第二十九篇、UICollectionView瀑布流

1.实现思路

　　>第一种方案：UIScrollView 镶嵌三个UITableView (不推荐使用)

　　>第二种方案：UIScrollView 镶嵌UIImageView （需要解决循环利用的问题）

　　>第三种方案：UICollectionView

2.基本骨架Layout:需要重写的方法

）- (void)prepareLayout
）- (CGSize)collectionViewContentSize
）- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
）- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect

.h

#import <UIKit/UIKit.h>

@class JQWaterflowLayout;

@protocol JQWaterflowLayoutDelegate <NSObject>
@required
- (CGFloat)waterflowLayout:(JQWaterflowLayout *)waterflowLayout heightForItemAtIndex:(NSUInteger)index itemWidth:(CGFloat)itemWidth;

@optional
- (CGFloat)columnCountInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
- (CGFloat)columnMarginInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
- (CGFloat)rowMarginInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
- (UIEdgeInsets)edgeInsetsInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
@end

@interface JQWaterflowLayout : UICollectionViewLayout
/** 代理 */
@property (nonatomic, weak) id<JQWaterflowLayoutDelegate> delegate;
@end

.m

#import "XMGWaterflowLayout.h"

/** 默认的列数 */
static const NSInteger JQDefaultColumnCount = ;
/** 每一列之间的间距 */
static const CGFloat JQDefaultColumnMargin = ;
/** 每一行之间的间距 */
static const CGFloat JQDefaultRowMargin = ;
/** 边缘间距 */
static const UIEdgeInsets JQDefaultEdgeInsets = {, , , };

@interface JQWaterflowLayout()
/** 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;

- (CGFloat)rowMargin;
- (CGFloat)columnMargin;
- (NSInteger)columnCount;
- (UIEdgeInsets)edgeInsets;
@end

@implementation JQWaterflowLayout

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

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

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

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

#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 = ;

    // 清除以前计算的所有高度
    [self.columnHeights removeAllObjects];
    for (NSInteger i = ; 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:];
    for (NSInteger i = ; i < count; i++) {
        // 创建位置
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:];
        // 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 - ) * 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 = ;
    CGFloat minColumnHeight = [self.columnHeights[] doubleValue];
    for (NSInteger i = ; 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(, self.contentHeight + self.edgeInsets.bottom);
}

@end