Now iOS 14 supports multiple asset picker by default. I recommend use PHPickerViewController from iOS 14 instead of this, it's a much better solution.

// to show
let picker = AssetsPickerViewController()
picker.pickerDelegate = self
present(picker, animated: true, completion: nil)

// to handle
extension SimpleExampleController: AssetsPickerViewControllerDelegate {
    
    func assetsPickerCannotAccessPhotoLibrary(controller: AssetsPickerViewController) {}
    func assetsPickerDidCancel(controller: AssetsPickerViewController) {}
    func assetsPicker(controller: AssetsPickerViewController, selected assets: [PHAsset]) {
        // do your job with selected assets
    }
    func assetsPicker(controller: AssetsPickerViewController, shouldSelect asset: PHAsset, at indexPath: IndexPath) -> Bool {
        return true
    }
    func assetsPicker(controller: AssetsPickerViewController, didSelect asset: PHAsset, at indexPath: IndexPath) {}
    func assetsPicker(controller: AssetsPickerViewController, shouldDeselect asset: PHAsset, at indexPath: IndexPath) -> Bool {
        return true
    }
    func assetsPicker(controller: AssetsPickerViewController, didDeselect asset: PHAsset, at indexPath: IndexPath) {}
}

pickerConfig.albumIsShowEmptyAlbum = false

pickerConfig.albumIsShowHiddenAlbum = true

pickerConfig.selectedAssets = self.assets

func assetsPicker(controller: AssetsPickerViewController, shouldSelect asset: PHAsset, at indexPath: IndexPath) -> Bool {   
    if controller.selectedAssets.count > 3 {
        // do your job here
        return false
    }
    return true
}

func assetsPicker(controller: AssetsPickerViewController, shouldSelect asset: PHAsset, at indexPath: IndexPath) -> Bool {   
    if controller.selectedAssets.count > 0 {
        controller.photoViewController.deselectAll()
    }
    return true
}

pickerConfig.assetsMaximumSelectionCount = 5

pickerConfig.albumCellType = CustomAlbumCell.classForCoder()
// and implement your own UICollectionViewCell which conforms to AssetsAlbumCellProtocol

pickerConfig.assetCellType = CustomAssetCell.classForCoder()
// and implement your own UICollectionViewCell which conforms to AssetsPhotoCellProtocol

let options = PHFetchOptions()
options.sortDescriptors = [NSSortDescriptor(key: "estimatedAssetCount", ascending: true)]
        
pickerConfig.albumFetchOptions = [
    .smartAlbum: options
]

pickerConfig.albumComparator = { (albumType, leftEntry, rightEntry) -> Bool in
    // return: Is leftEntry ordered before the rightEntry?
    switch albumType {
    case .smartAlbum:
        return leftEntry.album.assetCollectionSubtype.rawValue < rightEntry.album.assetCollectionSubtype.rawValue
    case .album:
        return leftEntry.result.count < rightEntry.result.count     // ascending order by asset count
    case .moment:
        return true
    }
}

let options = PHFetchOptions()
options.sortDescriptors = [
    NSSortDescriptor(key: "pixelWidth", ascending: true),
    NSSortDescriptor(key: "pixelHeight", ascending: true)
]

pickerConfig.assetFetchOptions = [
    .smartAlbum: options
]

let options = PHFetchOptions()
options.predicate = NSPredicate(format: "estimatedAssetCount = 0")
pickerConfig.albumFetchOptions = [.smartAlbum: options]

// return true to include, false to discard.
let smartAlbumFilter: ((PHAssetCollection, PHFetchResult<PHAsset>) -> Bool) = { (album, fetchResult) in
    // filter by album object
    if album.assetCollectionSubtype == .smartAlbumBursts { return false }
    if album.assetCollectionSubtype == .smartAlbumTimelapses { return false }
    if album.assetCollectionSubtype == .smartAlbumFavorites { return false }
            
    // filter by fetch result
    if fetchResult.count > 50 {
        return true     // only shows albums that contains more than 50 assets
    } else {
        return false    //
    }
}
pickerConfig.albumFilter = [
    .smartAlbum: smartAlbumFilter
]

let options = PHFetchOptions()
options.predicate = NSPredicate(format: "mediaType = %d", PHAssetMediaType.video.rawValue)
options.sortDescriptors = [NSSortDescriptor(key: "duration", ascending: true)]
        
pickerConfig.assetFetchOptions = [
    .smartAlbum: options,
    .album: options
]

AssetsPickerConfig.customStringConfig = [
    .cancel: "Cancel",
    .done: "Done",
    .titleAlbums: "Albums",
    .titleSectionMyAlbums: "My Albums",
    .footerPhotos: "%@ Photos",
    .footerVideos: "%@ Videos",
    .footerItems: "%@ Photos, %@ Videos",
    .titleSelectedPhoto: "%@ Photo Selected",
    .titleSelectedPhotos: "%@ Photos Selected",
    .titleSelectedVideo: "%@ Video Selected",
    .titleSelectedVideos: "%@ Videos Selected",
    .titleSelectedItems: "%@ Items Selected",
    .titleNoItems: "No Photos or Videos",
    .messageNoItems: "You can take photos and videos using the camera, or sync photos and videos onto your %@ using iTunes.",
    .messageNoItemsCamera: "You can sync photos and videos onto your %@ using iTunes.",
    .titleNoPermission: "This app does not have access to your photos or videos.",
    .messageNoPermission: "You can enable access in Privacy Settings.",
]

AssetsPickerConfig.customStringConfig = [
    .titleNoItems: "No Photos or Videos",
    .messageNoItems: "You can take photos and videos using the camera, or sync photos and videos onto your %@ using iTunes.",
    .messageNoItemsCamera: "You can sync photos and videos onto your %@ using iTunes.",
    .titleNoPermission: "This app does not have access to your photos or videos.",
    .messageNoPermission: "You can enable access in Privacy Settings.",
]