UICollectionView详解（二）：自定义UICollectionViewLayout

@property (nonatomic) CGRect frame;
@property (nonatomic) CGPoint center;
@property (nonatomic) CGSize size;
@property (nonatomic) CATransform3D transform3D;
@property (nonatomic) CGRect bounds;
@property (nonatomic) CGAffineTransform transform ;
@property (nonatomic) CGFloat alpha;
@property (nonatomic) NSInteger zIndex; // default is 0
@property (nonatomic, getter=isHidden) BOOL hidden; 
@property (nonatomic, strong) NSIndexPath *indexPath;

#import <UIKit/UIKit.h>
@interface My_1Layout : UICollectionViewLayout
@end

#import "My_1Layout.h"

@interface My_1Layout()
{
    UIEdgeInsets _edgeInset;//内边距
    CGFloat _lineSpacing;//行间距
    CGFloat _columnsSpacing;//列间距
    NSInteger _columnsNum;//列数
    NSMutableArray *_columnsHeightArray;//用来存放所有列的高度
    CGFloat _maxHeight;//collectionContent最大高度
}
@property (nonatomic,strong) NSMutableArray *attributesArray;//用来存放所有的cell的布局

@end

@implementation My_1Layout

- (instancetype)init{
    if ([super init]) {
        _edgeInset = UIEdgeInsetsMake(5, 10, 5, 10);
        _lineSpacing = 10;
        _columnsSpacing = 10;
        _columnsNum = 3;
        _maxHeight = _edgeInset.top;
        _columnsHeightArray = [NSMutableArray new];
        _columnsHeightArray = [NSMutableArray arrayWithCapacity:_columnsNum];
        }
    return self;
}

- (void)prepareLayout{
    /**
     切记，一定要先调用父类的prepareLayout
     */
    [super prepareLayout];
    
    [_columnsHeightArray removeAllObjects];
    for (int i = 0; i < _columnsNum ; i ++) {
        [_columnsHeightArray addObject:[NSNumber numberWithInteger:_edgeInset.top]];
    }
    
    [self.attributesArray removeAllObjects];
    /**
     调用layoutAttributesForItemAtIndexPath：方法，根据collectionView中cell的个数，使用for循环，创建对应个数的cell的attributes，并存放到_columnsHeightArray数组中（也可以将该过程放到layoutAttributesForElementsInRect：中去执行）
     */
    NSInteger cellNum = [self.collectionView numberOfItemsInSection:0];
    for (int i = 0; i < cellNum; i ++) {
        NSIndexPath*indexPath=[NSIndexPath indexPathForItem:i inSection:0];
        UICollectionViewLayoutAttributes *attri = [self layoutAttributesForItemAtIndexPath:indexPath];
        [self.attributesArray addObject:attri];
    }
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds{
    return YES;
}

-(NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect{
    /**
     直接返回之前存放好的所有cell的attributes（也可以将prepareLayout方法中for循环创建attributes的过程放到这里执行）
     */
    return self.attributesArray;
}

- (UICollectionViewLayoutAttributes*)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath{
    UICollectionViewLayoutAttributes*attributes=[UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    
    CGFloat cellW = (kScreenWidth-_edgeInset.left-_edgeInset.right-(_columnsNum-1)*_columnsSpacing)/_columnsNum;
    CGFloat cellH = indexPath.item%2==0?160:125;
    // 应该添加cell的列号
    NSInteger minHeightColumn = 0;
    // 应该添加cell的列的高度
    CGFloat minColumnHeight = [_columnsHeightArray[minHeightColumn] doubleValue];
    //循环  获取最小的列的高度和该列的列号
    for (int i = 1; i < _columnsHeightArray.count; i ++ ) {
        CGFloat tempH = [_columnsHeightArray[i] floatValue];
        if (minColumnHeight > tempH) {
            minColumnHeight = tempH;
            minHeightColumn = i;
        }
    }
    //为高度最小的列添加cell
    CGFloat cellY = [_columnsHeightArray[minHeightColumn] floatValue]+_lineSpacing;
    CGFloat cellX = _edgeInset.left + minHeightColumn * (cellW + _columnsSpacing);
    attributes.frame = CGRectMake(cellX, cellY, cellW, cellH);
    //保存最新的高度
    CGFloat newHeight = cellY+cellH;
    [_columnsHeightArray replaceObjectAtIndex:minHeightColumn withObject:[NSNumber numberWithInteger:newHeight]];
    //返回布局信息
    return attributes;
}

- (CGSize)collectionViewContentSize{
    //根据最高的列 设置collectionContentSize
    [_columnsHeightArray enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
        CGFloat maxHeight = [_columnsHeightArray[idx]floatValue];
        if (maxHeight > _maxHeight) {
            _maxHeight = maxHeight;
        }
    }];
    return CGSizeMake(kScreenWidth, _maxHeight);
}

- (NSMutableArray *)attributesArray{
    if (!_attributesArray) {
        _attributesArray = [NSMutableArray new];
    }
    return _attributesArray;
}

My_1Layout *layout = [[My_1Layout alloc]init];
    UICollectionView* collectionView = [[UICollectionView alloc]initWithFrame:CGRectMake(0, 64, kScreenWidth, kScreenHeight-64) collectionViewLayout:layout];

/**
 *prepareLayout会频繁调用，所以只做一些简单的初始化操作
 */
- (void)prepareLayout{
    [super prepareLayout];
    // 水平滚动
    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    //设置cell大小（可放到外面的控制器中）
    self.itemSize = CGSizeMake(200, 200);
    //设置内边距
    CGFloat inset = (self.collectionView.frame.size.width - self.itemSize.width) * 0.5;
    self.sectionInset = UIEdgeInsetsMake(0, inset, 0, inset);
}

/**
 *设置为YES,collectionView的显示范围发生变化时，就要刷新布局
 */
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds{
    return YES;
}

/**
 *返回所有cell的布局属性
 */
- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect{
    //获取UICollectionViweFlowLayout已经做好的布局
    NSArray *attrbutesArray = [super layoutAttributesForElementsInRect:rect];
    //计算collectionView可视范围的中心点所对应的collectionView的x值
    CGFloat centerX = self.collectionView.contentOffset.x + self.collectionView.frame.size.width * 0.5;
    //以每个cell中心点到centerX的距离为参考，对cell进行缩放
    for (UICollectionViewLayoutAttributes *attributes in attrbutesArray) {
        //cell的中心点到centerX的距离
        CGFloat distance = ABS(attributes.center.x - centerX);
        //根据distance计算cell的缩放比例
        CGFloat scale = 1 - (distance / self.collectionView.frame.size.width);
        //设置缩放比例
        attributes.transform3D = CATransform3DMakeScale(scale, scale, scale);
    }
    // 返回调整之后的布局属性数组
    return attrbutesArray;
}

/**
 * 在重新刷新布局时调用
 */
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes *attr = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    return attr;
}

/**
 * collectionView停止滑动时调用，可以手动设置collectionView的偏移量
 * proposedContentOffset collectionView原本的偏移量
 */
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity {
    //计算出最终显示的矩形框
    CGRect endRect;
    endRect.origin.x = proposedContentOffset.x;
    endRect.origin.y = 0;
    endRect.size = CGSizeMake(self.collectionView.frame.size.width, self.collectionView.frame.size.height);
    
    //获得所有cell的布局属性
    NSArray *attributesArr = [super layoutAttributesForElementsInRect:endRect];
    
    //计算collectionView最中心点的x值
    CGFloat centerX = proposedContentOffset.x + self.collectionView.frame.size.width * 0.5;
    //循环所有的布局属性，得到距离中心点最近的cell到中心点的距离
    CGFloat minDelta = MAXFLOAT;
    for (UICollectionViewLayoutAttributes *attr in attributesArr) {
        if(ABS(minDelta) > ABS(attr.center.x - centerX)) {
            minDelta = attr.center.x - centerX;
        }
    }
    //原来偏移量的x+距离中心点最近的cell到中心点的距离 将其设置为偏移量,该cell就会到中心点
    proposedContentOffset.x += minDelta;
    return proposedContentOffset;
}