convert UIImage to NSData and back to UIImage

Try With this:

Use this for convert UIImage to NSData.

  NSData *imagedata = UIImagePNGRepresentation(thumbnail);

Try this for image data convert back to UIImage

  UIImage *image= [UIImage imageWithData:imagedata];

in Swift version:

    let imageData = image!.pngData()
    let imageFromData = UIImage(data: imageData)

See: https://developer.apple.com/documentation/uikit/uiimage/1624096-pngdata

To convert UIImage to NSData, use either UIImageJPEGRepresentation(UIImage *image, CGFloat compressionQuality) or UIImagePNGRepresentation(UIImage *image)

To convert NSData to UIImage, use [UIImage imageWithData:imageData]

So your example could look like this:

cell.textLabel.text = [[offlineImageListArray objectAtIndex:indexPath.row] imageName];
UIImage *thumbnail = [self retrieveImageFromDevice:[[offlineImageListArray objectAtIndex:indexPath.row] imageName]];
NSData* data = UIImagePNGRepresentation(thumbnail);
ImageData *imageDataObject = [[ImageData alloc] initWithImageId:[[offlineImageListArray objectAtIndex:indexPath.row]imageId] imageName:[[offlineImageListArray objectAtIndex:indexPath.row] imageName] imageData:data];
[imagesArray addObject:imageDataObject];

References: https://developer.apple.com/LIBRARY/IOS/documentation/UIKit/Reference/UIKitFunctionReference/Reference/reference.html https://developer.apple.com/Library/ios/documentation/UIKit/Reference/UIImage_Class/Reference/Reference.html