IOS实现自定义布局瀑布流

- (void)prepareLayout
{
  //通常在该方法中完成布局的初始化操作
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
  //默认返回YES
}

- (nullable NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
  //该方法需要返回rect区域中所有元素布局属性的数组
}
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
  //该方法返回indexPath位置的元素的布局属性
}

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
  //返回值是，UICollectionView最终停留的点
}

- (void)prepareLayout
{
  [super prepareLayout];
  //设置滚动方向
  self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
  //设置内边距
  CGFloat inset = (self.collectionView.frame.size.width - self.itemSize.width) * 0.5;
  self.sectionInset = UIEdgeInsetsMake(0, inset, 0, inset);
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
  return YES;
}

- (nullable NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
  //获取rect区域中所有元素的布局属性
  NSArray *array = [super layoutAttributesForElementsInRect:rect];

  //获取UICollectionView的中点，以contentView的左上角为原点
  CGFloat centerX = self.collectionView.contentOffset.x + self.collectionView.frame.size.width * 0.5;

  //重置rect区域中所有元素的布局属性，即基于他们距离UICollectionView的中点的剧烈，改变其大小
  for (UICollectionViewLayoutAttributes *attribute in array)
  {
    //获取距离中点的距离
    CGFloat delta = ABS(attribute.center.x - centerX);
    //计算缩放比例
    CGFloat scale = 1 - delta / self.collectionView.bounds.size.width;
    //设置布局属性
    attribute.transform = CGAffineTransformMakeScale(scale, scale);
  }

  //返回所有元素的布局属性
  return [array copy];
}

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
  //计算最终显示的矩形框
  CGRect rect;
  rect.origin.x = proposedContentOffset.x;
  rect.origin.y = 0;
  rect.size = self.collectionView.frame.size;

  //获取最终显示在矩形框中的元素的布局属性
  NSArray *array = [super layoutAttributesForElementsInRect:rect];

  //获取UICollectionView的中点，以contentView的左上角为原点
  CGFloat centerX = proposedContentOffset.x + self.collectionView.frame.size.width * 0.5;

  //获取所有元素到中点的最短距离
  CGFloat minDelta = MAXFLOAT;
  for (UICollectionViewLayoutAttributes *attribute in array)
  {
    CGFloat delta = attribute.center.x - centerX;
    if (ABS(minDelta) > ABS(delta))
    {
      minDelta = delta;
    }
  }

  //改变UICollectionView的偏移量
  proposedContentOffset.x += minDelta;
  return proposedContentOffset;
}

@property (nonatomic, strong) NSMutableArray *attrsArray;

- (NSMutableArray *)attrsArray
{
  if (_attrsArray == nil)
  {
    _attrsArray = [NSMutableArray array];
  }
  return _attrsArray;
}

- (void)prepareLayout
{
  [super prepareLayout];

  //移除所有旧的布局属性
  [self.attrsArray removeAllObjects];

  //获取元素的个数
  NSInteger count = [self.collectionView numberOfItemsInSection:0];
  //布局所有的元素
  for (NSInteger i = 0; i<count; i++)
  {
    NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
    //设置并获取indexPath位置元素的布局属性
    UICollectionViewLayoutAttributes *attrs = [self layoutAttributesForItemAtIndexPath:indexPath];
    //将indexPath位置元素的布局属性添加到所有布局属性数组中
    [self.attrsArray addObject:attrs];
  }
}

- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(nonnull NSIndexPath *)indexPath
{
  //获取元素的个数
  NSInteger count = [self.collectionView numberOfItemsInSection:0];

  /**设置圆心布局*/
  //设置圆形的半径
  CGFloat radius = 70;
  //圆心的位置
  CGFloat oX = self.collectionView.frame.size.width * 0.5;
  CGFloat oY = self.collectionView.frame.size.height * 0.5;
  //获取indexPath位置的元素的布局属性
  UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
  //设置尺寸
  attrs.size = CGSizeMake(50, 50);
  //设置位置
  if (count == 1)
  {
    attrs.center = CGPointMake(oX, oY);
  }
  else
  {
    CGFloat angle = (2 * M_PI / count) * indexPath.item;
    CGFloat centerX = oX + radius * sin(angle);
    CGFloat centerY = oY + radius * cos(angle);
    attrs.center = CGPointMake(centerX, centerY);
  }
  //返回indexPath位置元素的布局属性
  return attrs;
}

- (nullable NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
  //返回所有元素的布局属性
  return self.attrsArray;
}

@property (nonatomic, copy) NSString *imageName;

- (void)awakeFromNib
{
  //通过Layer设置边框
  self.imageView.layer.borderColor = [UIColor whiteColor].CGColor;
  self.imageView.layer.borderWidth = 6;
  self.imageView.layer.cornerRadius = 3;
}

- (void)setImageName:(NSString *)imageName
{
  _imageName = [imageName copy];
  self.imageView.image = [UIImage imageNamed:imageName];
}

/**所有的图片*/
@property (nonatomic, strong) NSMutableArray *imageNames;

- (NSMutableArray *)imageNames
{
  if (_imageNames == nil)
  {
    NSMutableArray *imageNames = [NSMutableArray array];
    for (NSInteger i = 0; i<20; i++)
    {
      NSString *imageName = [NSString stringWithFormat:@"%zd", i + 1];
      [imageNames addObject:imageName];
    }
    _imageNames = imageNames;
  }
  return _imageNames;
}

@property (nonatomic, weak) UICollectionView *collectionView;

- (void)setupCollectionView
{
  //设置frame
  CGFloat collectionViewW = self.view.bounds.size.width;
  CGFloat collectionViewH = 200;
  CGRect frame = CGRectMake(0, 150, collectionViewW, collectionViewH);

  //创建布局
  LYPCircleLayout *layout = [[LYPCircleLayout alloc] init];

  //创建collectionView
  UICollectionView *collectionView = [[UICollectionView alloc] initWithFrame:frame collectionViewLayout:layout];
  self.collectionView = collectionView;

  //设置collectionView的数据源和代理
  collectionView.dataSource = self;
  collectionView.delegate = self;

  //添加collectionView到控制器的view中
  [self.view addSubview:collectionView];
}

/**设置重用表示*/
static NSString *const ID = @"photo";
- (void)viewDidLoad
{
  [super viewDidLoad];

  [self setupCollectionView];

  //注册cell
  [self.collectionView registerNib:[UINib nibWithNibName:NSStringFromClass([LYPPhotoCell class]) bundle:nil] forCellWithReuseIdentifier:ID];
}

- (NSInteger)collectionView:(nonnull UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
  return self.imageNames.count;
}

- (UICollectionViewCell *)collectionView:(nonnull UICollectionView *)collectionView cellForItemAtIndexPath:(nonnull NSIndexPath *)indexPath
{
  //根据重用标示从缓存池中取出cell，若缓存池中没有，则自动创建
  LYPPhotoCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:ID forIndexPath:indexPath];
  //设置cell的imageName属性
  cell.imageName = self.imageNames[indexPath.item];

  //返回cell
  return cell;
}

- (void)collectionView:(nonnull UICollectionView *)collectionView didSelectItemAtIndexPath:(nonnull NSIndexPath *)indexPath
{
  //将图片名从数组中移除
  [self.imageNames removeObjectAtIndex:indexPath.item];
  //删除collectionView中的indexPath位置的元素
  [self.collectionView deleteItemsAtIndexPaths:@[indexPath]];
}

- (void)touchesBegan:(nonnull NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event
{
  //判断当前布局的种类
  if ([self.collectionView.collectionViewLayout isKindOfClass:[LYPLineLayout class]])
  {
    //流水布局，切换至圆形布局
    [self.collectionView setCollectionViewLayout:[[LYPCircleLayout alloc] init] animated:YES];
  } else
  {
    //圆形布局，切换至流水布局
    LYPLineLayout *layout = [[LYPLineLayout alloc] init];
    //设置元素的尺寸，若不设置，将使用自动计算尺寸
    layout.itemSize = CGSizeMake(130, 130);
    [self.collectionView setCollectionViewLayout:layout animated:YES]; 
  }
}