这是ViewController中CollectionView的代码，但是它不会使Cells处于CollectionView的中心，结果如下：

extension DeviceAndEquipmentViewController : UICollectionViewDelegate,UICollectionViewDataSource,UICollectionViewDelegateFlowLayout {

func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 1
}

func collectionView(_ collectionView: UICollectionView,numberOfItemsInSection section: Int) -> Int {

    if collectionView === deviceCollectionView {
        guard selectedDeviceList.count != 0 else {
            selectedDeviceList.insert("nil",at: 0)
            return 1
        }
        return selectedDeviceList.count
    } else if collectionView === equipmentCollectionView {
        guard selectedEquipmentList.count != 0 else {
            selectedEquipmentList.insert("nil",at: 0)
            return 1
        }
        return selectedEquipmentList.count
    } else if collectionView === softwareCollectionView {
        guard selectedSoftwareList.count != 0 else {
            selectedSoftwareList.insert("nil",at: 0)
            return 1
        }
        return selectedSoftwareList.count
    } else {
        return 0
    }

}


func collectionView(_ collectionView: UICollectionView,layout collectionViewLayout: UICollectionViewLayout,insetforSectionAt section: Int) -> UIEdgeInsets {

    let inset:CGFloat = 10
    return UIEdgeInsets(top: inset,left: inset,bottom: inset,right: inset)

}

func collectionView(_ collectionView: UICollectionView,sizeForItemAt indexPath: IndexPath) -> CGSize {

                let numberOfItemsPerRow:CGFloat = 1
                 let spacingBetweenCells:CGFloat = 16

                let totalSpacing = (2 * self.spacing) + ((numberOfItemsPerRow - 1) * spacingBetweenCells)

                let width = (deviceCollectionView.bounds.width - totalSpacing) / numberOfItemsPerRow

    if collectionView == deviceCollectionView {

            let device = self.selectedDeviceList[indexPath.row]

            if device != "nil" {
                    return CGSize(width: width,height: 45)
            } else {
                    return CGSize(width: width + 15,height: 55)
        }
    } else if collectionView == equipmentCollectionView {

            let equipment = self.selectedEquipmentList[indexPath.row]
            if equipment != "nil" {
                return CGSize(width: width,height: 45)
            } else {
                return CGSize(width: width + 15,height: 55)
        }
    } else {

        let software = self.selectedSoftwareList[indexPath.row]

            if software != "nil" {
                return CGSize(width: width,height: 55)
        }
    }


}
}

这是每个CollectionView FlowLayout的设置

let layout = UICollectionViewFlowLayout()
    layout.sectionInset = UIEdgeInsets(top: spacing,left: spacing,bottom: spacing,right: spacing)
    layout.minimumLinespacing = spacing
    layout.minimumInteritemSpacing = spacing
    self.equipmentCollectionView?.collectionViewLayout = layout

我看过其他主题，但是并没有解决我的问题，我对每个集合视图单元格使用XIB，并且没有在Code中完成全部操作，如果您能帮助我解决问题，我将不胜感激。