从屏幕返回数据

在某些情况下，您可能希望从新屏幕返回数据。例如，假设您推送一个新屏幕，该屏幕向用户提供两个选项。当用户点击一个选项时，您希望通知第一个屏幕用户的选择，以便它可以根据该信息采取行动。

您可以使用以下步骤使用 Navigator.pop() 方法执行此操作

1. 定义主屏幕
2. 添加一个启动选择屏幕的按钮
3. 显示带有两个按钮的选择屏幕
4. 当按钮被点击时，关闭选择屏幕
5. 在主屏幕上显示一个带有选择的 Snackbar

主屏幕显示一个按钮。点击时，它会启动选择屏幕。

class HomeScreen extends StatelessWidget {
  const HomeScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Returning Data Demo'),
      ),
      // Create the SelectionButton widget in the next step.
      body: const Center(
        child: SelectionButton(),
      ),
    );
  }
}

现在，创建 SelectionButton，它执行以下操作

• 点击时启动选择屏幕。
• 等待选择屏幕返回结果。

class SelectionButton extends StatefulWidget {
  const SelectionButton({super.key});

  @override
  State<SelectionButton> createState() => _SelectionButtonState();
}

class _SelectionButtonState extends State<SelectionButton> {
  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: () {
        _navigateAndDisplaySelection(context);
      },
      child: const Text('Pick an option, any option!'),
    );
  }

  Future<void> _navigateAndDisplaySelection(BuildContext context) async {
    // Navigator.push returns a Future that completes after calling
    // Navigator.pop on the Selection Screen.
    final result = await Navigator.push(
      context,
      // Create the SelectionScreen in the next step.
      MaterialPageRoute(builder: (context) => const SelectionScreen()),
    );
  }
}

现在，构建一个包含两个按钮的选择屏幕。当用户点击一个按钮时，该应用程序会关闭选择屏幕并让主屏幕知道哪个按钮被点击了。

此步骤定义了 UI。下一步将添加代码以返回数据。

class SelectionScreen extends StatelessWidget {
  const SelectionScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Pick an option'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Padding(
              padding: const EdgeInsets.all(8),
              child: ElevatedButton(
                onPressed: () {
                  // Pop here with "Yep"...
                },
                child: const Text('Yep!'),
              ),
            ),
            Padding(
              padding: const EdgeInsets.all(8),
              child: ElevatedButton(
                onPressed: () {
                  // Pop here with "Nope"...
                },
                child: const Text('Nope.'),
              ),
            )
          ],
        ),
      ),
    );
  }
}

现在，更新两个按钮的 onPressed() 回调。要将数据返回到第一个屏幕，请使用 Navigator.pop() 方法，该方法接受一个可选的第二个参数，称为 result。任何结果都将返回到 SelectionButton 中的 Future。

ElevatedButton(
  onPressed: () {
    // Close the screen and return "Yep!" as the result.
    Navigator.pop(context, 'Yep!');
  },
  child: const Text('Yep!'),
)

ElevatedButton(
  onPressed: () {
    // Close the screen and return "Nope." as the result.
    Navigator.pop(context, 'Nope.');
  },
  child: const Text('Nope.'),
)

现在您正在启动选择屏幕并等待结果，您将需要对返回的信息做些什么。

在这种情况下，使用 SelectionButton 中的 _navigateAndDisplaySelection() 方法显示一个显示结果的 SnackBar。

// A method that launches the SelectionScreen and awaits the result from
// Navigator.pop.
Future<void> _navigateAndDisplaySelection(BuildContext context) async {
  // Navigator.push returns a Future that completes after calling
  // Navigator.pop on the Selection Screen.
  final result = await Navigator.push(
    context,
    MaterialPageRoute(builder: (context) => const SelectionScreen()),
  );

  // When a BuildContext is used from a StatefulWidget, the mounted property
  // must be checked after an asynchronous gap.
  if (!context.mounted) return;

  // After the Selection Screen returns a result, hide any previous snackbars
  // and show the new result.
  ScaffoldMessenger.of(context)
    ..removeCurrentSnackBar()
    ..showSnackBar(SnackBar(content: Text('$result')));
}

import 'package:flutter/material.dart';

void main() {
  runApp(
    const MaterialApp(
      title: 'Returning Data',
      home: HomeScreen(),
    ),
  );
}

class HomeScreen extends StatelessWidget {
  const HomeScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Returning Data Demo'),
      ),
      body: const Center(
        child: SelectionButton(),
      ),
    );
  }
}

class SelectionButton extends StatefulWidget {
  const SelectionButton({super.key});

  @override
  State<SelectionButton> createState() => _SelectionButtonState();
}

class _SelectionButtonState extends State<SelectionButton> {
  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: () {
        _navigateAndDisplaySelection(context);
      },
      child: const Text('Pick an option, any option!'),
    );
  }

  // A method that launches the SelectionScreen and awaits the result from
  // Navigator.pop.
  Future<void> _navigateAndDisplaySelection(BuildContext context) async {
    // Navigator.push returns a Future that completes after calling
    // Navigator.pop on the Selection Screen.
    final result = await Navigator.push(
      context,
      MaterialPageRoute(builder: (context) => const SelectionScreen()),
    );

    // When a BuildContext is used from a StatefulWidget, the mounted property
    // must be checked after an asynchronous gap.
    if (!context.mounted) return;

    // After the Selection Screen returns a result, hide any previous snackbars
    // and show the new result.
    ScaffoldMessenger.of(context)
      ..removeCurrentSnackBar()
      ..showSnackBar(SnackBar(content: Text('$result')));
  }
}

class SelectionScreen extends StatelessWidget {
  const SelectionScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Pick an option'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Padding(
              padding: const EdgeInsets.all(8),
              child: ElevatedButton(
                onPressed: () {
                  // Close the screen and return "Yep!" as the result.
                  Navigator.pop(context, 'Yep!');
                },
                child: const Text('Yep!'),
              ),
            ),
            Padding(
              padding: const EdgeInsets.all(8),
              child: ElevatedButton(
                onPressed: () {
                  // Close the screen and return "Nope." as the result.
                  Navigator.pop(context, 'Nope.');
                },
                child: const Text('Nope.'),
              ),
            )
          ],
        ),
      ),
    );
  }
}