#import <UIKit/UIKit.h>// 類的前置聲明@class WaterfallFlowLayout;@protocol WaterfallFlowLayoutDelegate <NSObject>// 動態獲取 item 寬度- (CGFloat) WaterfallFlowLayout:(WaterfallFlowLayout *) layout widthForItemAtIndexPath:(NSIndexPath *) indexPath;@end@interface WaterfallFlowLayout : UICollectionViewLayout@property (nonatomic,assign) id <WaterfallFlowLayoutDelegate> delegate;@property (nonatomic) NSInteger numberOfColumns;@property (nonatomic) CGFloat minimumLineSpacing;@property (nonatomic) CGFloat minimumInteritemSpacing;@property (nonatomic) UIEdgeInsets sectionInset;@end

#import "WaterfallFlowLayout.h"@interface WaterfallFlowLayout (){  // 用于記錄每一列布局到的寬度  NSMutableArray * _widthOfColumns;  // 用于保存所有item的屬性 (frame)  NSMutableArray * _itemsAttributes;}@end@implementation WaterfallFlowLayout- (void) setNumberOfColumns:(NSInteger)numberOfColumns {  if (_numberOfColumns != numberOfColumns) {    _numberOfColumns = numberOfColumns;    // 讓原有布局失效，需要重新布局    [self invalidateLayout];  }}- (void)setMinimumLineSpacing:(CGFloat)minimumLineSpacing {  if (_minimumLineSpacing != minimumLineSpacing) {    _minimumLineSpacing = minimumLineSpacing;    [self invalidateLayout];  }}- (void)setMinimumInteritemSpacing:(CGFloat)minimumInteritemSpacing {  if (_minimumInteritemSpacing != minimumInteritemSpacing) {    _minimumInteritemSpacing = minimumInteritemSpacing;    [self invalidateLayout];  }}- (void)setSectionInset:(UIEdgeInsets)sectionInset {  if (!UIEdgeInsetsEqualToEdgeInsets(_sectionInset, sectionInset)) {    _sectionInset = sectionInset;    [self invalidateLayout];  }}//重寫方法 1: 準備布局-(void)prepareLayout {  [super prepareLayout];  // 真正的布局在這里完成  if (_itemsAttributes) {    [_itemsAttributes removeAllObjects];  }else {    _itemsAttributes = [[NSMutableArray alloc] init];  }  if (_widthOfColumns) {    [_widthOfColumns removeAllObjects];  }else {    _widthOfColumns = [[NSMutableArray alloc] init];  }  for (NSInteger i = 0; i < self.numberOfColumns; i++) {    // 初始化每一列的寬度(默認為上邊距)//    _heightOfColumns[i] = @(self.sectionInset.top);    [_widthOfColumns addObject:@(self.sectionInset.left)];  }  // item的總數  NSInteger count = [self.collectionView numberOfItemsInSection:0];//  CGFloat itemWidth = (self.collectionView.frame.size.width - self.sectionInset.left - self.sectionInset.right - (_numberOfColumns-1) * _minimumInteritemSpacing )/_numberOfColumns;  // 總的高度 (集合視圖的寬度)  CGFloat totalHeight = self.collectionView.frame.size.height;  // 有效的高度 (出去間隔及邊界)  CGFloat validHeight = totalHeight - self.sectionInset.top - self.self.sectionInset.bottom - (self.numberOfColumns-1) * self.minimumInteritemSpacing;  // 每一個item的高度  CGFloat itemHeight = validHeight/self.numberOfColumns;  // 設置item的默認寬度  CGFloat itemWidth = itemHeight;  for (NSInteger i = 0; i<count; i++) {    // 最短列的下標    NSInteger index = [self indexOfShortestColumn];    CGFloat originY = self.sectionInset.top + index * (itemHeight +self.minimumInteritemSpacing);    CGFloat originX = [_widthOfColumns[index] floatValue];    // 構造 indexPath    NSIndexPath * indexPath = [NSIndexPath indexPathForItem:i inSection:0];    // 動態的獲取寬度    if ([self.delegate respondsToSelector:@selector(WaterfallFlowLayout:widthForItemAtIndexPath:)]) {      itemWidth = [self.delegate WaterfallFlowLayout:self widthForItemAtIndexPath:indexPath];    }    UICollectionViewLayoutAttributes * attr = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];    attr.frame = CGRectMake(originX, originY, itemWidth, itemHeight);    // 保存 item 的屬性 到數組中    [_itemsAttributes addObject:attr];    // 更新布局到的一列(最短列) 的高度    _widthOfColumns[index] = @(originX + itemWidth + self.minimumLineSpacing);  }  // 刷新顯示  [self.collectionView reloadData];}//重寫方法 2: 返回指定區域的item的屬性(frame)- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {  NSMutableArray * array = [NSMutableArray array];  for (UICollectionViewLayoutAttributes * attr in _itemsAttributes) {    // 判斷兩個矩形是否有交集    if (CGRectIntersectsRect(attr.frame, rect)) {      [array addObject:attr];    }  }  return array;}//重寫方法 3: 返回內容的尺寸-(CGSize)collectionViewContentSize {  CGFloat height = self.collectionView.frame.size.height;  NSInteger index = [self indexOfLongestColumn];  CGFloat width = [_widthOfColumns[index] floatValue] + self.sectionInset.right - self.minimumLineSpacing;  return CGSizeMake(width, height);}- (NSInteger) indexOfLongestColumn {  NSInteger index = 0;  for (NSInteger i = 0; i<_numberOfColumns; i++) {    if ([_widthOfColumns[i] floatValue] > [_widthOfColumns[index] floatValue]) {      index = i;    }  }  return index;}- (NSInteger) indexOfShortestColumn {  NSInteger index = 0;  for (NSInteger i = 0; i<_numberOfColumns; i++) {    if ([_widthOfColumns[i] floatValue] < [_widthOfColumns[index] floatValue]) {      index = i;    }  }  return index;}@end

#import "RootViewController.h"#import "WaterfallFlowLayout.h"@interface RootViewController () <UICollectionViewDataSource,UICollectionViewDelegateFlowLayout,WaterfallFlowLayoutDelegate>{  UICollectionView * _collectionView;}@end@implementation RootViewController- (void)dealloc {  [_collectionView release];  [super dealloc];}- (void)viewDidLoad {  [super viewDidLoad];  // 創建集合視圖  [self createCollectionView];}- (UICollectionViewLayout *)createLayout {#if 1  WaterfallFlowLayout * layout = [[WaterfallFlowLayout alloc] init];  layout.sectionInset = UIEdgeInsetsMake(20, 20, 20, 20);  layout.minimumLineSpacing = 10;  layout.minimumInteritemSpacing = 20;  layout.numberOfColumns = 3;  layout.delegate = self;  [self performSelector:@selector(changeLayout:) withObject:layout afterDelay:3];#else  UICollectionViewFlowLayout * layout = [[UICollectionViewFlowLayout alloc] init];  layout.minimumLineSpacing = 10;  layout.itemSize = CGSizeMake(150, 100);  layout.sectionInset = UIEdgeInsetsMake(10, 10, 10, 10);#endif  return [layout autorelease];}- (void)changeLayout:(WaterfallFlowLayout *)layout {  layout.numberOfColumns = 3;}- (void)createCollectionView {  CGRect frame = CGRectMake(0, 20, VIEW_WIDTH, VIEW_HEIGHT-20);  _collectionView = [[UICollectionView alloc] initWithFrame:frame collectionViewLayout:[self createLayout]];  _collectionView.backgroundColor = [UIColor cyanColor];  // 設置代理  _collectionView.delegate = self;  _collectionView.dataSource = self;  // 注冊cell 類型 及 復用標識  [_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellId"];  [self.view addSubview:_collectionView];}#pragma mark - UICollectionViewDataSource- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {  return 102;}- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {  UICollectionViewCell * cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cellId" forIndexPath:indexPath];  UILabel * label = nil;  NSArray * array = cell.contentView.subviews;  if (array.count) {    label = array[0];  }else {    label = [[UILabel alloc] init];//    label.frame = cell.bounds;    label.textAlignment = NSTextAlignmentCenter;    label.font = [UIFont systemFontOfSize:50];    [cell.contentView addSubview:label];    [label release];  }  label.frame = cell.bounds;  label.text = [NSString stringWithFormat:@"%ld",indexPath.item];  label.textColor = [UIColor whiteColor];  cell.backgroundColor = RandomColor;  return cell;}- (CGSize) collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {  return CGSizeMake( arc4random()%100+200, 110);}-(CGFloat) WaterfallFlowLayout:(WaterfallFlowLayout *)layout widthForItemAtIndexPath:(NSIndexPath *)indexPath{  return arc4random()%150+50;}-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{  NSLog(@"點擊了第 %ld 組，第 %ld 行",indexPath.section,indexPath.row);}- (void)didReceiveMemoryWarning {  [super didReceiveMemoryWarning];}@end