UICollectionViewLayout layoutAttributesForElementsInRect和layoutAttributesForItemAtIndexPath [英] UICollectionViewLayout layoutAttributesForElementsInRect and layoutAttributesForItemAtIndexPath

问题描述

我正在实施自定义流程布局。它有两种主要的方法来覆盖以确定单元格的位置： layoutAttributesForElementsInRect 和 layoutAttributesForItemAtIndexPath 。

在我的代码中，调用了 layoutAttributesForElementsInRect ，但 layoutAttributesForItemAtIndexPath 不是。什么决定了什么叫？ layoutAttributesForItemAtIndexPath 在哪里被调用？

解决方案

layoutAttributesForElementsInRect：不一定调用 layoutAttributesForItemAtIndexPath：。

事实上，如果你子类 UICollectionViewFlowLayout ，流布局将准备布局并缓存生成的属性。因此，当调用 layoutAttributesForElementsInRect：时，它不会询问 layoutAttributesForItemAtIndexPath：，而只是使用缓存的值。 / p>

如果要确保始终根据布局修改布局属性，请为 layoutAttributesForElementsInRect：和 layoutAttributesForItemAtIndexPath：：

   - （NSArray *）layoutAttributesForElementsInRect :( CGRect）rect 
 {
 NSArray * attributesInRect = [super layoutAttributesForElementsInRect：rect]; 
 for（UICollectionViewLayoutAttributes * attributesInRect中的cellAttributes）{
 [self modifyLayoutAttributes：cellAttributes]; 
} 
返回attributesInRect; 
} 
 
  - （UICollectionViewLayoutAttributes *）layoutAttributesForItemAtIndexPath：（NSIndexPath *）indexPath 
 {
 UICollectionViewLayoutAttributes * attributes = [super layoutAttributesForItemAtIndexPath：indexPath]; 
 [self modifyLayoutAttributes：attributes]; 
返回属性; 
} 
 
  - （void）modifyLayoutAttributes：（UICollectionViewLayoutAttributes *）attributes 
 {
 //调整标准属性大小，居中，变换等
 //或者子类UICollectionViewLayoutAttributes并添加其他属性。 
 //注意，子类将要求您覆盖copyWithZone和isEqual。 
 //你需要告诉你的布局在+（Class）中使用你的子类layoutAttributesClass 
} 
  




I'm implementing a custom flow layout. It has 2 main methods for overriding to determine placement of cells: layoutAttributesForElementsInRect and layoutAttributesForItemAtIndexPath.

In my code, layoutAttributesForElementsInRect is called, but layoutAttributesForItemAtIndexPath isn't. What determines which gets called? Where does layoutAttributesForItemAtIndexPath get called?

解决方案

layoutAttributesForElementsInRect: doesn't necessarily call layoutAttributesForItemAtIndexPath:.

In fact, if you subclass UICollectionViewFlowLayout, the flow layout will prepare the layout and cache the resulting attributes. So, when layoutAttributesForElementsInRect: is called, it won't ask layoutAttributesForItemAtIndexPath:, but just uses the cached values.

If you want to ensure that the layout attributes are always modified according to your layout, implement a modifier for both 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
}

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