Dart泛型编程

Question

我正在尝试为Flutter项目实现一个块模式，我的代码是大量可重用的，所以我决定使用泛型编程来实现它。

首先，我定义了事件类：

import 'package:flutter_bloc/flutter_bloc.dart';

abstract class AbstractSelectionEvent<T> {
  final T obj;

  AbstractSelectionEvent(this.obj);
}

class SelectedEvent<T> extends AbstractSelectionEvent<T> {
  SelectedEvent(T obj) : super(obj);
}

class UnselectedEvent<T> extends AbstractSelectionEvent<T> {
  UnselectedEvent(T obj) : super(obj);
}

class ChangeSelectionEvent<T> extends AbstractSelectionEvent<T> {
  ChangeSelectionEvent(T obj) : super(obj);
}

然后是状态类：

abstract class AbstractSelectionState<T> {
  final Set<T> selected;

  AbstractSelectionState(this.selected);

  AbstractSelectionState<T> select(T obj);

  AbstractSelectionState<T> unselect(T obj);

  AbstractSelectionState<T> changeSelect(T obj);

  AbstractSelectionState<T> reset(T obj);
}

class SelectedState<T> extends AbstractSelectionState<T> {
  SelectedState(Set<T> selected) : super(selected);

  SelectedState<T> select(T obj) => SelectedState<T>(selected..add(obj));

  SelectedState<T> unselect(T obj) => SelectedState<T>(selected..remove(obj));

  SelectedState<T> changeSelect(T obj) =>
      selected.contains(obj) ? unselect(obj) : select(obj);

  SelectedState<T> reset(T obj) => SelectedState<T>(<T>{});
}

最后，当我编写BLOC类时，我遇到了一些麻烦。

abstract class SelectionBloc<
    T,
    E extends AbstractSelectionEvent<T>,
    S extends AbstractSelectionState<T>
  > extends Bloc<E, S> {
  SelectionBloc(S initialState) : super(initialState) {
    
    // ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ The first error
    on<SelectedEvent<T>>((event, emit) {
      
      //   ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ The second error
      emit(state.select(event.obj));
      
    });
    
  }
}

第一个错误是

'SelectedEvent<T>' doesn't conform to the bound 'E' of the type parameter 'E'.


Try using a type that is or is a subclass of 'E'.

第二个错误：

The argument type 'AbstractSelectionState<T>' can't be assigned to the parameter type 'S'.

对于第二个错误，我知道select、unselect、changeSelect、reset函数返回AbstractSelectionState<T>。

SelectionBloc期望emit函数的状态为S，它是AbstractSelectionState<T>的子类。

但是从另一个角度来看，AbstractSelectionState<T>类是抽象的，这意味着函数的返回值必须是AbstractSelectionState<T>子类的实例。

这个问题可以解决吗？

Answer 1

这应该是可行的：

abstract class SelectionBloc<T>
    extends Bloc<AbstractSelectionEvent<T>, AbstractSelectionState<T>> {

  SelectionBloc(AbstractSelectionState<T> initialState) : super(initialState) {
    
    on<AbstractSelectionEvent<T>>((
      event,
      emit,
    ) {
      emit(state.select(event.obj));
    });
    
  }
}