class ImageInputAdapter {
  /// Initialize from either a URL or a file, but not both.
  ImageInputAdapter({
    this.file,
    this.url
  }) : assert(file != null || url != null), assert(file != null && url == null), assert(file == null && url != null);

  /// An image file
  final File file;
  /// A direct link to the remote image
  final String url;

  /// Render the image from a file or from a remote source.
  Widget widgetize() {
    if (file != null) {
      return Image.file(file);
    } else {
      return FadeInImage(
        image: NetworkImage(url),
        placeholder: AssetImage("assets/images/placeholder.png"),
        fit: BoxFit.contain,
      );
    }
  }
}

import 'package:image_form_field/image_form_field.dart';

ImageFormField<ImageInputAdapter>(
  previewImageBuilder: (_, ImageInputAdapter image) =>
    image.widgetize(),
  buttonBuilder: (_, int count) =>
    Container(
      child: Text(
        count == null || count < 1 ? "Upload Image" : "Upload More"
      )
    )
  initializeFileAsImage: (File file) =>
    ImageInputAdapter(file: file),
  initialValue: existingPhotoUrl == null ? null : (List<ImageInputImageAdapter>()..add(ImageInputImageAdapter(url: existingPhotoUrl))),
  // Even if `shouldAllowMultiple` is true, images will always be a `List` of the declared type (i.e. `ImageInputAdater`).
  onSaved: (images) _images = images,
)