void addListener(ImageListener listener)

Adds a listener callback that is called whenever a concrete ImageInfo object is available. If a concrete image is already available, this object will call the listener synchronously.

Source 

void addListener(ImageListener listener) {
  if (_completer != null)
    return _completer.addListener(listener);
  _listeners ??= <ImageListener>[];
  _listeners.add(listener);
}