未来已完成的Flutter错误

Question

我在找出“错误状态：未来已经完成”的错误时遇到了麻烦。我基本上有一个使用BottomNavigationBar的应用程序，并且在我的一个屏幕上，我正在使用使用Completer类的此类。

再次创建Web视图时会发生这种情况：

      body: WebView(
        initialUrl: 'https://www.google.com/',
        onWebViewCreated: (WebViewController webViewController) {
          _controller.complete(webViewController);
        },
      )

在第一个视图上一切正常，但是如果您选择另一个BottomNavigationBarItem并返回到该类，则将收到状态为Exception的异常：Future已经完成。下面是完整的代码。

import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'package:url_launcher/url_launcher.dart';
import 'dart:async';

 class myClass extends StatelessWidget {
  Completer<WebViewController> _controller = Completer<WebViewController>();
  final Set<String> _favorites = Set<String>();

@override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        actions: <Widget>[
          NavigationControls(_controller.future),
          Menu(_controller.future, () => _favorites),
        ],
      ),
      //EXCEPTION OCCURS HERE
      //BAD STATE: FUTURE ALREADY COMPLETED
      body: WebView(
        initialUrl: 'https://www.google.com/',
        onWebViewCreated: (WebViewController webViewController) {
          _controller.complete(webViewController);
        },
      ),
      floatingActionButton: _bookmarkButton(),
    );
  }

  _bookmarkButton() {
    return FutureBuilder<WebViewController>(
      future: _controller.future,
      builder:
          (BuildContext context, AsyncSnapshot<WebViewController> controller) {
        if (controller.hasData) {
          return FloatingActionButton(
            onPressed: () async {
              var url = await controller.data.currentUrl();
              _favorites.add(url);
              Scaffold.of(context).showSnackBar(
                SnackBar(content: Text('Saved $url for later reading.')),
              );
            },
            child: Icon(Icons.favorite),
          );
        }
        return Container();
      },
    );
  }
}

class Menu extends StatelessWidget {
  Menu(this._webViewControllerFuture, this.favoritesAccessor);
  final Future<WebViewController> _webViewControllerFuture;
  final Function favoritesAccessor;

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: _webViewControllerFuture,
      builder:
          (BuildContext context, AsyncSnapshot<WebViewController> controller) {
        if (!controller.hasData) return Container();
        return PopupMenuButton<String>(
          onSelected: (String value) async {
            if (value == 'Email link') {
              var url = await controller.data.currentUrl();
              await launch(
                  'mailto:?subject=hello&body=$url');
            } else {
              var newUrl = await Navigator.push(context,
                  MaterialPageRoute(builder: (BuildContext context) {
                return FavoritesPage(favoritesAccessor());
              }));
              Scaffold.of(context).removeCurrentSnackBar();
              if (newUrl != null) controller.data.loadUrl(newUrl);
            }
          },
          itemBuilder: (BuildContext context) => <PopupMenuItem<String>>[
                const PopupMenuItem<String>(
                  value: 'Email link',
                  child: Text('Email link'),
                ),
                const PopupMenuItem<String>(
                  value: 'See Favorites',
                  child: Text('See Favorites'),
                ),
              ],
        );
      },
    );
  }
}

class FavoritesPage extends StatelessWidget {
  FavoritesPage(this.favorites);
  final Set<String> favorites;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Favorite pages')),
      body: ListView(
          children: favorites
              .map((url) => ListTile(
                  title: Text(url), onTap: () => Navigator.pop(context, url)))
              .toList()),
    );
  }
}

class NavigationControls extends StatelessWidget {
  const NavigationControls(this._webViewControllerFuture)
      : assert(_webViewControllerFuture != null);
  final Future<WebViewController> _webViewControllerFuture;
  @override
  Widget build(BuildContext context) {
    return FutureBuilder<WebViewController>(
      future: _webViewControllerFuture,
      builder:
          (BuildContext context, AsyncSnapshot<WebViewController> snapshot) {
        final bool webViewReady =
            snapshot.connectionState == ConnectionState.done;
        final WebViewController controller = snapshot.data;
        return Row(
          children: <Widget>[
            IconButton(
              icon: const Icon(Icons.arrow_back_ios),
              onPressed: !webViewReady
                  ? null
                  : () => navigate(context, controller, goBack: true),
            ),
            IconButton(
              icon: const Icon(Icons.arrow_forward_ios),
              onPressed: !webViewReady
                  ? null
                  : () => navigate(context, controller, goBack: false),
            ),
          ],
        );
      },
    );
  }

  navigate(BuildContext context, WebViewController controller,
      {bool goBack: false}) async {
    bool canNavigate =
        goBack ? await controller.canGoBack() : await controller.canGoForward();
    if (canNavigate) {
      goBack ? controller.goBack() : controller.goForward();
    } else {
      Scaffold.of(context).showSnackBar(
        SnackBar(
            content: Text("No ${goBack ? 'back' : 'forward'} history item")),
      );
    }
  }
}

Answer 1

试图重现该问题，但未能做到。您可以使用底部导航栏等显示代码吗？我是这样解释的，它在这里起作用：

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:url_launcher/url_launcher.dart';
import 'package:webview_flutter/webview_flutter.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      initialRoute: "/",
      routes: {
        "/": (context) => Home(),
      },
    );
  }
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  final navigatorKey = GlobalKey<NavigatorState>();
  int _currentIndex = 0;
  final pagesRouteFactories = {
    'page1': (RouteSettings settings) => MaterialPageRoute<dynamic>(
          builder: (BuildContext context) => HomePage(),
          settings: settings,
        ),
    'page2': (RouteSettings settings) => MaterialPageRoute<dynamic>(
          builder: (BuildContext context) => MyClass(),
          settings: settings,
        ),
  };

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: MaterialApp(
        navigatorKey: navigatorKey,
        onGenerateRoute: (RouteSettings route) =>
            pagesRouteFactories[route.name](route),
        initialRoute: 'page1',
      ),
      bottomNavigationBar: BottomNavigationBar(
        backgroundColor: Colors.white,
        type: BottomNavigationBarType.fixed,
        currentIndex: _currentIndex,
        items: <BottomNavigationBarItem>[
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            title: FittedBox(
              child: Text(
                'Home',
                style: TextStyle(fontWeight: FontWeight.w600),
              ),
            ),
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.web),
            title: FittedBox(
              child: Text(
                'Web',
                style: TextStyle(fontWeight: FontWeight.w600),
              ),
            ),
          ),
        ],
        onTap: (int index) {
          setState(() {
            _currentIndex = index;
          });
          navigatorKey.currentState
              .pushReplacementNamed(pagesRouteFactories.keys.toList()[index]);
        },
      ),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Text('hi'),
      ),
    );
  }
}

class MyClass extends StatelessWidget {
  Completer<WebViewController> _controller = Completer<WebViewController>();
  final Set<String> _favorites = Set<String>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        actions: <Widget>[
          NavigationControls(_controller.future),
          Menu(_controller.future, () => _favorites),
        ],
      ),
      //EXCEPTION OCCURS HERE
      //BAD STATE: FUTURE ALREADY COMPLETED
      body: WebView(
        initialUrl: 'https://www.google.com/',
        onWebViewCreated: (WebViewController webViewController) {
          _controller.complete(webViewController);
        },
      ),
      floatingActionButton: _bookmarkButton(),
    );
  }

  _bookmarkButton() {
    return FutureBuilder<WebViewController>(
      future: _controller.future,
      builder:
          (BuildContext context, AsyncSnapshot<WebViewController> controller) {
        if (controller.hasData) {
          return FloatingActionButton(
            onPressed: () async {
              var url = await controller.data.currentUrl();
              _favorites.add(url);
              Scaffold.of(context).showSnackBar(
                SnackBar(content: Text('Saved $url for later reading.')),
              );
            },
            child: Icon(Icons.favorite),
          );
        }
        return Container();
      },
    );
  }
}

class Menu extends StatelessWidget {
  Menu(this._webViewControllerFuture, this.favoritesAccessor);
  final Future<WebViewController> _webViewControllerFuture;
  final Function favoritesAccessor;

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: _webViewControllerFuture,
      builder:
          (BuildContext context, AsyncSnapshot<WebViewController> controller) {
        if (!controller.hasData) return Container();
        return PopupMenuButton<String>(
          onSelected: (String value) async {
            if (value == 'Email link') {
              var url = await controller.data.currentUrl();
              await launch('mailto:?subject=hello&body=$url');
            } else {
              var newUrl = await Navigator.push(context,
                  MaterialPageRoute(builder: (BuildContext context) {
                return FavoritesPage(favoritesAccessor());
              }));
              Scaffold.of(context).removeCurrentSnackBar();
              if (newUrl != null) controller.data.loadUrl(newUrl);
            }
          },
          itemBuilder: (BuildContext context) => <PopupMenuItem<String>>[
            const PopupMenuItem<String>(
              value: 'Email link',
              child: Text('Email link'),
            ),
            const PopupMenuItem<String>(
              value: 'See Favorites',
              child: Text('See Favorites'),
            ),
          ],
        );
      },
    );
  }
}

class FavoritesPage extends StatelessWidget {
  FavoritesPage(this.favorites);
  final Set<String> favorites;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Favorite pages')),
      body: ListView(
          children: favorites
              .map((url) => ListTile(
                  title: Text(url), onTap: () => Navigator.pop(context, url)))
              .toList()),
    );
  }
}

class NavigationControls extends StatelessWidget {
  const NavigationControls(this._webViewControllerFuture)
      : assert(_webViewControllerFuture != null);
  final Future<WebViewController> _webViewControllerFuture;
  @override
  Widget build(BuildContext context) {
    return FutureBuilder<WebViewController>(
      future: _webViewControllerFuture,
      builder:
          (BuildContext context, AsyncSnapshot<WebViewController> snapshot) {
        final bool webViewReady =
            snapshot.connectionState == ConnectionState.done;
        final WebViewController controller = snapshot.data;
        return Row(
          children: <Widget>[
            IconButton(
              icon: const Icon(Icons.arrow_back_ios),
              onPressed: !webViewReady
                  ? null
                  : () => navigate(context, controller, goBack: true),
            ),
            IconButton(
              icon: const Icon(Icons.arrow_forward_ios),
              onPressed: !webViewReady
                  ? null
                  : () => navigate(context, controller, goBack: false),
            ),
          ],
        );
      },
    );
  }

  navigate(BuildContext context, WebViewController controller,
      {bool goBack: false}) async {
    bool canNavigate =
        goBack ? await controller.canGoBack() : await controller.canGoForward();
    if (canNavigate) {
      goBack ? controller.goBack() : controller.goForward();
    } else {
      Scaffold.of(context).showSnackBar(
        SnackBar(
            content: Text("No ${goBack ? 'back' : 'forward'} history item")),
      );
    }
  }
}