obtainKey method
Converts a pictureProvider's settings plus a pictureConfiguration to a key that describes the precise picture to load.
The type of the key is determined by the subclass. It is a value that unambiguously identifies the picture (including its scale) that the load method will fetch. Different PictureProviders given the same constructor arguments and PictureConfiguration objects should return keys that are '==' to each other (possibly by using a class for the key that itself implements ==).
Implementation
@override
Future<StringPicture> obtainKey(PictureConfiguration picture) {
return SynchronousFuture<StringPicture>(this);
}