.width;
	
	[rows enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
		NSArray *cellAttributes = (NSArray *)obj;

		// Figure out the total width of the cells available in the row, plus gutters.
		double gutter = 10;
		__block double totalWidth = 0;
		[cellAttributes enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
			UICollectionViewLayoutAttributes *attributes = (UICollectionViewLayoutAttributes *)obj;
			totalWidth += attributes.frame.size.width;
			totalWidth += gutter;
		}];
		totalWidth -= gutter;
		
		// Start at offsetX, then iterate through all the cells in the row, and update their frames.
		__block double offsetX = (rowWidth - totalWidth) / 2;
		[cellAttributes enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
			UICollectionViewLayoutAttributes *attributes = (UICollectionViewLayoutAttributes *)obj;
			CGRect frame = attributes.frame;
			frame.origin.x = round(offsetX);
			attributes.frame = frame;
			offsetX += attributes.frame.size.width + gutter;
		}];
	}];
	
	return attributes;
}

@end

MLFWCenteredCellLayout.h

//
//  CHTCenteredCellLayout.h
//
//  Created by Steve Streza on 10/25/12.
//  Use however you like.
//

#import <UIKit/UIKit.h>

@interface CHTCenteredCellLayout : UICollectionViewFlowLayout

@end