UICollectionViewFlowLayout
1.直接设置FlowLayout对象
创建UICollectionViewLayout对象，通过设置UICollectionViewLayout对象属性的值可以设置item的基本布局，包括大小，间距，内边距、滚动方向等
UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
layout.minimumLineSpacing = 10;
layout.minimumInteritemSpacing = 10;
layout.scrollDirection = UICollectionViewScrollDirectionVertical;
layout.sectionInset = UIEdgeInsetsMake(10, 10, 10, 10);
2.自定义布局
当系统的布局无论怎么设置都无法满足需求的时候，我们就需要自定义布局，重写其计算布局的方法。
图片.png
图片.png
上图是系统默认布局，下图是自定义布局，可以注意到系统布局是将item两端对齐，间距根据剩余的宽度自己缩放，UICollectionViewFlowLayout的minimumInteritemSpacing属性设置的是最小间距，但如果要设置间距相等，系统布局就实现不了了。
3.下面通过两种方案实现
两种方案都是通过继承UICollectionViewFlowLayout，这两种方案本质上都是通过修改对应位置的item来实现的
第一种方案：重写layoutAttributesForElementsInRect:方法返回指定区域cellt的布局, 此方法会多次调用，为了更好的性能，在这个方法当中，我们使用的UICollectionViewLayoutAttributes最好是在prepareLayout已经布局好的信息。
1.//获取指定位置的cell的布局
- (NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
NSArray *originAttributes = [super layoutAttributesForElementsInRect:rect];
NSMutableArray *updatedAttributes = [NSMutableArray arrayWithArray:originAttributes];
for (UICollectionViewLayoutAttributes *attributes in originAttributes) {
if (!attributes.representedElementKind) {
NSUInteger index = [updatedAttributes indexOfObject:attributes];
updatedAttributes[index] = [self layoutAttributesForItemAtIndexPath:attributes.indexPath];
}
}
return updatedAttributes ;
}
2.重写layoutAttributesForItemAtIndexPath:方法返回指定indexPath的布局
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
UICollectionViewLayoutAttributes *currentAttributes = [super layoutAttributesForItemAtIndexPath:indexPath];
UIEdgeInsets sectionInset = self.sectionInset;
BOOL isFirstItemInSection = indexPath.item == 0;
CGFloat layoutWidth = CGRectGetWidth(self.collectionView.frame) - sectionInset.left - sectionInset.right;
if (isFirstItemInSection) {
[currentAttributes leftAlignFrameWithSectionInset:sectionInset];
return currentAttributes;
}
NSIndexPath *previousIndexPath = [NSIndexPath indexPathForItem:indexPath.item - 1 inSection:indexPath.section];
CGRect previousFrame = [self layoutAttributesForItemAtIndexPath:previousIndexPath].frame;
CGRect currentFrame = currentAttributes.frame;
CGFloat previousFrameRightPoint = previousFrame.origin.x + previousFrame.size.width;
CGRect strecthedCurrentFrame = CGRectMake(sectionInset.left, currentFrame.origin.y, layoutWidth, currentFrame.size.height);
//previousFrame和strecthedCurrentFrame不相交证明不在一行
BOOL isFirstItemInRow = !CGRectIntersectsRect(previousFrame, strecthedCurrentFrame);
if (isFirstItemInRow) {
[currentAttributes leftAlignFrameWithSectionInset:sectionInset];
return currentAttributes;
}
CGRect frame = currentAttributes.frame;
frame.origin.x = previousFrameRightPoint + self.minimumInteritemSpacing;
currentAttributes.frame = frame;
return currentAttributes;
}
第二种方案
1.重写prepareLayout，每次更新布局的时候collectionView都回先调用这个方法，为将要开始的更新做准备，此时会将计算好的布局存储起来
//重写其布局方法
- (void)prepareLayout {
[super prepareLayout];
NSMutableArray *layoutInfoArr = [NSMutableArray array];
//获取布局信息
NSInteger numberOfSections = [self.collectionView numberOfSections];
for (NSInteger section = 0; section < numberOfSections; section++) {
NSInteger numberOfItems = [self.collectionView numberOfItemsInSection:section];
NSMutableArray *subArr = [NSMutableArray arrayWithCapacity:numberOfItems];
for (NSInteger item = 0; item < numberOfItems; item++) {
NSIndexPath *indexPath = [NSIndexPath indexPathForItem:item inSection:section];
UICollectionViewLayoutAttributes *attributes = [self layoutAttributesForItemAtIndexPath:indexPath];
[subArr addObject:attributes];
}
[layoutInfoArr addObject:[subArr copy]];
}
self.attributesArray = [layoutInfoArr copy];
}
2.重写layoutAttributesForElementsInRect:方法返回指定区域的布局
//指出了与指定区域有交接的UICollectionViewLayoutAttributes对象放到一个数组中，然后返回
- (NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
NSMutableArray *layoutAttributesArr = [NSMutableArray array];
[self.attributesArray enumerateObjectsUsingBlock:^(NSArray * _Nonnull array, NSUInteger idx, BOOL * _Nonnull stop) {
[array enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
if (CGRectIntersectsRect(obj.frame, rect)) { //如果item在rect内
[layoutAttributesArr addObject:obj];
}
}];
}];
return layoutAttributesArr;
}
3.重写并调用layoutAttributesForItemAtIndexPath方法计算布局
//计算每一个item的布局
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
//indexPath对应的系统为我们计算的布局
UICollectionViewLayoutAttributes *oldAttributes = [super layoutAttributesForItemAtIndexPath:indexPath];
//创建一个我们期望的布局
UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
CGFloat itemX = self.sectionInset.left;//默认X值
CGFloat itemY = oldAttributes.frame.origin.y;//Y值直接用系统算的
CGSize itemSize = oldAttributes.size; //大小代理直接返回的
//同一行
BOOL line = oldAttributes.frame.origin.y == self.lastFrame.origin.y;
//无需换行&&indexpath.row != 0调整X值，(indexPath.row = 0)时self.lastFrame还未赋值
if (oldAttributes.frame.origin.x != itemX && indexPath.row != 0 && line) {
itemX = self.lastFrame.origin.x + self.lastFrame.size.width + self.minimumLineSpacing;
}
attributes.frame = CGRectMake(itemX, itemY, itemSize.width, itemSize.height);
//更新上一个item的位置
self.lastFrame = CGRectMake(itemX, itemY, itemSize.width, itemSize.height);
return attributes;
}
4.如果需要自定义分区头部和尾部可以重写下面两个方法，并在prepareLayout里面做相应的处理
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath;
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForDecorationViewOfKind:(NSString*)elementKind atIndexPath:(NSIndexPath *)indexPath;
demo:https://github.com/SunshineLily/CollectionViewFlowLayout
参考：https://blog.csdn.net/lg767201403/article/details/90518273
参考：https://www.jianshu.com/p/5ee9333644ed
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