“value.isInitialized”：不是 true || Flutter桌面错误

Question

我正在为 flutter 桌面窗口开发 Blockly（Blockly 是编程语言 JavaScript 的客户端库，用于创建基于块的可视化编程语言和编辑器）。 我正在使用 webview_windows 包在网络视图中显示“https://developers.google.com/blockly”。

下面是我的代码

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'dart:async';

import 'package:webview_windows/webview_windows.dart';

 final navigatorKey = GlobalKey<NavigatorState>();

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

 class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
   return MaterialApp(navigatorKey: navigatorKey, home: ExampleBrowser());
  }
}

 class ExampleBrowser extends StatefulWidget {
 @override
 State<ExampleBrowser> createState() => _ExampleBrowser();
}

class _ExampleBrowser extends State<ExampleBrowser> {
final _controller = WebviewController();
final _textController = TextEditingController();
 bool _isWebviewSuspended = false;

 @override
 void initState() {
     super.initState();
      initPlatformState();
     }

     Future<void> initPlatformState() async {
       try {
        await _controller.initialize();
        _controller.url.listen((url) {
         _textController.text = URL;
         });

      await _controller.setBackgroundColor(Colors.transparent);
      await _controller.setPopupWindowPolicy(WebviewPopupWindowPolicy.deny);
      await _controller.loadUrl('https://developers.google.com/blockly');

  if (!mounted) return;
  setState(() {});
} on PlatformException catch (e) {
  WidgetsBinding.instance?.addPostFrameCallback((_) {
    showDialog(
        context: context,
        builder: (_) => AlertDialog(
              title: Text('Error'),
              content: Column(
                mainAxisSize: MainAxisSize.min,
                crossAxisAlignment: CrossAxisAlignment.start,
                children: [
                  Text('Code: ${e.code}'),
                  Text('Message: ${e.message}'),
                ],
              ),
              actions: [
                TextButton(
                  child: Text('Continue'),
                  onPressed: () {
                    Navigator.of(context).pop();
                  },
                )
              ],
            ));
   });
  }
 }

 Widget compositeView() {
  if (!_controller.value.isInitialized) {
  return const Text(
    'Not Initialized',
    style: TextStyle(
      fontSize: 24.0,
      fontWeight: FontWeight.w900,
    ),
  );
} else {
  return Padding(
    padding: EdgeInsets.all(20),
    child: Column(
      children: [
        Card(
          elevation: 0,
          child: TextField(
            decoration: InputDecoration(
                hintText: 'URL',
                contentPadding: EdgeInsets.all(10.0),
                suffixIcon: IconButton(
                  icon: Icon(Icons.refresh),
                  onPressed: () {
                    _controller.reload();
                  },
                )),
            textAlignVertical: TextAlignVertical.center,
            controller: _textController,
            onSubmitted: (val) {
              _controller.loadUrl(val);
            },
          ),
        ),
        Expanded(
            child: Card(
                color: Colors.transparent,
                elevation: 0,
                clipBehavior: Clip.antiAliasWithSaveLayer,
                child: Stack(
                  children: [
                    Webview(
                      _controller,
                      permissionRequested: _onPermissionRequested,
                    ),
                    StreamBuilder<LoadingState>(
                        stream: _controller.loadingState,
                        builder: (context, snapshot) {
                          if (snapshot.hasData &&
                              snapshot.data == LoadingState.loading) {
                            return LinearProgressIndicator();
                          } else {
                            return SizedBox();
                          }
                        }),
                  ],
                ))),
      ],
    ),
    );
  }
  }

  @override
    Widget build(BuildContext context) {
    return Scaffold(
    floatingActionButton: FloatingActionButton(
    tooltip: _isWebviewSuspended ? 'Resume webview' : 'Suspend webview',
    onPressed: () async {
      if (_isWebviewSuspended) {
        await _controller.resume();
      } else {
        await _controller.suspend();
      }
      setState(() {
        _isWebviewSuspended = !_isWebviewSuspended;
      });
    },
    child: Icon(_isWebviewSuspended ? Icons.play_arrow : Icons.pause),
  ),
  appBar: AppBar(
      title: StreamBuilder<String>(
    stream: _controller.title,
    builder: (context, snapshot) {
      return Text(
          snapshot.hasData ? snapshot.data! : 'WebView (Windows) Example');
    },
  )),
    body: Center(
      child: compositeView(),
      ),
     );
  }

Future<WebviewPermissionDecision> _onPermissionRequested(
    String url, WebviewPermissionKind kind, bool isUserInitiated) async {
    final decision = await showDialog<WebviewPermissionDecision>(
    context: navigatorKey.currentContext!,
    builder: (BuildContext context) => AlertDialog(
    title: const Text('WebView permission requested'),
    content: Text('WebView has requested permission \'$kind\''),
    actions: <Widget>[
      TextButton(
        onPressed: () =>
            Navigator.pop(context, WebviewPermissionDecision.deny),
        child: const Text('Deny'),
      ),
      TextButton(
        onPressed: () =>
            Navigator.pop(context, WebviewPermissionDecision.allow),
        child: const Text('Allow'),
      ),
    ],
  ),
  );

    return decision ?? WebviewPermissionDecision.none;
  }
}

，这是我在控制台中的错误

Answer 1

我在不同的场景中遇到了同样的问题，我所做的是对控制器内的其他函数执行 1 秒的 Future.delayed ，在我的情况下，异常在 setBackgroundColor 中抛出（这是在 initialize() 之后调用）说 value.isInitialized 在控制器初始化后不成立，这里是一个例子：

    Future.wait([
      _windowsWebViewController.initialize(),
      _windowsWebViewController.setBackgroundColor(Colors.transparent),
      _windowsWebViewController.setPopupWindowPolicy(WebviewPopupWindowPolicy.allow),
      _windowsWebViewController.loadUrl(element.url.toString().indexOf('.pdf') != -1
          ? 'https://docs.google.com/gview?embedded=true&url=${element.url.toString()}'
          : element.url.toString()),
    ]);

这里是解决方案：

Future.wait([
      _windowsWebViewController.initialize(),
    ]);

    Future.delayed(Duration(seconds: 1), () {

      _windowsWebViewController.setBackgroundColor(Colors.transparent);
      _windowsWebViewController.setPopupWindowPolicy(WebviewPopupWindowPolicy.allow);
      _windowsWebViewController.loadUrl(element.url.toString().indexOf('.pdf') != -1
      ? 'https://docs.google.com/gview?embedded=true&url=${element.url.toString()}'
          : element.url.toString());

    });

出于某种原因，即使我们初始化了控制器, 其余的即使我们使用await来完成初始化，它内部的方法在控制器内部的值发生变化时也不会收到通知。