14.collectionView

1.你要继承自CollectionViewLayout这个家伙做事情，一般要实现下面四个方法：

image.png

2 重写一些父类方法：
重写prepareLayout方法
作用:在这个方法做一些初始化操作。
重写layoutAttributesForElementsInRect方法
作用：返回当前屏幕视图框内item的属性，可以直接返回所有item属性,指定区域的cell布局对象.定新的区域的时候调用
重写collectionViewContentSize方法
作用:决定collectionView的可滚动范围

#import "CYWaterFlowLayout.h"

/** 默认的列数 */
static const NSInteger CYDefaultColumnCount = 3;
/** 每一列之间的间距 */
static const CGFloat CYDefaultColumnMargin = 10;
/** 每一行之间的间距 */
static const CGFloat CYDefaultRowMargin = 10;
/** 边缘间距 */
static const UIEdgeInsets CYDefaultEdgeInsets = {10, 10, 10, 10};

@interface CYWaterFlowLayout()
/** 存放所有cell的布局属性 */
@property (nonatomic, strong) NSMutableArray *attrsArray;
/** 存放所有列的当前高度 */
@property (nonatomic, strong) NSMutableArray *columnHeights;
/** 内容的高度 */
@property (nonatomic, assign) CGFloat contentHeight;

// 声明一下，点语法就能调用了
- (CGFloat)rowMargin;
- (CGFloat)columnMargin;
- (NSInteger)columnCount;
- (UIEdgeInsets)edgeInsets;
@end

@implementation CYWaterFlowLayout

#pragma mark - 常见数据处理
- (CGFloat)rowMargin
{
    if ([self.delegate respondsToSelector:@selector(rowMarginInWaterflowLayout:)]) {
        return [self.delegate rowMarginInWaterflowLayout:self];
    } else {
        return CYDefaultRowMargin;
    }
}

- (CGFloat)columnMargin
{
    if ([self.delegate respondsToSelector:@selector(columnMarginInWaterflowLayout:)]) {
        return [self.delegate columnMarginInWaterflowLayout:self];
    } else {
        return CYDefaultColumnMargin;
    }
}

- (NSInteger)columnCount
{
    if ([self.delegate respondsToSelector:@selector(columnCountInWaterflowLayout:)]) {
        return [self.delegate columnCountInWaterflowLayout:self];
    } else {
        return CYDefaultColumnCount;
    }
}

- (UIEdgeInsets)edgeInsets
{
    if ([self.delegate respondsToSelector:@selector(edgeInsetsInWaterflowLayout:)]) {
        return [self.delegate edgeInsetsInWaterflowLayout:self];
    } else {
        return CYDefaultEdgeInsets;
    }
}

#pragma mark - 懒加载
- (NSMutableArray *)columnHeights
{
    if (!_columnHeights) {
        _columnHeights = [NSMutableArray array];
    }
    return _columnHeights;
}

- (NSMutableArray *)attrsArray
{
    if (!_attrsArray) {
        _attrsArray = [NSMutableArray array];
    }
    return _attrsArray;
}

/**
 * 初始化
 */
- (void)prepareLayout
{
    [super prepareLayout];

    self.contentHeight = 0;

    // 清除以前计算的所有高度
    [self.columnHeights removeAllObjects];
    for (NSInteger i = 0; i < self.columnCount; i++) {
        [self.columnHeights addObject:@(self.edgeInsets.top)];
    }

    // 清除之前所有的布局属性
    [self.attrsArray removeAllObjects];
    // 开始创建每一个cell对应的布局属性
    NSInteger count = [self.collectionView numberOfItemsInSection:0];
    for (NSInteger i = 0; i < count; i++) {
        // 创建位置
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
        // 获取indexPath位置cell对应的布局属性
        UICollectionViewLayoutAttributes *attrs = [self layoutAttributesForItemAtIndexPath:indexPath];
        [self.attrsArray addObject:attrs];
    }
}

/**
 * 决定cell的排布
 */
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    return self.attrsArray;
}

/**
 * 返回indexPath位置cell对应的布局属性
 */
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    // 创建布局属性
    UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

    // collectionView的宽度
    CGFloat collectionViewW = self.collectionView.frame.size.width;

    // 设置布局属性的frame
    CGFloat w = (collectionViewW - self.edgeInsets.left - self.edgeInsets.right - (self.columnCount - 1) * self.columnMargin) / self.columnCount;
    CGFloat h = [self.delegate waterflowLayout:self heightForItemAtIndex:indexPath.item itemWidth:w];

    // 找出高度最短的那一列
    NSInteger destColumn = 0;
    CGFloat minColumnHeight = [self.columnHeights[0] doubleValue];
    for (NSInteger i = 1; i < self.columnCount; i++) {
        // 取得第i列的高度
        CGFloat columnHeight = [self.columnHeights[i] doubleValue];

        if (minColumnHeight > columnHeight) {
            minColumnHeight = columnHeight;
            destColumn = i;
        }
    }

    CGFloat x = self.edgeInsets.left + destColumn * (w + self.columnMargin);
    CGFloat y = minColumnHeight;
    if (y != self.edgeInsets.top) {
        y += self.rowMargin;
    }
    attrs.frame = CGRectMake(x, y, w, h);

    // 更新最短那列的高度
    self.columnHeights[destColumn] = @(CGRectGetMaxY(attrs.frame));

    // 记录内容的高度
    CGFloat columnHeight = [self.columnHeights[destColumn] doubleValue];
    if (self.contentHeight < columnHeight) {
        self.contentHeight = columnHeight;
    }
    return attrs;
}

- (CGSize)collectionViewContentSize
{
    //    CGFloat maxColumnHeight = [self.columnHeights[0] doubleValue];
    //    for (NSInteger i = 1; i < self.columnCount; i++) {
    //        // 取得第i列的高度
    //        CGFloat columnHeight = [self.columnHeights[i] doubleValue];
    //
    //        if (maxColumnHeight < columnHeight) {
    //            maxColumnHeight = columnHeight;
    //        }
    //    }
    return CGSizeMake(0, self.contentHeight + self.edgeInsets.bottom);
}

@end

瀑布流小框架

https://www.jianshu.com/p/83f2d6ac7e68

CollectionView详解

collectionView瀑布流的设计与实现