feat(ui): add possibility to limit attachments in MessageInput

Signed-off-by: xsahil03x <[email protected]>
2021-08-11 17:01:50 +05:30
parent 2317115350
commit 9dba5a61d2
3 changed files with 268 additions and 244 deletions
@@ -106,7 +106,7 @@ class ChannelPage extends StatelessWidget {
            Expanded(
              child: MessageListView(),
            ),
-            MessageInput(),
+            MessageInput(attachmentLimit: 3),
          ],
        ),
      );
@@ -37,6 +37,16 @@ typedef ErrorListener = void Function(
  StackTrace? stackTrace,
 );

+/// A callback that can be passed to [MessageInput.onAttachmentLimitExceed].
+///
+/// This callback should not throw.
+///
+/// It exists merely for showing custom error, and should not be used otherwise.
+typedef AttachmentLimitExceedListener = void Function(
+  int limit,
+  String error,
+);
+
 /// Builder for attachment thumbnails
 typedef AttachmentThumbnailBuilder = Widget Function(
  BuildContext,
@@ -164,7 +174,13 @@ class MessageInput extends StatefulWidget {
    this.compressedVideoQuality = VideoQuality.DefaultQuality,
    this.compressedVideoFrameRate = 30,
    this.onError,
-  }) : super(key: key);
+    this.attachmentLimit = 10,
+    this.onAttachmentLimitExceed,
+  })  : assert(
+          initialMessage == null || editMessage == null,
+          "Can't provide both `initialMessage` and `editMessage`",
+        ),
+        super(key: key);

  /// Message to edit
  final Message? editMessage;
@@ -247,6 +263,11 @@ class MessageInput extends StatefulWidget {
  /// A callback for error reporting
  final ErrorListener? onError;

+  final int attachmentLimit;
+
+  /// A callback for error reporting
+  final AttachmentLimitExceedListener? onAttachmentLimitExceed;
+
  @override
  MessageInputState createState() => MessageInputState();

@@ -270,7 +291,6 @@ class MessageInputState extends State<MessageInput> {
  final _imagePicker = ImagePicker();
  late final FocusNode _focusNode;
  bool _inputEnabled = true;
-  bool _messageIsPresent = false;
  bool _commandEnabled = false;
  OverlayEntry? _commandsOverlay, _mentionsOverlay, _emojiOverlay;
  late Iterable<String> _emojiNames;
@@ -285,13 +305,15 @@ class MessageInputState extends State<MessageInput> {
      KeyboardVisibilityController();

  /// The editing controller passed to the input TextField
-  late final TextEditingController textEditingController;
+  late final TextEditingController _textEditingController;

  late StreamChatThemeData _streamChatTheme;
  late MessageInputThemeData _messageInputTheme;

  bool get _hasQuotedMessage => widget.quotedMessage != null;

+  bool get _messageIsPresent => _textEditingController.text.trim().isNotEmpty;
+
  @override
  void initState() {
    super.initState();
@@ -303,19 +325,19 @@ class MessageInputState extends State<MessageInput> {
      _keyboardListener =
          _keyboardVisibilityController.onChange.listen((visible) {
        if (_focusNode.hasFocus) {
-          _onChanged(context, textEditingController.text);
+          _onChanged(context, _textEditingController.text);
        }
      });
    }

-    textEditingController =
+    _textEditingController =
        widget.textEditingController ?? TextEditingController();
    if (widget.editMessage != null || widget.initialMessage != null) {
      _parseExistingMessage(widget.editMessage ?? widget.initialMessage!);
    }

-    textEditingController.addListener(() {
-      _onChanged(context, textEditingController.text);
+    _textEditingController.addListener(() {
+      _onChanged(context, _textEditingController.text);
    });

    _focusNode.addListener(() {
@@ -582,7 +604,7 @@ class MessageInputState extends State<MessageInput> {
                    maxLines: null,
                    onSubmitted: (_) => sendMessage(),
                    keyboardType: widget.keyboardType,
-                    controller: textEditingController,
+                    controller: _textEditingController,
                    focusNode: _focusNode,
                    style: _messageInputTheme.inputTextStyle,
                    autofocus: widget.autofocus,
@@ -728,7 +750,6 @@ class MessageInputState extends State<MessageInput> {
            .catchError((e) {});

        setState(() {
-          _messageIsPresent = s.trim().isNotEmpty;
          _actionsShrunk = s.trim().isNotEmpty &&
              ((widget.actions?.length ?? 0) +
                      (widget.showCommandsButton ? 1 : 0) +
@@ -764,15 +785,15 @@ class MessageInputState extends State<MessageInput> {

  void _checkEmoji(String s, BuildContext context) {
    if (s.isNotEmpty &&
-        textEditingController.selection.baseOffset > 0 &&
-        textEditingController.text
+        _textEditingController.selection.baseOffset > 0 &&
+        _textEditingController.text
            .substring(
              0,
-              textEditingController.selection.baseOffset,
+              _textEditingController.selection.baseOffset,
            )
            .contains(':')) {
-      final textToSelection = textEditingController.text
-          .substring(0, textEditingController.value.selection.start);
+      final textToSelection = _textEditingController.text
+          .substring(0, _textEditingController.value.selection.start);
      final splits = textToSelection.split(':');
      final query = splits[splits.length - 2].toLowerCase();
      final emoji = Emoji.byName(query);
@@ -791,9 +812,9 @@ class MessageInputState extends State<MessageInput> {

  void _checkMentions(String s, BuildContext context) {
    if (s.isNotEmpty &&
-        textEditingController.selection.baseOffset > 0 &&
-        textEditingController.text
-            .substring(0, textEditingController.selection.baseOffset)
+        _textEditingController.selection.baseOffset > 0 &&
+        _textEditingController.text
+            .substring(0, _textEditingController.selection.baseOffset)
            .split(' ')
            .last
            .contains('@')) {
@@ -816,8 +837,7 @@ class MessageInputState extends State<MessageInput> {

      if (matchedCommandsList.length == 1) {
        _chosenCommand = matchedCommandsList[0];
-        textEditingController.clear();
-        _messageIsPresent = false;
+        _textEditingController.clear();
        setState(() {
          _commandEnabled = true;
        });
@@ -833,7 +853,7 @@ class MessageInputState extends State<MessageInput> {
  }

  OverlayEntry? _buildCommandsOverlayEntry() {
-    final text = textEditingController.text.trimLeft();
+    final text = _textEditingController.text.trimLeft();
    final commands = StreamChannel.of(context)
            .channel
            .config
@@ -1037,7 +1057,7 @@ class MessageInputState extends State<MessageInput> {
                  onPressed: _attachmentContainsFile && _attachments.isNotEmpty
                      ? null
                      : () {
-                          pickFile(DefaultAttachmentTypes.image, true);
+                          pickFile(DefaultAttachmentTypes.image, camera: true);
                        },
                ),
                IconButton(
@@ -1048,7 +1068,7 @@ class MessageInputState extends State<MessageInput> {
                  onPressed: _attachmentContainsFile && _attachments.isNotEmpty
                      ? null
                      : () {
-                          pickFile(DefaultAttachmentTypes.video, true);
+                          pickFile(DefaultAttachmentTypes.video, camera: true);
                        },
                ),
              ],
@@ -1107,7 +1127,7 @@ class MessageInputState extends State<MessageInput> {
                      if (_attachments.containsKey(media.id)) {
                        setState(() => _attachments.remove(media.id));
                      } else {
-                        _addAttachment(media);
+                        _addAssetAttachment(media);
                      }
                    },
                  ),
@@ -1119,15 +1139,13 @@ class MessageInputState extends State<MessageInput> {
    );
  }

-  void _addAttachment(AssetEntity medium) async {
+  void _addAssetAttachment(AssetEntity medium) async {
    final mediaFile = await medium.originFile.timeout(
      const Duration(seconds: 5),
      onTimeout: () => medium.originFile,
    );

-    if (mediaFile == null) {
-      return;
-    }
+    if (mediaFile == null) return;

    var file = AttachmentFile(
      path: mediaFile.path,
@@ -1166,11 +1184,12 @@ class MessageInputState extends State<MessageInput> {
    }

    setState(() {
-      _attachments[medium.id] = Attachment(
+      final attachment = Attachment(
        id: medium.id,
        file: file,
        type: medium.type == AssetType.image ? 'image' : 'video',
      );
+      _addAttachments([attachment]);
    });
  }

@@ -1251,8 +1270,8 @@ class MessageInputState extends State<MessageInput> {
  }

  OverlayEntry? _buildMentionsOverlayEntry() {
-    final splits = textEditingController.text
-        .substring(0, textEditingController.value.selection.start)
+    final splits = _textEditingController.text
+        .substring(0, _textEditingController.value.selection.start)
        .split('@');
    final query = splits.last.toLowerCase();

@@ -1314,10 +1333,10 @@ class MessageInputState extends State<MessageInput> {
                          splits[splits.length - 1] = m.user!.name;
                          final rejoin = splits.join('@');

-                          textEditingController.value = TextEditingValue(
+                          _textEditingController.value = TextEditingValue(
                            text: rejoin +
-                                textEditingController.text.substring(
-                                    textEditingController.selection.start),
+                                _textEditingController.text.substring(
+                                    _textEditingController.selection.start),
                            selection: TextSelection.collapsed(
                              offset: rejoin.length,
                            ),
@@ -1361,8 +1380,8 @@ class MessageInputState extends State<MessageInput> {
  }

  OverlayEntry? _buildEmojiOverlay() {
-    final splits = textEditingController.text
-        .substring(0, textEditingController.value.selection.start)
+    final splits = _textEditingController.text
+        .substring(0, _textEditingController.value.selection.start)
        .split(':');
    final query = splits.last.toLowerCase();

@@ -1473,10 +1492,10 @@ class MessageInputState extends State<MessageInput> {
  void _chooseEmoji(List<String> splits, Emoji emoji) {
    final rejoin = splits.sublist(0, splits.length - 1).join(':') + emoji.char!;

-    textEditingController.value = TextEditingValue(
+    _textEditingController.value = TextEditingValue(
      text: rejoin +
-          textEditingController.text
-              .substring(textEditingController.selection.start),
+          _textEditingController.text
+              .substring(_textEditingController.selection.start),
      selection: TextSelection.collapsed(
        offset: rejoin.length,
      ),
@@ -1487,11 +1506,10 @@ class MessageInputState extends State<MessageInput> {
  }

  void _setCommand(Command c) {
-    textEditingController.clear();
+    _textEditingController.clear();
    setState(() {
      _chosenCommand = c;
      _commandEnabled = true;
-      _messageIsPresent = false;
    });
    _commandsOverlay?.remove();
    _commandsOverlay = null;
@@ -1682,7 +1700,7 @@ class MessageInputState extends State<MessageInput> {
  }

  Widget _buildCommandButton() {
-    final s = textEditingController.text.trim();
+    final s = _textEditingController.text.trim();

    return IconButton(
      icon: StreamSvgIcon.lightning(
@@ -1768,87 +1786,100 @@ class MessageInputState extends State<MessageInput> {
      });
    } else {
      showModalBottomSheet(
-          clipBehavior: Clip.hardEdge,
-          shape: const RoundedRectangleBorder(
-            borderRadius: BorderRadius.only(
-              topLeft: Radius.circular(32),
-              topRight: Radius.circular(32),
-            ),
+        clipBehavior: Clip.hardEdge,
+        shape: const RoundedRectangleBorder(
+          borderRadius: BorderRadius.only(
+            topLeft: Radius.circular(32),
+            topRight: Radius.circular(32),
          ),
-          context: context,
-          isScrollControlled: true,
-          builder: (_) => Column(
-                mainAxisSize: MainAxisSize.min,
-                children: <Widget>[
-                  ListTile(
-                    title: Text(
-                      context.translations.addAFileLabel,
-                      style: const TextStyle(
-                        fontWeight: FontWeight.bold,
-                      ),
-                    ),
-                  ),
-                  ListTile(
-                    leading: const Icon(Icons.image),
-                    title: Text(context.translations.uploadAPhotoLabel),
-                    onTap: () {
-                      pickFile(DefaultAttachmentTypes.image);
-                      Navigator.pop(context);
-                    },
-                  ),
-                  ListTile(
-                    leading: const Icon(Icons.video_library),
-                    title: Text(context.translations.uploadAVideoLabel),
-                    onTap: () {
-                      pickFile(DefaultAttachmentTypes.video);
-                      Navigator.pop(context);
-                    },
-                  ),
-                  if (!kIsWeb)
-                    ListTile(
-                      leading: const Icon(Icons.camera_alt),
-                      title: Text(context.translations.photoFromCameraLabel),
-                      onTap: () {
-                        pickFile(DefaultAttachmentTypes.image, true);
-                        Navigator.pop(context);
-                      },
-                    ),
-                  if (!kIsWeb)
-                    ListTile(
-                      leading: const Icon(Icons.videocam),
-                      title: Text(context.translations.videoFromCameraLabel),
-                      onTap: () {
-                        pickFile(DefaultAttachmentTypes.video, true);
-                        Navigator.pop(context);
-                      },
-                    ),
-                  ListTile(
-                    leading: const Icon(Icons.insert_drive_file),
-                    title: Text(context.translations.uploadAFileLabel),
-                    onTap: () {
-                      pickFile(DefaultAttachmentTypes.file);
-                      Navigator.pop(context);
-                    },
-                  ),
-                ],
-              ));
+        ),
+        context: context,
+        isScrollControlled: true,
+        builder: (_) => Column(
+          mainAxisSize: MainAxisSize.min,
+          children: <Widget>[
+            ListTile(
+              title: Text(
+                context.translations.addAFileLabel,
+                style: const TextStyle(
+                  fontWeight: FontWeight.bold,
+                ),
+              ),
+            ),
+            ListTile(
+              leading: const Icon(Icons.image),
+              title: Text(context.translations.uploadAPhotoLabel),
+              onTap: () {
+                pickFile(DefaultAttachmentTypes.image);
+                Navigator.pop(context);
+              },
+            ),
+            ListTile(
+              leading: const Icon(Icons.video_library),
+              title: Text(context.translations.uploadAVideoLabel),
+              onTap: () {
+                pickFile(DefaultAttachmentTypes.video);
+                Navigator.pop(context);
+              },
+            ),
+            if (!kIsWeb)
+              ListTile(
+                leading: const Icon(Icons.camera_alt),
+                title: Text(context.translations.photoFromCameraLabel),
+                onTap: () {
+                  pickFile(DefaultAttachmentTypes.image, camera: true);
+                  Navigator.pop(context);
+                },
+              ),
+            if (!kIsWeb)
+              ListTile(
+                leading: const Icon(Icons.videocam),
+                title: Text(context.translations.videoFromCameraLabel),
+                onTap: () {
+                  pickFile(DefaultAttachmentTypes.video, camera: true);
+                  Navigator.pop(context);
+                },
+              ),
+            ListTile(
+              leading: const Icon(Icons.insert_drive_file),
+              title: Text(context.translations.uploadAFileLabel),
+              onTap: () {
+                pickFile(DefaultAttachmentTypes.file);
+                Navigator.pop(context);
+              },
+            ),
+          ],
+        ),
+      );
    }
  }

-  /// Add an attachment to the sending message
-  /// Use this to add custom type attachments
-  void addAttachment(Attachment attachment) {
-    setState(() {
-      _attachments[attachment.id] = attachment.copyWith(
-        uploadState: attachment.uploadState,
+  /// Adds an attachment to the [_attachments] map
+  void _addAttachments(Iterable<Attachment> attachments) {
+    final length = _attachments.length + attachments.length;
+    if (length > widget.attachmentLimit) {
+      final onAttachmentLimitExceed = widget.onAttachmentLimitExceed;
+      if (onAttachmentLimitExceed != null) {
+        return onAttachmentLimitExceed(
+          widget.attachmentLimit,
+          'Attachment Limit crossed ${widget.attachmentLimit}',
+        );
+      }
+      return _showErrorAlert(
+        'Attachment Limit crossed ${widget.attachmentLimit}',
      );
-    });
+    }
+    for (final attachment in attachments) {
+      _attachments[attachment.id] = attachment;
+    }
  }

  /// Pick a file from the device
  /// If [camera] is true then the camera will open
-  // ignore: avoid_positional_boolean_parameters
-  void pickFile(DefaultAttachmentTypes fileType, [bool camera = false]) async {
+  void pickFile(
+    DefaultAttachmentTypes fileType, {
+    bool camera = false,
+  }) async {
    setState(() => _inputEnabled = false);

    AttachmentFile? file;
@@ -1947,16 +1978,14 @@ class MessageInputState extends State<MessageInput> {
      }
    }

-    _attachments[attachment.id] = attachment;
-
    setState(() {
-      _attachments.update(
-          attachment.id,
-          (it) => it.copyWith(
-                file: file,
-                extraData: {...it.extraData}
-                  ..update('file_size', ((_) => file!.size!)),
-              ));
+      _addAttachments([
+        attachment.copyWith(
+          file: file,
+          extraData: {...attachment.extraData}
+            ..update('file_size', ((_) => file!.size!)),
+        ),
+      ]);
    });
  }

@@ -2005,7 +2034,7 @@ class MessageInputState extends State<MessageInput> {

  /// Sends the current message
  Future<void> sendMessage() async {
-    var text = textEditingController.text.trim();
+    var text = _textEditingController.text.trim();
    if (text.isEmpty && _attachments.isEmpty) {
      return;
    }
@@ -2018,12 +2047,11 @@ class MessageInputState extends State<MessageInput> {

    final attachments = [..._attachments.values];

-    textEditingController.clear();
+    _textEditingController.clear();
    _attachments.clear();
    widget.onQuotedMessageCleared?.call();

    setState(() {
-      _messageIsPresent = false;
      _commandEnabled = false;
    });

@@ -2153,7 +2181,8 @@ class MessageInputState extends State<MessageInput> {
                child: Text(
                  context.translations.okLabel,
                  style: _streamChatTheme.textTheme.bodyBold.copyWith(
-                      color: _streamChatTheme.colorTheme.accentPrimary),
+                    color: _streamChatTheme.colorTheme.accentPrimary,
+                  ),
                ),
              ),
            ],
@@ -2164,13 +2193,8 @@ class MessageInputState extends State<MessageInput> {
  }

  void _parseExistingMessage(Message message) {
-    textEditingController.text = message.text!;
-    _messageIsPresent = true;
-    for (final attachment in message.attachments) {
-      _attachments[attachment.id] = attachment.copyWith(
-        uploadState: attachment.uploadState,
-      );
-    }
+    _textEditingController.text = message.text!;
+    _addAttachments(message.attachments);
  }

  @override
@@ -2196,54 +2220,6 @@ class MessageInputState extends State<MessageInput> {
  }
 }

-/// Represents a 2-tuple, or pair.
-class Tuple2<T1, T2> {
-  /// Creates a new tuple value with the specified items.
-  const Tuple2(this.item1, this.item2);
-
-  /// Create a new tuple value with the specified list [items].
-  factory Tuple2.fromList(List items) {
-    if (items.length != 2) {
-      throw ArgumentError('items must have length 2');
-    }
-
-    return Tuple2<T1, T2>(items[0] as T1, items[1] as T2);
-  }
-
-  /// Returns the first item of the tuple
-  final T1 item1;
-
-  /// Returns the second item of the tuple
-  final T2 item2;
-
-  /// Returns a tuple with the first item set to the specified value.
-  Tuple2<T1, T2> withItem1(T1 v) => Tuple2<T1, T2>(v, item2);
-
-  /// Returns a tuple with the second item set to the specified value.
-  Tuple2<T1, T2> withItem2(T2 v) => Tuple2<T1, T2>(item1, v);
-
-  /// Creates a [List] containing the items of this [Tuple2].
-  ///
-  /// The elements are in item order. The list is variable-length
-  /// if [growable] is true.
-  List toList({bool growable = false}) =>
-      List.from([item1, item2], growable: growable);
-
-  @override
-  String toString() => '[$item1, $item2]';
-
-  @override
-  bool operator ==(Object other) =>
-      identical(this, other) ||
-      other is Tuple2 &&
-          runtimeType == other.runtimeType &&
-          item1 == other.item1 &&
-          item2 == other.item2;
-
-  @override
-  int get hashCode => item1.hashCode ^ item2.hashCode;
-}
-
 class _PickerWidget extends StatefulWidget {
  const _PickerWidget({
    Key? key,
@@ -2281,74 +2257,74 @@ class _PickerWidgetState extends State<_PickerWidget> {
      return const Offstage();
    }
    return FutureBuilder<bool>(
-        future: requestPermission,
-        builder: (context, snapshot) {
-          if (!snapshot.hasData) {
-            return const Center(child: CircularProgressIndicator());
-          }
+      future: requestPermission,
+      builder: (context, snapshot) {
+        if (!snapshot.hasData) {
+          return const Center(child: CircularProgressIndicator());
+        }

-          if (snapshot.data!) {
-            if (widget.containsFile) {
-              return GestureDetector(
-                onTap: () {
-                  widget.onAddMoreFilesClick(DefaultAttachmentTypes.file);
-                },
-                child: Container(
-                  constraints: const BoxConstraints.expand(),
-                  color: widget.streamChatTheme.colorTheme.inputBg,
-                  alignment: Alignment.center,
+        if (snapshot.data!) {
+          if (widget.containsFile) {
+            return GestureDetector(
+              onTap: () {
+                widget.onAddMoreFilesClick(DefaultAttachmentTypes.file);
+              },
+              child: Container(
+                constraints: const BoxConstraints.expand(),
+                color: widget.streamChatTheme.colorTheme.inputBg,
+                alignment: Alignment.center,
+                child: Text(
+                  context.translations.addMoreFilesLabel,
+                  style: TextStyle(
+                    color: widget.streamChatTheme.colorTheme.accentPrimary,
+                    fontWeight: FontWeight.bold,
+                  ),
+                ),
+              ),
+            );
+          }
+          return MediaListView(
+            selectedIds: widget.selectedMedias,
+            onSelect: widget.onMediaSelected,
+          );
+        }
+
+        return InkWell(
+          onTap: () async {
+            PhotoManager.openSetting();
+          },
+          child: Container(
+            color: widget.streamChatTheme.colorTheme.inputBg,
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: [
+                SvgPicture.asset(
+                  'svgs/icon_picture_empty_state.svg',
+                  package: 'stream_chat_flutter',
+                  height: 140,
+                  color: widget.streamChatTheme.colorTheme.disabled,
+                ),
+                Text(
+                  context.translations.enablePhotoAndVideoAccessMessage,
+                  style: widget.streamChatTheme.textTheme.body.copyWith(
+                      color: widget.streamChatTheme.colorTheme.textLowEmphasis),
+                  textAlign: TextAlign.center,
+                ),
+                const SizedBox(height: 6),
+                Center(
                  child: Text(
-                    context.translations.addMoreFilesLabel,
-                    style: TextStyle(
+                    context.translations.allowGalleryAccessMessage,
+                    style: widget.streamChatTheme.textTheme.bodyBold.copyWith(
                      color: widget.streamChatTheme.colorTheme.accentPrimary,
-                      fontWeight: FontWeight.bold,
                    ),
                  ),
                ),
-              );
-            }
-            return MediaListView(
-              selectedIds: widget.selectedMedias,
-              onSelect: widget.onMediaSelected,
-            );
-          }
-
-          return InkWell(
-            onTap: () async {
-              PhotoManager.openSetting();
-            },
-            child: Container(
-              color: widget.streamChatTheme.colorTheme.inputBg,
-              child: Column(
-                mainAxisAlignment: MainAxisAlignment.center,
-                crossAxisAlignment: CrossAxisAlignment.stretch,
-                children: [
-                  SvgPicture.asset(
-                    'svgs/icon_picture_empty_state.svg',
-                    package: 'stream_chat_flutter',
-                    height: 140,
-                    color: widget.streamChatTheme.colorTheme.disabled,
-                  ),
-                  Text(
-                    context.translations.enablePhotoAndVideoAccessMessage,
-                    style: widget.streamChatTheme.textTheme.body.copyWith(
-                        color:
-                            widget.streamChatTheme.colorTheme.textLowEmphasis),
-                    textAlign: TextAlign.center,
-                  ),
-                  const SizedBox(height: 6),
-                  Center(
-                    child: Text(
-                      context.translations.allowGalleryAccessMessage,
-                      style: widget.streamChatTheme.textTheme.bodyBold.copyWith(
-                        color: widget.streamChatTheme.colorTheme.accentPrimary,
-                      ),
-                    ),
-                  ),
-                ],
-              ),
+              ],
            ),
-          );
-        });
+          ),
+        );
+      },
+    );
  }
 }
@@ -340,3 +340,51 @@ Widget wrapAttachmentWidget(
      type: MaterialType.transparency,
      child: attachmentWidget,
    );
+
+/// Represents a 2-tuple, or pair.
+class Tuple2<T1, T2> {
+  /// Creates a new tuple value with the specified items.
+  const Tuple2(this.item1, this.item2);
+
+  /// Create a new tuple value with the specified list [items].
+  factory Tuple2.fromList(List items) {
+    if (items.length != 2) {
+      throw ArgumentError('items must have length 2');
+    }
+
+    return Tuple2<T1, T2>(items[0] as T1, items[1] as T2);
+  }
+
+  /// Returns the first item of the tuple
+  final T1 item1;
+
+  /// Returns the second item of the tuple
+  final T2 item2;
+
+  /// Returns a tuple with the first item set to the specified value.
+  Tuple2<T1, T2> withItem1(T1 v) => Tuple2<T1, T2>(v, item2);
+
+  /// Returns a tuple with the second item set to the specified value.
+  Tuple2<T1, T2> withItem2(T2 v) => Tuple2<T1, T2>(item1, v);
+
+  /// Creates a [List] containing the items of this [Tuple2].
+  ///
+  /// The elements are in item order. The list is variable-length
+  /// if [growable] is true.
+  List toList({bool growable = false}) =>
+      List.from([item1, item2], growable: growable);
+
+  @override
+  String toString() => '[$item1, $item2]';
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is Tuple2 &&
+          runtimeType == other.runtimeType &&
+          item1 == other.item1 &&
+          item2 == other.item2;
+
+  @override
+  int get hashCode => item1.hashCode ^ item2.hashCode;
+}