從螢幕返回資料

在某些情況下，你可能希望從新螢幕返回資料。例如，假設你推送了一個新螢幕，它向用戶顯示兩個選項。當用戶輕觸一個選項時，你希望將使用者的選擇告知第一個螢幕，以便它能夠根據這些資訊採取行動。

你可以使用 Navigator.pop() 方法，按照以下步驟實現：

定義主螢幕
新增一個啟動選擇螢幕的按鈕
顯示帶有兩個按鈕的選擇螢幕
輕觸按鈕時，關閉選擇螢幕
在主螢幕上顯示一個帶有選擇結果的 Snackbar

主螢幕顯示一個按鈕。輕觸時，它會啟動選擇螢幕。

dartclass 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()),
    );
  }
}

現在，建立 SelectionButton，它執行以下操作：

輕觸時啟動 SelectionScreen。
等待 SelectionScreen 返回結果。

dartclass 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<String>(builder: (context) => const SelectionScreen()),
    );
  }
}

現在，構建一個包含兩個按鈕的選擇螢幕。當用戶輕觸一個按鈕時，應用程式會關閉選擇螢幕，並讓主螢幕知道哪個按鈕被輕觸了。

此步驟定義了 UI。下一步將新增程式碼以返回資料。

dartclass 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.'),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

現在，更新兩個按鈕的 onPressed() 回撥。要將資料返回到第一個螢幕，請使用 Navigator.pop() 方法，該方法接受一個可選的第二個引數，稱為 result。任何結果都將返回到 SelectionButton 中的 Future。

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

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

現在你已經啟動了一個選擇螢幕並等待結果，你將希望對返回的資訊做一些事情。

在這種情況下，透過使用 SelectionButton 中的 _navigateAndDisplaySelection() 方法，顯示一個顯示結果的 Snackbar。

dart// 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<String>(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<String>(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.'),
              ),
            ),
          ],
        ),
      ),
    );
  }
}