UICollectionViewLayoutlayoutAttributesForElementsInRect和layoutAttributesForItemAtIndexPath

我正在实现自定义流程布局。它有两种主要方法来覆盖以确定单元格的位置：layoutAttributesForElementsInRect and layoutAttributesForItemAtIndexPath.

在我的代码中，layoutAttributesForElementsInRect被称为，但是layoutAttributesForItemAtIndexPath不是。什么决定哪个被调用？哪里有layoutAttributesForItemAtIndexPath接到电话吗？

layoutAttributesForElementsInRect:不一定会打电话layoutAttributesForItemAtIndexPath:.

事实上，如果你子类化UICollectionViewFlowLayout，流布局将准备布局并缓存结果属性。所以，当layoutAttributesForElementsInRect:被调用，它不会询问layoutAttributesForItemAtIndexPath:，但仅使用缓存的值。

如果您想确保布局属性始终根据您的布局进行修改，请为两者实现修饰符layoutAttributesForElementsInRect: and layoutAttributesForItemAtIndexPath::

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
  NSArray *attributesInRect = [super layoutAttributesForElementsInRect:rect];
  for (UICollectionViewLayoutAttributes *cellAttributes in attributesInRect) {
    [self modifyLayoutAttributes:cellAttributes];
  }
  return attributesInRect;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
  UICollectionViewLayoutAttributes *attributes = [super layoutAttributesForItemAtIndexPath:indexPath];
  [self modifyLayoutAttributes:attributes];
  return attributes;
}

- (void)modifyLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes
{
  // Adjust the standard properties size, center, transform etc.
  // Or subclass UICollectionViewLayoutAttributes and add additional attributes.
  // Note, that a subclass will require you to override copyWithZone and isEqual.
  // And you'll need to tell your layout to use your subclass in +(Class)layoutAttributesClass
}