get thumbnail images in list and full size image when click on the item of list

So I am using below code to fetch all the images from library which is working fine :

func grabPhotos(){

   let imgManager = PHImageManager.default()
   let requestOptions = PHImageRequestOptions()
   requestOptions.isSynchronous = true
   requestOptions.deliveryMode = .highQualityFormat
   let fetchOptions = PHFetchOptions()
   fetchOptions.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: true)]
   if let fetchResults : PHFetchResult = PHAsset.fetchAssets(with: .image, options: fetchOptions){

      if fetchResults.count>0{

       for i in 0..<fetchResults.count{

         imgManager.requestImage(for: fetchResults.object(at: i), targetSize: CGSize(width:100, height: 100), contentMode: .aspectFill, options: requestOptions, resultHandler: {

         image, error in
        self.Galleryimages.append(image!)

        print("array count is ",self.Galleryimages.count)
        self.photoCollectionview.reloadData()
      })
      }

    }
  }
}

I am showing all the images in my UICollectionView, but I didn't find any way to get original image whenever clicking on any thumbnail image. I want to fetch the original image (full size image) when user clicks on any thumbnail image which is populated in UICollectionView.

Thank you.

To load thumbnail image.

Got the solution after doing too much stuff, may be it can help to others. Below are the steps to do this.

Step 1 : Declare object of PHFetchResult

var Galleryimages: PHFetchResult<PHAsset>!

Step 2 : Fetch results from gallery using below code:

func grabPhotos(){

Galleryimages = PHAsset.fetchAssets(with: PHAssetMediaType.image, options: nil)

}

Step 3 : Show the thumbnail images in your UI (collectionview/Tableview) using below code :

let imageview = cell.viewWithTag(1) as! UIImageView

PHImageManager.default().requestImage(for: (Galleryimages?[indexPath.row])!, targetSize: CGSize(width: 200, height: 200), contentMode: .aspectFill, options: nil) { (image: UIImage?, info: [AnyHashable: Any]?) -> Void in
  imageview.image = image
}

Step 4 : And finally get the full size image using below code.

let options = PHImageRequestOptions()
options.deliveryMode = .highQualityFormat
options.resizeMode = .exact

PHImageManager.default().requestImage(for: (Galleryimages[indexPath.row]), targetSize: PHImageManagerMaximumSize, contentMode: .aspectFill, options: options) { (image: UIImage?, info: [AnyHashable: Any]?) -> Void in
  if let image = image {
    //Use this originan image
  }
}