UICollectionView实现的标签选择器

近来，在项目中需要实现一个类似兴趣标签的选择器。由于标签的文字长度不定，所以标签的显示长度就不定。为了实现效果，就使用了UICollectionView来实现了每行的标签数量不定、cell的宽度自适应的效果。先在此分享出来：

2017-09-07 update

现已支持多section效果和记录上一次选中的标签的效果，并解决UICollectionView在滑动过程中有些cell一会显示一会不显示的bug。bug详情见：UICollectionView滚动的时候会出现cell消失的情况，代码已更新到GitHub，欢迎star。

1、自适应UICollectionViewCell

这里只是在自适应UICollectionViewCell上放一个和UICollectionViewCell保持一样大小的按钮，当选中和取消选中时改变按钮的文字颜色和边框颜色：

#pragma mark---标签cell
@implementation YLTagsCollectionViewCell
-(instancetype)initWithFrame:(CGRect)frame
{
    if(self = [super initWithFrame:frame]){
        self.backgroundColor = [UIColor clearColor];
        _btn = [UIButton buttonWithType:UIButtonTypeCustom];
        //此处可以根据需要自己使用自动布局代码实现
        _btn.frame = CGRectMake(0, 0, frame.size.width, frame.size.height);
        _btn.backgroundColor = [UIColor whiteColor];
        _btn.titleLabel.font = [UIFont systemFontOfSize:14];
        _btn.layer.borderWidth = 1.f;
        _btn.layer.cornerRadius = frame.size.height/2.0;
        _btn.layer.masksToBounds = YES;
        [_btn setTitleColor:HEXCOLOR(0x666666) forState:UIControlStateNormal];
        _btn.layer.borderColor = HEXCOLOR(0xdddddd).CGColor;
        _btn.userInteractionEnabled = NO;
        [self.contentView addSubview:_btn];
    }
    return self;
}

-(void)layoutSubviews
{
    [super layoutSubviews];
    _btn.frame = CGRectMake(0, 0, self.contentView.frame.size.width, self.contentView.frame.size.height);
}

-(void)setSelected:(BOOL)selected
{
    [super setSelected:selected];
    _btn.layer.borderColor = selected?HEXCOLOR(0xffb400).CGColor:HEXCOLOR(0xdddddd).CGColor;
    [_btn setTitleColor:selected?HEXCOLOR(0xffb400):HEXCOLOR(0x666666) forState:UIControlStateNormal];
}

-(void)setHighlighted:(BOOL)highlighted
{
    [super setHighlighted:highlighted];
    _btn.layer.borderColor = highlighted?HEXCOLOR(0xffb400).CGColor:HEXCOLOR(0xdddddd).CGColor;
    [_btn setTitleColor:highlighted?HEXCOLOR(0xffb400):HEXCOLOR(0x666666) forState:UIControlStateNormal];
}

@end

2、UICollectionViewFlowLayout子类--YLWaterFlowLayout的实现

.h头文件

#import <UIKit/UIKit.h>

@class YLWaterFlowLayout;
@protocol  YLWaterFlowLayoutDelegate <NSObject>
/**通过代理获得每个cell的宽度*/
- (CGFloat)waterFlowLayout:(YLWaterFlowLayout *)layout 
widthAtIndexPath:(NSIndexPath *)indexPath;

@end

@interface YLWaterFlowLayout : UICollectionViewFlowLayout
@property (nonatomic,assign) id<YLWaterFlowLayoutDelegate> delegate;
@property(nonatomic,assign)CGFloat rowHeight;///< 固定行高

@end

.m文件

#import "YLWaterFlowLayout.h"

@interface YLWaterFlowLayout()
@property(nonatomic,strong)NSMutableArray *originxArray;
@property(nonatomic,strong)NSMutableArray *originyArray;
@end

@implementation YLWaterFlowLayout
#pragma mark - 初始化属性
- (instancetype)init {
    self = [super init];
    if (self) {
        [self setupLayout];
        _originxArray = [NSMutableArray array];
        _originyArray = [NSMutableArray array];
    }
    return self;
}

- (void)setupLayout
{
    self.minimumInteritemSpacing = 5;//同一行不同cell间距
    self.minimumLineSpacing = 5;//行间距
    self.headerReferenceSize = CGSizeMake(0, 50);//设置section header 固定高度，如果需要的话
    self.sectionInset = UIEdgeInsetsMake(10, 10, 10, 10);
    self.scrollDirection = UICollectionViewScrollDirectionVertical;
}

#pragma mark - 重写父类的方法，实现瀑布流布局
#pragma mark - 当尺寸有所变化时，重新刷新
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    return NO;
}

- (void)prepareLayout {
    [super prepareLayout];
}

#pragma mark - 所有cell和view的布局属性
//sectionheader sectionfooter decorationview collectionviewcell的属性都会走这个方法
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray *array = [super layoutAttributesForElementsInRect:rect];
    for(UICollectionViewLayoutAttributes *attrs in array){
        //类型判断
        if(attrs.representedElementCategory == UICollectionElementCategoryCell){
            UICollectionViewLayoutAttributes *theAttrs = [self layoutAttributesForItemAtIndexPath:attrs.indexPath];
            attrs.frame = theAttrs.frame;
        }
    }
    return array;
}

#pragma mark - 指定cell的布局属性
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CGFloat x = self.sectionInset.left;
    //如果有sectionheader需要加上sectionheader高度
    CGFloat y = self.headerReferenceSize.height + self.sectionInset.top;
    //判断获得前一个cell的x和y
    NSInteger preRow = indexPath.row - 1;
    if(preRow >= 0){
        if(_originyArray.count > preRow){
            x = [_originxArray[preRow]floatValue];
            y = [_originyArray[preRow]floatValue];
        }
        NSIndexPath *preIndexPath = [NSIndexPath indexPathForItem:preRow inSection:indexPath.section];
        CGFloat preWidth = [self.delegate waterFlowLayout:self widthAtIndexPath:preIndexPath];
        x += preWidth + self.minimumInteritemSpacing;
    }
    
    CGFloat currentWidth = [self.delegate waterFlowLayout:self widthAtIndexPath:indexPath];
    //保证一个cell不超过最大宽度
    currentWidth = MIN(currentWidth, self.collectionView.frame.size.width - self.sectionInset.left - self.sectionInset.right);
    if(x + currentWidth > self.collectionView.frame.size.width - self.sectionInset.right){
        //超出范围，换行
        x = self.sectionInset.left;
        y += _rowHeight + self.minimumLineSpacing;
    }
    // 创建属性
    UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    attrs.frame = CGRectMake(x, y, currentWidth, _rowHeight);
    _originxArray[indexPath.row] = @(x);
    _originyArray[indexPath.row] = @(y);
    return attrs;
}

#pragma mark - CollectionView的滚动范围
- (CGSize)collectionViewContentSize
{
    CGFloat width = self.collectionView.frame.size.width;
    __block CGFloat maxY = 0;
    [_originyArray enumerateObjectsUsingBlock:^(NSNumber *number, NSUInteger idx, BOOL * _Nonnull stop) {
        CGFloat y = [number floatValue];
        if (y > maxY) {
            maxY = y;
        }
    }];
    return CGSizeMake(width, maxY + _rowHeight + self.sectionInset.bottom);
}

@end

实现思路：在YLWaterFlowLayout中使用originxArray和originyArray两个个数组记录了每一个自定义YLTagsCollectionViewCell的位置x和y。

在 -(UICollectionViewLayoutAttributes *) layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath方法中通获得与当前YLTagsCollectionViewCell临近的“上一个YLTagsCollectionViewCell”的位置和尺寸信息，将上一个cell的x加上上一个cell的width来得到当前cell的x。同时还要判断当前cell的x+width是否会超越出屏幕右边缘，如果超出，则表明需要换行显示了，这时候就要修改y的值了。

3、效果图

大家可以参考这种思路来实现其他类似的标签选择视图。
demo地址YLTagsChooser