flutter - 编译时间错误 : Type arguments in partial instantiations must be instantiated and are therefore not allowed to depend on type parameters

Question

当我尝试调用 listenEventReducer 时，出现了这个奇怪的错误。该方法知道类型，但仍然不确定我哪里出错了。

import 'package:test/test.dart';

enum ObjectChangeType { added, modified, removed }

typedef UpdateItem<T extends ListenEventItem> = T Function(T oldItem, T newItem);

T _defaultUpdate<T extends ListenEventItem>(T oldItem, T newItem) => newItem;

abstract class ListenEventItem {
  String get id;

  ObjectChangeType get changeType;
}

abstract class ListenEvent<T extends ListenEventItem> {
  List<T> get data;

  Map<ObjectChangeType, List<T>> get typedData {
    return data.fold(
      <ObjectChangeType, List<T>>{},
      (Map<ObjectChangeType, List<T>> map, T element) {
        map[element.changeType] = (map[element.changeType] ?? <T>[])..add(element);

        return map;
      },
    );
  }
}

Map<String, T> listenEventReducer<T extends ListenEventItem>(Map<String, T> state, ListenEvent<T> action,
    [UpdateItem<T> update = _defaultUpdate]) {
  update ??= _defaultUpdate;
  final Map<String, T> b = Map<String, T>.from(state);

  for (ObjectChangeType type in ObjectChangeType.values) {
    final List<T> data = action.typedData[type];
    if (data == null || data.isEmpty) {
      continue;
    }

    if (type == ObjectChangeType.added) {
      for (T item in data) {
        b[item.id] == item;
      }
    } else if (type == ObjectChangeType.modified) {
      for (T item in data) {
        b[item.id] = update(b[item.id], item);
      }
    } else if (type == ObjectChangeType.removed) {
      final List<String> ids = data.map((T it) => it.id).toList();
      b.removeWhere((String id, _) => ids.contains(id));
    }
  }

  return b;
}

class Data extends ListenEventItem {
  Data(this.id, this.changeType, this.metadata);

  final String metadata;

  @override
  final String id;

  @override
  final ObjectChangeType changeType;
}

class DataEvent extends ListenEvent<Data> {
  DataEvent(this.data);

  @override
  final List<Data> data;
}

void main() {
  final List<Data> items = List<Data>();
  items.add(Data('0', ObjectChangeType.added, 'a'));
  items.add(Data('1', ObjectChangeType.added, 'b'));
  items.add(Data('0', ObjectChangeType.removed, 'a'));
  items.add(Data('2', ObjectChangeType.added, 'c'));
  items.add(Data('1', ObjectChangeType.modified, 'd'));
  items.add(Data('3', ObjectChangeType.added, 'e'));
  final DataEvent event = DataEvent(items);

  final Map<String, Data> state = Map<String, Data>();
  final Map<String, Data> newState = listenEventReducer<Data>(state, event);

  expect(newState, isNotNull);
  expect(newState.length, 3);
  expect(newState['0'], isNull);
  expect(newState['1'].metadata, 'd');
  expect(newState['2'].metadata, 'c');
  expect(newState['3'].metadata, 'e');
}

Answer 1

问题是 typedef和函数类型 _defaultUpdate不完全匹配并向编译器提供有关类型泛型的完整信息。

初始化时[UpdateItem<T> update = _defaultUpdate] ，编译器需要类型 T对于 UpdateItem 但它不知道是否 _defaultUpdate实际上正在返回 T其中 T extends ListenEventItem , _defaultUpdate可以返回任何类型 S其中 S extends ListenEventItem .

例如，假设您有两个扩展 ListenEventItem 的类

class A extends ListenEventItem {}

和

class B extends ListenEventItem {}

现在，_defaultUpdate函数可以返回 A或 B当没有传递通用参数时。当你尝试像这样初始化它时

UpdateItem<A> update = _defaultUpdate

编译器不知道是否 _defaultUpdate返回 A或 B使其在编译时失败。

---

您可以修复此问题，使 typedef 函数接受通用参数，这样编译器就知道期望任何扩展 ListenEventItem 的类

例子:

typedef UpdateItem = T Function<T extends ListenEventItem>(T oldItem, T newItem);

T _defaultUpdate<T extends ListenEventItem>(T oldItem, T newItem) => newItem;

完整代码:

import 'package:test/test.dart';

enum ObjectChangeType { added, modified, removed }

typedef UpdateItem = T Function<T extends ListenEventItem>(T oldItem, T newItem);

T _defaultUpdate<T extends ListenEventItem>(T oldItem, T newItem) => newItem;

abstract class ListenEventItem {
  String get id;

  ObjectChangeType get changeType;
}

abstract class ListenEvent<T extends ListenEventItem> {
  List<T> get data;

  Map<ObjectChangeType, List<T>> get typedData {
    return data.fold(
      <ObjectChangeType, List<T>>{},
      (Map<ObjectChangeType, List<T>> map, T element) {
        map[element.changeType] = (map[element.changeType] ?? <T>[])
          ..add(element);

        return map;
      },
    );
  }
}

Map<String, T> listenEventReducer<T extends ListenEventItem>(
    Map<String, T> state, ListenEvent<T> action,
    [UpdateItem update = _defaultUpdate]) {
  update ??= _defaultUpdate;
  final Map<String, T> b = Map<String, T>.from(state);

  for (ObjectChangeType type in ObjectChangeType.values) {
    final List<T> data = action.typedData[type];
    if (data == null || data.isEmpty) {
      continue;
    }

    if (type == ObjectChangeType.added) {
      for (T item in data) {
        b[item.id] = item;
      }
    } else if (type == ObjectChangeType.modified) {
      for (T item in data) {
        b[item.id] = update(b[item.id], item);
      }
    } else if (type == ObjectChangeType.removed) {
      final List<String> ids = data.map((T it) => it.id).toList();
      b.removeWhere((String id, _) => ids.contains(id));
    }
  }

  return b;
}

class Data extends ListenEventItem {
  Data(this.id, this.changeType, this.metadata);

  final String metadata;

  @override
  final String id;

  @override
  final ObjectChangeType changeType;
}

class DataEvent extends ListenEvent<Data> {
  DataEvent(this.data);

  @override
  final List<Data> data;
}

void main() {
  final List<Data> items = List<Data>();
  items.add(Data('0', ObjectChangeType.added, 'a'));
  items.add(Data('1', ObjectChangeType.added, 'b'));
  items.add(Data('0', ObjectChangeType.removed, 'a'));
  items.add(Data('2', ObjectChangeType.added, 'c'));
  items.add(Data('1', ObjectChangeType.modified, 'd'));
  items.add(Data('3', ObjectChangeType.added, 'e'));
  final DataEvent event = DataEvent(items);

  final Map<String, Data> state = Map<String, Data>();
  final Map<String, Data> newState = listenEventReducer<Data>(state, event);

  test('Test', () {
    expect(newState, isNotNull);
    expect(newState.length, 3);
    expect(newState['0'], isNull);
    expect(newState['1'].metadata, 'd');
    expect(newState['2'].metadata, 'c');
    expect(newState['3'].metadata, 'e');
  });
}