1. override
Future<AssetBundleImageKey> obtainKey(ImageConfiguration configuration)

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 load 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 operator ==).

Source 

@override
Future<AssetBundleImageKey> obtainKey(ImageConfiguration configuration) {
  // This function tries to return a SynchronousFuture if possible. We do this
  // because otherwise showing an image would always take at least one frame,
  // which would be sad. (This code is called from inside build/layout/paint,
  // which all happens in one call frame; using native Futures would guarantee
  // that we resolve each future in a new call frame, and thus not in this
  // build/layout/paint sequence.)
  final AssetBundle chosenBundle = bundle ?? configuration.bundle ?? rootBundle;
  Completer<AssetBundleImageKey> completer;
  Future<AssetBundleImageKey> result;
  chosenBundle.loadStructuredData(_kAssetManifestFileName, _manifestParser).then(
    (Map<String, List<String>> manifest) {
      final String chosenName = _chooseVariant(
        name,
        configuration,
        manifest == null ? null : manifest[name]
      );
      final double chosenScale = _parseScale(chosenName);
      final AssetBundleImageKey key = new AssetBundleImageKey(
        bundle: chosenBundle,
        name: chosenName,
        scale: chosenScale
      );
      if (completer != null) {
        // We already returned from this function, which means we are in the
        // asynchronous mode. Pass the value to the completer. The completer's
        // future is what we returned.
        completer.complete(key);
      } else {
        // We haven't yet returned, so we must have been called synchronously
        // just after loadStructuredData returned (which means it provided us
        // with a SynchronousFuture). Let's return a SynchronousFuture
        // ourselves.
        result = new SynchronousFuture<AssetBundleImageKey>(key);
      }
    }
  ).catchError((dynamic error, StackTrace stack) {
    // We had an error. (This guarantees we weren't called synchronously.)
    // Forward the error to the caller.
    assert(completer != null);
    assert(result == null);
    completer.completeError(error, stack);
  });
  if (result != null) {
    // The code above ran synchronously, and came up with an answer.
    // Return the SynchronousFuture that we created above.
    return result;
  }
  // The code above hasn't yet run its "then" handler yet. Let's prepare a
  // completer for it to use when it does run.
  completer = new Completer<AssetBundleImageKey>();
  return completer.future;
}