obtainKey method
Converts an ImageProvider's settings plus an ImageConfiguration to a key that describes the precise image to load.
The type of the key is determined by the subclass. It is a value that unambiguously identifies the image (including its scale) that the loadImage method will fetch. Different ImageProviders given the same constructor arguments and ImageConfiguration objects should return keys that are '==' to each other (possibly by using a class for the key that itself implements ==).
If the result can be determined synchronously, this function should return a SynchronousFuture. This allows image resolution to progress synchronously during a frame rather than delaying image loading.
Implementation
@override
Future<CachedNetworkImageKey> obtainKey(ImageConfiguration configuration) {
return SynchronousFuture<CachedNetworkImageKey>(CachedNetworkImageKey(
url: url,
scale: scale
));
}