UICollectionViewCell 宽度自适应/自动居左

@implementation PolyCell

- (instancetype)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
        self.titleLabel = [[UILabel alloc] init];
        self.titleLabel.textAlignment =NSTextAlignmentCenter;
        self.titleLabel.textColor = color333333;
        self.titleLabel.backgroundColor = colorE3E3E3;
        self.titleLabel.font = kFont(13);
        self.titleLabel.cornerRadius = 5;
        [self.contentView addSubview:self.titleLabel];
        
        [self.titleLabel mas_makeConstraints:^(MASConstraintMaker *make) {
            make.edges.equalTo(self);
        }];
    }
    return self;
}

#pragma mark — 实现自适应文字宽度的关键步骤:item的layoutAttributes
- (UICollectionViewLayoutAttributes *)preferredLayoutAttributesFittingAttributes:(UICollectionViewLayoutAttributes *)layoutAttributes{
    
    UICollectionViewLayoutAttributes *attributes = [super preferredLayoutAttributesFittingAttributes:layoutAttributes];
    CGRect rect = [self.titleLabel.text boundingRectWithSize:CGSizeMake(CGFLOAT_MAX, cellHeight) options:NSStringDrawingTruncatesLastVisibleLine|NSStringDrawingUsesFontLeading |NSStringDrawingUsesLineFragmentOrigin attributes:@{NSFontAttributeName: self.titleLabel.font} context:nil];
    rect.size.width +=10;
    rect.size.height+=10;
    attributes.frame = rect;
    return attributes;
}
@end

- (UICollectionView *)collectionView{
    if (!_collectionView) {
        UICollectionViewFlowLayout  *layout = [[UICollectionViewFlowLayout alloc] init];
        layout.minimumLineSpacing = 10;
        layout. minimumInteritemSpacing  = 10;
        layout.scrollDirection =  UICollectionViewScrollDirectionVertical;

        // 设置item预估的大小
        layout.estimatedItemSize = CGSizeMake(20, cellHeight);
        
        _collectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.width, [UIScreen mainScreen].bounds.size.height) collectionViewLayout:layout];
        _collectionView.backgroundColor = [UIColor whiteColor];
        _collectionView.delegate = self;
        _collectionView.dataSource = self;
        [_collectionView registerClass:[PolyCell class] forCellWithReuseIdentifier:@"PolyCell"];
        
        //collectionView的item只剩下一个时自动左对齐
        SEL sel = NSSelectorFromString(@"_setRowAlignmentsOptions:");
        if ([_collectionView.collectionViewLayout respondsToSelector:sel]) {
            ((void(*)(id,SEL,NSDictionary*)) objc_msgSend)(_collectionView.collectionViewLayout,sel, @{@"UIFlowLayoutCommonRowHorizontalAlignmentKey":@(NSTextAlignmentLeft),@"UIFlowLayoutLastRowHorizontalAlignmentKey" : @(NSTextAlignmentLeft),@"UIFlowLayoutRowVerticalAlignmentKey" : @(NSTextAlignmentCenter)});
        }
        
    }
    return _collectionView;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
    
    PolyCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"PolyCell" forIndexPath:indexPath];
    cell.titleLabel.text = self.tags[indexPath.row];
    return cell;
}