refactor(lib): create structure with base and interface

lib/src/base/nearby_message_content_base.dart

@@ -0,0 +1,73 @@
+import 'package:nearby_service/nearby_service.dart';
+
+///
+/// Abstraction for the message content.
+/// The [type] is used to determine, what type of content is it.
+///
+abstract base class NearbyMessageContentBase {
+  const NearbyMessageContentBase();
+
+  ///
+  /// Contains the conditional logic of creating [NearbyMessageFilesRequest],
+  /// [NearbyMessageFilesResponse] or [NearbyMessageTextContent]
+  /// by `type` field of [json].
+  ///
+  static Content fromJson<Content extends NearbyMessageContentBase>(
+      Map<String, dynamic>? json) {
+    try {
+      final type = NearbyMessageContentType.values.firstWhere(
+        (e) => e.name == json?['type'],
+      );
+      if (type.isText) {
+        return NearbyMessageTextContent.fromJson(json) as Content;
+      } else if (type.isFilesRequest) {
+        return NearbyMessageFilesRequest.fromJson(json) as Content;
+      } else if (type.isFilesResponse) {
+        return NearbyMessageFilesResponse.fromJson(json) as Content;
+      } else {
+        throw NearbyServiceException.unsupportedDecoding(json);
+      }
+    } catch (e) {
+      throw NearbyServiceException(e);
+    }
+  }
+
+  NearbyMessageContentType get type;
+
+  ///
+  /// Check for the content if it is valid for sending or receiving.
+  ///
+  bool get isValid;
+
+  ///
+  /// * The [onText] callback returns this instance of [NearbyMessageContentBase],
+  /// cast as [NearbyMessageTextContent] if is a text.
+  ///
+  /// * The [onFilesRequest] callback returns this instance of [NearbyMessageContentBase],
+  /// cast as [NearbyMessageFilesRequest] if is a files pack request.
+  ///
+  /// * The [onFilesResponse] callback returns this instance of [NearbyMessageContentBase],
+  /// cast as [NearbyMessageFilesResponse] if is a files pack response.
+  ///
+  T? byType<T>({
+    T Function(NearbyMessageTextContent)? onText,
+    T Function(NearbyMessageFilesRequest)? onFilesRequest,
+    T Function(NearbyMessageFilesResponse)? onFilesResponse,
+  }) {
+    if (this is NearbyMessageTextContent && onText != null) {
+      return onText(this as NearbyMessageTextContent);
+    } else if (this is NearbyMessageFilesRequest && onFilesRequest != null) {
+      return onFilesRequest(this as NearbyMessageFilesRequest);
+    } else if (this is NearbyMessageFilesResponse && onFilesResponse != null) {
+      return onFilesResponse(this as NearbyMessageFilesResponse);
+    }
+    return null;
+  }
+
+  ///
+  /// Gets [Map] from [NearbyMessageContentBase]
+  ///
+  Map<String, dynamic> toJson() {
+    return {'type': type.name};
+  }
+}