https://github.com/LeonLeeboy/LBCollectionViewLayout (一个collectionViewLayout的asset )。

重写UICollectionViewLayout 的几个必不可少的步骤。

1. prepareLayout

 一般初始化（必要的）数据类型。

2. collecitonViewContentSize

UICollecitonView  说到底是UIScrollView ， 需要contentSize。

3. layoutAttributesForElementsInRect

Implement -layoutAttributesForElementsInRect: to return layout attributes for for supplementary or decoration views, or to perform layout in an as-needed-on-screen fashion.

4. layoutAttributesForItemAtIndexPath

You need to implement it, even though it won't get called in the "default" scenario.

UICollectionViewLayout 重写方法详细解

@interface UICollectionViewLayout (UISubclassingHooks)

#if UIKIT_DEFINE_AS_PROPERTIES
@property(class, nonatomic, readonly) Class layoutAttributesClass; // override this method to provide a custom class to be used when instantiating instances of UICollectionViewLayoutAttributes
@property(class, nonatomic, readonly) Class invalidationContextClass NS_AVAILABLE_IOS(7_0); // override this method to provide a custom class to be used for invalidation contexts
#else
+ (Class)layoutAttributesClass; // override this method to provide a custom class to be used when instantiating instances of UICollectionViewLayoutAttributes
+ (Class)invalidationContextClass NS_AVAILABLE_IOS(7_0); // override this method to provide a custom class to be used for invalidation contexts
#endif

// The collection view calls -prepareLayout once at its first layout as the first message to the layout instance.
// The collection view calls -prepareLayout again after layout is invalidated and before requerying the layout information.
// Subclasses should always call super if they override.
- (void)prepareLayout;

// UICollectionView calls these four methods to determine the layout information.
// Implement -layoutAttributesForElementsInRect: to return layout attributes for for supplementary or decoration views, or to perform layout in an as-needed-on-screen fashion.
// Additionally, all layout subclasses should implement -layoutAttributesForItemAtIndexPath: to return layout attributes instances on demand for specific index paths.
// If the layout supports any supplementary or decoration view types, it should also implement the respective atIndexPath: methods for those types.
- (nullable NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect; // return an array layout attributes instances for all the views in the given rect
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath;
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath;
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForDecorationViewOfKind:(NSString*)elementKind atIndexPath:(NSIndexPath *)indexPath;

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds; // return YES to cause the collection view to requery the layout for geometry information
- (UICollectionViewLayoutInvalidationContext *)invalidationContextForBoundsChange:(CGRect)newBounds NS_AVAILABLE_IOS(7_0);

- (BOOL)shouldInvalidateLayoutForPreferredLayoutAttributes:(UICollectionViewLayoutAttributes *)preferredAttributes withOriginalAttributes:(UICollectionViewLayoutAttributes *)originalAttributes NS_AVAILABLE_IOS(8_0);
- (UICollectionViewLayoutInvalidationContext *)invalidationContextForPreferredLayoutAttributes:(UICollectionViewLayoutAttributes *)preferredAttributes withOriginalAttributes:(UICollectionViewLayoutAttributes *)originalAttributes NS_AVAILABLE_IOS(8_0);

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity; // return a point at which to rest after scrolling - for layouts that want snap-to-point scrolling behavior
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset NS_AVAILABLE_IOS(7_0); // a layout can return the content offset to be applied during transition or update animations

#if UIKIT_DEFINE_AS_PROPERTIES
@property(nonatomic, readonly) CGSize collectionViewContentSize; // Subclasses must override this method and use it to return the width and height of the collection view’s content. These values represent the width and height of all the content, not just the content that is currently visible. The collection view uses this information to configure its own content size to facilitate scrolling.
#else
- (CGSize)collectionViewContentSize; // Subclasses must override this method and use it to return the width and height of the collection view’s content. These values represent the width and height of all the content, not just the content that is currently visible. The collection view uses this information to configure its own content size to facilitate scrolling.
#endif

@property (nonatomic, readonly) UIUserInterfaceLayoutDirection developmentLayoutDirection; // Default implementation returns the layout direction of the main bundle's development region; FlowLayout returns leftToRight. Subclasses may override this to specify the implementation-time layout direction of the layout.
@property(nonatomic, readonly) BOOL flipsHorizontallyInOppositeLayoutDirection; // Base implementation returns false. If your subclass’s implementation overrides this property to return true, a UICollectionView showing this layout will ensure its bounds.origin is always found at the leading edge, flipping its coordinate system horizontally if necessary.

@end

我们可能会遇到的问题

• 我们重写UICOllectionViewLayout , 会遇到cellForItemAtIndex 方法不调用。

一般情况下 都是 第三步里 出错，layoutAttributesInRect 里 没有进行实现，如果出现问题，大多数情况下是在LayoutAttributesInRect里面。

• UICollecitonViewLayoutAttributes 的初始化报错。

我们初始化这个NSObject类的时候，我们只能用类方法进行初始化。 用 alloc／init 会报elementIsnill 错误。

• when we subClass UICollectionViewLayout ,layoutAttributes doesn't called in some case (default)

layoutAttributesForItemAtIndexPath: is called in different scenarii, when you add new cells to the collection view, when you change the layout, when you reload specific index paths, etc.

You need to implement it, even though it won't get called in the "default" scenario.

demo 传送门：https://www.jianshu.com/p/1da2be9cc902