我可以使用 if else 语句在 BlocBuilder 中不传递任何内容吗

Question

如果 counterValue 数字小于 0，我不想将数据传递给文本小部件。此代码：

main.dart :

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:flutter_bloc_concepts/cubit/cubit/counter_cubit.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return BlocProvider<CounterCubit>(
      create: (context) => CounterCubit(),
      child: const MaterialApp(
        title: 'flutter_bloc Demo',
        debugShowCheckedModeBanner: false,
        home: HomePage(),
      ),
    );
  }
}

class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Flutter BLoC Concepts DEMO")),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text("You have pushed the button this many times:"),

            // Bloc Builder
            BlocBuilder<CounterCubit, CounterState>(
              builder: (context, state) {
                if (state.counterValue < 0) {
                  return NOTHING;
                } else {
                  return Text(
                    state.counterValue.toString(),
                    style: Theme.of(context).textTheme.headline4,
                  );
                }
              },
            ),

            const SizedBox(height: 20),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceEvenly,
              children: [
                FloatingActionButton(
                  onPressed: () {
                    BlocProvider.of<CounterCubit>(context).decrement();
                  },
                  tooltip: "Decrement",
                  child: const Icon(Icons.remove),
                ),
                FloatingActionButton(
                  onPressed: () {
                    BlocProvider.of<CounterCubit>(context).increment();
                  },
                  tooltip: "Increment",
                  child: const Icon(Icons.add),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

counter_cubit.dart :

import 'package:bloc/bloc.dart';
import 'package:meta/meta.dart';

part 'counter_state.dart';

class CounterCubit extends Cubit<CounterState> {
  CounterCubit() : super(CounterState(counterValue: 0));

  void increment() => emit(CounterState(counterValue: state.counterValue + 1));
  void decrement() => emit(CounterState(counterValue: state.counterValue - 1));
}

counter_state.dart :

part of 'counter_cubit.dart';

class CounterState {
  int counterValue;

  CounterState({
    required this.counterValue,
  });
}

当 counterValue 数字小于 0 时，我可以不向文本小部件传递任何内容吗？我不想显示负数，所以如果我在 counterValue = 0 显示数字时按减量按钮不是负数 -1,-2... / 我想保持在 0。我可以这样做吗

Answer 1

在 counter_cubit.dart 中添加 if else 语句：

void decrement() {
  if (state.counterValue <= 0) {
    state.counterValue = 0;
  } else {
    emit(CounterState(counterValue: state.counterValue - 1));
  }
}

我可以使用 if else 语句在 BlocBuilder 中不传递任何内容吗

Can i passing nothing in BlocBuilder with if else statement

dart

flutter

bloc

flutter-bloc

cubit