使用 Sqflite 对 flutter 上的 null 值使用 Null Check 运算符

Question

我正在使用 Sqflite 研究 flutter 并尝试制作一个小应用程序，但是当我运行代码时出现此错误，我几乎到处都在寻找它可能是什么。

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这里是使用Sqflite包的代码：

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    Future<Database> getDatabase() async {\n      final String path = join(await getDatabasesPath(), 'bytebank.db');\n      return openDatabase(path, onCreate: (db, version) {\n        db.execute('CREATE TABLE contacts'\n            '(id INTEGER PRIMARY KEY, '\n            'name TEXT, '\n            'account_number INTEGER)');\n      }, version: 1);\n    }\n    \n    Future<int> save(Contact contact) async {\n      final Database db = await getDatabase();\n      final Map<String, dynamic> contactMap = Map();\n      contactMap['name'] = contact.name;\n      contactMap['account_number'] = contact.account;\n      contactMap['id'] = contact.id;\n      return db.insert('contacts', contactMap);\n    }\n    \n    Future<List<Contact>> findAll() async {\n      final Database db = await getDatabase();\n      final List<Map<String, dynamic>> result = await db.query('contacts');\n      final List<Contact> contacts = [];\n      for (Map<String, dynamic> row in result) {\n        final Contact contact = Contact(\n          row['id'],\n          row['name'],\n          row['account_number'],\n        );\n        contacts.add(contact);\n      }\n      return contacts;\n    }\n

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这是我使用此功能的唯一一个地方：

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void main() {\n  save(Contact('William', 2, 13456)).then((id) {\n  findAll().then((contacts) => print(contacts));\n  });\n  runApp(ByteBankApp());\n}\n

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型号类别：

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class Contact {\n  final String name;\n  final int account;\n  final int id;\n  Contact( this.id,\n    this.name,\n    this.account,\n);\n\n  @override\n  String toString() {\n    return 'Contact{name: $name, account: $account}';\n  }\n}\n

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错误回溯：

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Launching lib\\main.dart on sdk gphone x86 arm in debug mode...\nRunning Gradle task 'assembleDebug'...\n\xe2\x88\x9a  Built build\\app\\outputs\\flutter-apk\\app-debug.apk.\nInstalling build\\app\\outputs\\flutter-apk\\app.apk...\nDebug service listening on ws://127.0.0.1:52296/07rtnvW9tlQ=/ws\nSyncing files to device sdk gphone x86 arm...\nE/flutter (28649): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value\nE/flutter (28649): #0      MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:142:86)\nE/flutter (28649): #1      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:148:36)\nE/flutter (28649): #2      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:331:12)\nE/flutter (28649): #3      invokeMethod (package:sqflite/src/sqflite_impl.dart:17:13)\nE/flutter (28649): #4      SqfliteDatabaseFactoryImpl.invokeMethod (package:sqflite/src/factory_impl.dart:82:7)\nE/flutter (28649): #5      SqfliteDatabaseFactoryMixin.safeInvokeMethod.<anonymous closure> (package:sqflite_common/src/factory_mixin.dart:41:38)\nE/flutter (28649): #6      wrapDatabaseException (package:sqflite/src/exception_impl.dart:7:32)\nE/flutter (28649): #7      SqfliteDatabaseFactoryImpl.wrapDatabaseException (package:sqflite/src/factory_impl.dart:78:7)\nE/flutter (28649): #8      SqfliteDatabaseFactoryMixin.safeInvokeMethod (package:sqflite_common/src/factory_mixin.dart:41:7)\nE/flutter (28649): #9      SqfliteDatabaseFactoryMixin.getDatabasesPath (package:sqflite_common/src/factory_mixin.dart:153:26)\nE/flutter (28649): #10     getDatabasesPath (package:sqflite/sqflite.dart:161:54)\nE/flutter (28649): #11     getDatabase (package:bytebank/database/app_database.dart:6:34)\nE/flutter (28649): #12     save (package:bytebank/database/app_database.dart:16:29)\nE/flutter (28649): #13     main (package:bytebank/main.dart:7:3)\nE/flutter (28649): #14     _runMainZoned.<anonymous closure>.<anonymous closure> (dart:ui/hooks.dart:142:25)\nE/flutter (28649): #15     _rootRun (dart:async/zone.dart:1354:13)\nE/flutter (28649): #16     _CustomZone.run (dart:async/zone.dart:1258:19)\nE/flutter (28649): #17     _runZoned (dart:async/zone.dart:1789:10)\nE/flutter (28649): #18     runZonedGuarded (dart:async/zone.dart:1777:12)\nE/flutter (28649): #19     _runMainZoned.<anonymous closure> (dart:ui/hooks.dart:138:5)\nE/flutter (28649): #20     _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:283:19)\nE/flutter (28649): #21     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)\nE/flutter (28649): \n

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Answer 1

我不确定是否有帮助，但在异步“主”方法中初始化数据库时我遇到了类似的问题。添加以下内容：

WidgetsFlutterBinding.ensureInitialized();

在初始化数据库的代码之前的方法内部解决了问题。