在 Flutter Web 应用程序中使用 rootBundle 加载资产 - 未执行的空值错误

Question

rootBundle.loadString($path)function 调用有问题。我想从 yaml 文件资产加载配置，并在我的 Web 应用程序 的基础上创建 Config 自定义对象。但我收到错误Unexpected null value。我处理不了，有人遇到过类似问题吗？

颤振医生：

[✓] Flutter (Channel master, 2.3.0-17.0.pre.225, on Ubuntu 18.04.5 LTS 5.4.0-74-generic, locale pl_PL.UTF-8)
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错误：

Error: Unexpected null value.
    at Object.throw_ [as throw] (http://localhost:40177/dart_sdk.js:5056:11)
    at Object.nullCheck (http://localhost:40177/dart_sdk.js:5381:30)
    at load (http://localhost:40177/packages/flutter/src/services/system_channels.dart.lib.js:2240:33)
    at load.next (<anonymous>)
    at runBody (http://localhost:40177/dart_sdk.js:37463:34)
    at Object._async [as async] (http://localhost:40177/dart_sdk.js:37494:7)
    at asset_bundle.PlatformAssetBundle.new.load (http://localhost:40177/packages/flutter/src/services/system_channels.dart.lib.js:2238:20)
    at asset_bundle.PlatformAssetBundle.new.loadString (http://localhost:40177/packages/flutter/src/services/system_channels.dart.lib.js:2110:32)
    at loadString.next (<anonymous>)
    at runBody (http://localhost:40177/dart_sdk.js:37463:34)
    at Object._async [as async] (http://localhost:40177/dart_sdk.js:37494:7)
    at asset_bundle.PlatformAssetBundle.new.loadString (http://localhost:40177/packages/flutter/src/services/system_channels.dart.lib.js:2109:20)
    at http://localhost:40177/packages/flutter/src/services/system_channels.dart.lib.js:2191:83
    at IdentityMap.new.putIfAbsent (http://localhost:40177/dart_sdk.js:25663:21)
    at asset_bundle.PlatformAssetBundle.new.loadString (http://localhost:40177/packages/flutter/src/services/system_channels.dart.lib.js:2191:57)
    at create (http://localhost:40177/packages/egm/config/config.dart.lib.js:25:60)
    at create.next (<anonymous>)
    at runBody (http://localhost:40177/dart_sdk.js:37463:34)
    at Object._async [as async] (http://localhost:40177/dart_sdk.js:37494:7)
    at Function.create (http://localhost:40177/packages/egm/config/config.dart.lib.js:24:20)
    at main$ (http://localhost:40177/packages/egm/main.dart.lib.js:230:42)
    at main$.next (<anonymous>)
    at runBody (http://localhost:40177/dart_sdk.js:37463:34)
    at Object._async [as async] (http://localhost:40177/dart_sdk.js:37494:7)
    at main$ (http://localhost:40177/packages/egm/main.dart.lib.js:228:18)
    at main (http://localhost:40177/web_entrypoint.dart.lib.js:33:29)
    at main.next (<anonymous>)
    at http://localhost:40177/dart_sdk.js:37444:33
    at _RootZone.runUnary (http://localhost:40177/dart_sdk.js:37315:59)
    at _FutureListener.thenAwait.handleValue (http://localhost:40177/dart_sdk.js:32550:29)
    at handleValueCallback (http://localhost:40177/dart_sdk.js:33077:49)
    at Function._propagateToListeners (http://localhost:40177/dart_sdk.js:33115:17)
    at _Future.new.[_completeWithValue] (http://localhost:40177/dart_sdk.js:32963:23)
    at http://localhost:40177/dart_sdk.js:32214:46
    at _RootZone.runUnary (http://localhost:40177/dart_sdk.js:37315:59)
    at _FutureListener.then.handleValue (http://localhost:40177/dart_sdk.js:32550:29)
    at handleValueCallback (http://localhost:40177/dart_sdk.js:33077:49)
    at Function._propagateToListeners (http://localhost:40177/dart_sdk.js:33115:17)
    at _Future.new.[_completeWithValue] (http://localhost:40177/dart_sdk.js:32963:23)
    at async._AsyncCallbackEntry.new.callback (http://localhost:40177/dart_sdk.js:32984:35)
    at Object._microtaskLoop (http://localhost:40177/dart_sdk.js:37567:13)
    at _startMicrotaskLoop (http://localhost:40177/dart_sdk.js:37573:13)
    at http://localhost:40177/dart_sdk.js:33323:9

配置类代码：

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

class Config {
  /// private constructor
  Config._(dynamic yamlConfig)
      : communication =
            CommunicationConfig.fromYaml(yamlConfig["communication"]);

  /// Factory method
  static Future<Config> create(final String path) async {
    dynamic yamlRawConfig = await rootBundle.loadString(path); //! ERROR
    dynamic yamlConfig = loadYaml(yamlRawConfig);
    return Config._(yamlConfig);
  }

  CommunicationConfig communication;
}

class CommunicationConfig {
  CommunicationConfig.fromYaml(dynamic communication)
      : address = communication["address"];
  String address = "";
}

在 main 中创建配置：

void main() async {
  dynamic app = MyApp();
  Config config = await Config.create("assets/config.yaml");
  Communication(messagesHandler: MessagesHandler(), config: config);
  runApp(app);
}

Answer 1

您需要在 main 函数的开头调用 WidgetsFlutterBinding.ensureInitialized()。

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  // Your code here...
  runApp(MyApp());
}

Answer 2

对于您想要做的工作，您需要确保 pubspec.yaml 具有正确声明的必要颤振资产