iOS UICollectionView自定义流水布局(一)

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。

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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