override func viewDidLoad() {
        super.viewDidLoad()
    
        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = .vertical //.horizontal
        layout.minimumLineSpacing = 0
        layout.minimumInteritemSpacing = 0
        popularServicesCollectionView.setCollectionViewLayout(layout, animated: true)
  }

extension ViewController : UICollectionViewDelegateFlowLayout {
  
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return UIEdgeInsets(top: 1.0, left: 1.0, bottom: 1.0, right: 1.0)//here your custom value for spacing
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let lay = collectionViewLayout as! UICollectionViewFlowLayout
    let widthPerItem = collectionView.frame.width / 2 - lay.minimumInteritemSpacing
    
    return CGSize(width:widthPerItem, height:100)
}
}

```flow
st=>start: 开始框
op=>operation: 处理框
cond=>condition: 判断框(是或否?)
sub1=>subroutine: 子流程
io=>inputoutput: 输入输出框
e=>end: 结束框
st(right)->op(right)->cond
cond(yes)->io(bottom)->e
cond(no)->sub1(right)->op
```

```flow
st=>start: 开始框
op=>operation: 处理框
cond=>condition: 判断框(是或否?)
sub1=>subroutine: 子流程
io=>inputoutput: 输入输出框
e=>end: 结束框
st->op->cond
cond(yes)->io->e
cond(no)->sub1(right)->op
```