UICollectionView中的cell 左对齐

项目中使用UICollectionView做布局，会发现当某个section只有一个cell的时候cell会居中显示，而项目中都是居左显示，这就需要对UICollectionView的布局做些处理，首先创建一个类继承UICollectionViewFlowLayout

添加一个属性 cell之间的距离

@interface YTCustomFlowLayout : UICollectionViewFlowLayout
@property (nonatomic) CGFloat maximumInteritemSpacing;
@end

然后在.m文件中重写- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect方法如下：

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray * layoutAttributes_t = [super layoutAttributesForElementsInRect:rect];
    NSArray * layoutAttributes = [[NSArray alloc]initWithArray:layoutAttributes_t copyItems:YES];
    //用来临时存放一行的Cell数组
    NSMutableArray * layoutAttributesTemp = [[NSMutableArray alloc]init];
    for (NSUInteger index = 0; index < layoutAttributes.count ; index++) {
        
        UICollectionViewLayoutAttributes *currentAttr = layoutAttributes[index]; // 当前cell的位置信息
        UICollectionViewLayoutAttributes *previousAttr = index == 0 ? nil : layoutAttributes[index-1]; // 上一个cell 的位置信
        UICollectionViewLayoutAttributes *nextAttr = index + 1 == layoutAttributes.count ?
        nil : layoutAttributes[index+1];//下一个cell 位置信息
        
        //加入临时数组
        [layoutAttributesTemp addObject:currentAttr];
        
        
        CGFloat previousY = previousAttr == nil ? 0 : CGRectGetMaxY(previousAttr.frame);
        CGFloat currentY = CGRectGetMaxY(currentAttr.frame);
        CGFloat nextY = nextAttr == nil ? 0 : CGRectGetMaxY(nextAttr.frame);
        //如果当前cell是单独一行
        if (currentY != previousY && currentY != nextY){
            if ([currentAttr.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
                [layoutAttributesTemp removeAllObjects];
                
            }else if ([currentAttr.representedElementKind isEqualToString:UICollectionElementKindSectionFooter]){
                [layoutAttributesTemp removeAllObjects];
                
            }else{
                [self setCellFrameWith:layoutAttributesTemp];
            }
        }
        //如果下一个不cell在本行，则开始调整Frame位置
        else if( currentY != nextY) {
            [self setCellFrameWith:layoutAttributesTemp];
        }
    }
    return layoutAttributes;
}

-(void)setCellFrameWith:(NSMutableArray*)layoutAttributes{
    CGFloat nowWidth = 0.0;
    
    nowWidth = self.sectionInset.left;
    for (UICollectionViewLayoutAttributes * attributes in layoutAttributes) {
        CGRect nowFrame = attributes.frame;
        nowFrame.origin.x = nowWidth;
        attributes.frame = nowFrame;
        nowWidth += nowFrame.size.width + self.maximumInteritemSpacing;
    }
    
    [layoutAttributes removeAllObjects];
    
}

至此就可以居左显示cell了