UICollectionViewLeftAlignedLayout是第三方的左对齐布局管理类，其继承自UICollectionViewFlowLayout，使用其可以方便的进行左对齐的瀑布流界面布局。

    UICollectionViewLeftAlignedLayout的git地址如下：https://github.com/mokagio/UICollectionViewLeftAlignedLayout。

    使用示例如下：

import <UICollectionViewLeftAlignedLayout.h>

@interface ViewController ()<UICollectionViewDataSource,UICollectionViewDelegateFlowLayout>
@property(nonatomic,strong)UICollectionView * collectionView;
@end

@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  UICollectionViewLeftAlignedLayout* layout = [[UICollectionViewLeftAlignedLayout alloc]init];
  self.collectionView = [[UICollectionView alloc]initWithFrame:self.view.frame collectionViewLayout:layout];
  self.collectionView.dataSource = self;
  self.collectionView.delegate=self;
  [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellID"];
  [self.view addSubview:self.collectionView];

}

-(CGSize)collectionView:(UICollectionView )collectionView layout:(UICollectionViewLayout )collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath{

return CGSizeMake(arc4random()%100+50, 100);

}

-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView{

return 1;

}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{

return 10;

}
-(UICollectionViewCell )collectionView:(UICollectionView )collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{

UICollectionViewCell * cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cellID" forIndexPath:indexPath];
cell.backgroundColor = [UIColor colorWithRed:arc4random()%255/255.0 green:arc4random()%255/255.0 blue:arc4random()%255/255.0 alpha:1];
return cell;

}
@end