转自:https://blog.csdn.net/QiuHaoZhou/article/details/54582741
需求：固定高度一个区域，里面左右分页显示很多个图标，在每一页中的图标先从左往右排，排满后再从上往下排。这一页排满后排下一页。
自定义的UICollectionViewLayout
UICollectionViewLayout的功能为向UICollectionView提供布局信息，不仅包括cell的布局信息，也包括追加视图和装饰视图的布局信息。实现一个自定义layout的常规做法是继承UICollectionViewLayout类，然后重载下列方法：
-(CGSize)collectionViewContentSize
- 返回collectionView的内容的尺寸
-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
- 返回rect中的所有的元素的布局属性
- 返回的是包含UICollectionViewLayoutAttributes的NSArray
UICollectionViewLayoutAttributes可以是cell，追加视图或装饰视图的信息，通过不同的UICollectionViewLayoutAttributes初始化方法可以得到不同类型的UICollectionViewLayoutAttributes：
- layoutAttributesForCellWithIndexPath:
- layoutAttributesForSupplementaryViewOfKind:withIndexPath:
- layoutAttributesForDecorationViewOfKind:withIndexPath:
-(UICollectionViewLayoutAttributes )layoutAttributesForItemAtIndexPath:(NSIndexPath )indexPath
- 返回对应于indexPath的位置的cell的布局属性
-(UICollectionViewLayoutAttributes )layoutAttributesForSupplementaryViewOfKind:(NSString )kind atIndexPath:(NSIndexPath *)indexPath
- 返回对应于indexPath的位置的追加视图的布局属性，如果没有追加视图可不重载
-(UICollectionViewLayoutAttributes * )layoutAttributesForDecorationViewOfKind:(NSString)decorationViewKind atIndexPath:(NSIndexPath )indexPath
- 返回对应于indexPath的位置的装饰视图的布局属性，如果没有装饰视图可不重载
-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
- 当边界发生改变时，是否应该刷新布局。如果YES则在边界变化（一般是scroll到其他地方）时，将重新计算需要的布局信息。
#import <UIKit/UIKit.h>
@interface BCCollectionViewHorizontalLayout : UICollectionViewFlowLayout
// 一行中 cell 的个数
@property (nonatomic,assign) NSUInteger itemCountPerRow;
// 一页显示多少行
@property (nonatomic,assign) NSUInteger rowCount;
@end
#import "BCCollectionViewHorizontalLayout.h"
@interface BCCollectionViewHorizontalLayout () <UICollectionViewDelegateFlowLayout>
@property (strong, nonatomic) NSMutableArray *allAttributes;
@end
@implementation BCCollectionViewHorizontalLayout
-(instancetype)init
{
if (self = [super init])
{
}
return self;
}
- (void)prepareLayout
{
[super prepareLayout];
self.allAttributes = [NSMutableArray array];
NSInteger sections = [self.collectionView numberOfSections];
for (int i = 0; i < sections; i++)
{
NSMutableArray * tmpArray = [NSMutableArray array];
NSUInteger count = [self.collectionView numberOfItemsInSection:i];
for (NSUInteger j = 0; j<count; j++) {
NSIndexPath *indexPath = [NSIndexPath indexPathForItem:j inSection:i];
UICollectionViewLayoutAttributes *attributes = [self layoutAttributesForItemAtIndexPath:indexPath];
[tmpArray addObject:attributes];
}
[self.allAttributes addObject:tmpArray];
}
}
- (CGSize)collectionViewContentSize
{
return [super collectionViewContentSize];
}
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
NSUInteger item = indexPath.item;
NSUInteger x;
NSUInteger y;
[self targetPositionWithItem:item resultX:&x resultY:&y];
NSUInteger item2 = [self originItemAtX:x y:y];
NSIndexPath *theNewIndexPath = [NSIndexPath indexPathForItem:item2 inSection:indexPath.section];
UICollectionViewLayoutAttributes *theNewAttr = [super layoutAttributesForItemAtIndexPath:theNewIndexPath];
theNewAttr.indexPath = indexPath;
return theNewAttr;
}
- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
NSArray *attributes = [super layoutAttributesForElementsInRect:rect];
NSMutableArray *tmp = [NSMutableArray array];
for (UICollectionViewLayoutAttributes *attr in attributes) {
for (NSMutableArray *attributes in self.allAttributes)
{
for (UICollectionViewLayoutAttributes *attr2 in attributes) {
if (attr.indexPath.item == attr2.indexPath.item) {
[tmp addObject:attr2];
break;
}
}
}
}
return tmp;
}
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
return YES;
}
// 根据 item 计算目标item的位置
// x 横向偏移 y 竖向偏移
- (void)targetPositionWithItem:(NSUInteger)item
resultX:(NSUInteger *)x
resultY:(NSUInteger *)y
{
NSUInteger page = item/(self.itemCountPerRow*self.rowCount);
NSUInteger theX = item % self.itemCountPerRow + page * self.itemCountPerRow;
NSUInteger theY = item / self.itemCountPerRow - page * self.rowCount;
if (x != NULL) {
*x = theX;
}
if (y != NULL) {
*y = theY;
}
}
// 根据偏移量计算item
- (NSUInteger)originItemAtX:(NSUInteger)x
y:(NSUInteger)y
{
NSUInteger item = x * self.rowCount + y;
return item;
}
@end