[Async Attachment Upload] Initial implementation

Signed-off-by: Sahil Kumar <[email protected]>

packages/stream_chat/lib/src/models/attachment_file.dart

@@ -0,0 +1,82 @@
+import 'dart:typed_data';
+
+import 'package:meta/meta.dart';
+import 'package:freezed_annotation/freezed_annotation.dart';
+
+part 'attachment_file.freezed.dart';
+
+part 'attachment_file.g.dart';
+
+///
+@freezed
+abstract class UploadState with _$UploadState {
+  ///
+  const factory UploadState.inProgress({int uploaded, int total}) = InProgress;
+
+  ///
+  const factory UploadState.success() = Success;
+
+  ///
+  const factory UploadState.failed({@required String error}) = Failed;
+
+  /// Creates a new instance from a json
+  factory UploadState.fromJson(Map<String, dynamic> json) =>
+      _$UploadStateFromJson(json);
+}
+
+///
+extension UploadStateX on UploadState {
+  ///
+  bool get isInProgress => this is InProgress;
+
+  ///
+  bool get isSuccess => this is Success;
+
+  ///
+  bool get isFailed => this is Failed;
+}
+
+Uint8List _fromString(String bytes) => Uint8List.fromList(bytes.codeUnits);
+
+String _toString(Uint8List bytes) => String.fromCharCodes(bytes);
+
+///
+@JsonSerializable()
+class AttachmentFile {
+  ///
+  const AttachmentFile({
+    this.path,
+    this.name,
+    this.bytes,
+    this.size,
+  });
+
+  /// The absolute path for a cached copy of this file. It can be used to create a
+  /// file instance with a descriptor for the given path.
+  /// ```
+  /// final File myFile = File(platformFile.path);
+  /// ```
+  final String path;
+
+  /// File name including its extension.
+  final String name;
+
+  /// Byte data for this file. Particularly useful if you want to manipulate its data
+  /// or easily upload to somewhere else.
+  @JsonKey(toJson: _toString, fromJson: _fromString)
+  final Uint8List bytes;
+
+  /// The file size in bytes.
+  final int size;
+
+  /// File extension for this file.
+  String get extension => name?.split('.')?.last;
+
+  /// Create a new instance from a json
+  factory AttachmentFile.fromJson(Map<String, dynamic> json) {
+    return _$AttachmentFileFromJson(json);
+  }
+
+  /// Serialize to json
+  Map<String, dynamic> toJson() => _$AttachmentFileToJson(this);
+}