getApplicationDocumentsDirectory 在 Flutter 中对 null 运算符进行 null 检查

Question

几天来一直在研究此代码，试图学习如何在颤振项目中正确地与 SQFLite 交互，在我的编码过程中，我遇到了一个错误并检查了整个 StackOverflow，我什至用谷歌搜索了它但没有答案.虽然我在 StackOverflow 上发现了一个类似问题的问题，但没有给出答案（很抱歉，如果我问这个问题，因为已经存在一个问题，但因为它没有得到回答）

正在做什么

我实际上是在尝试从互联网上提取数据并将其直接保存到本地存储 (sqflite)，所以每次我运行我的代码时，数据都会插入到本地存储中，但我会收到关于特定行的错误

我的代码

DatabaseHelper 类

class DatabaseHelper {
  DatabaseHelper._createInstance();
  static DatabaseHelper databaseHelper = new DatabaseHelper._createInstance();

  static Database? _database = null;

  static final _databaseName = 'prodcuts.db';
  static final _databaseVersion = 1;
  String productTable = 'product_table';
  String colId = 'id';
  String colTitle = 'title';
  String colUrl = 'url';
  String colThubmnail = 'thumbnailUrl';
  String colalbumId = 'albumId';

  Future<Directory?>? _appDocumentDirectory;

  factory DatabaseHelper() {
    if (databaseHelper != null) {
      databaseHelper = DatabaseHelper._createInstance();
    }
    return databaseHelper;
  }

  // Future<Database> get database async {
  //   print('Learning App 1 - ${_database}');
  //   var ini = await initializeDatabase();
  //   print('Learning App 2 -${ini}');
  //   return _database ??= await initializeDatabase();
  // }

  Future<Database> get database async {
    if (_database != null) {
      return _database!;
    }
    _database = await initializeDatabase();
    return _database!;
  }

  Future<Database> initializeDatabase() async {
    Directory directory = await getApplicationDocumentsDirectory();

    String path = join(directory.path, _databaseName);

    print("Learning App 3 -${directory}");

    var productsDatabase = await openDatabase(path,
        version: _databaseVersion, onCreate: _createDb);
    return productsDatabase;
  }

  void _createDb(Database db, int newVersion) async {
    await db
        .execute('CREATE TABLE $productTable($colId INTEGER, $colTitle TEXT, '
            '$colUrl TEXT,  $colThubmnail TEXT, $colalbumId INTEGER )');
  }

  Future<List<Map<String, dynamic>>> getProductMapList() async {
    Database db = await databaseHelper.database;
    var result = await db.query(productTable, orderBy: '$colId ASC');
    return result;
  }

  // Insert Operation: Insert a Note object to database
  // Future<int> insertProduct(Product product) async {
  //   Database db = await this.database;
  //   var result = await db.insert(productTable, product.toJson());
  //   return result;
  // }

  Future<List<Product>> insertProduct(List<Product> product) async {
    Database db = await databaseHelper.database;

    product.forEach((element) async {
      await db.insert(productTable, element.toJson());
    });

    print('was inserted');
    return product;
  }

  Future<int> updateProduct(Product product) async {
    var db = await databaseHelper.database;
    var result = await db.update(productTable, product.toJson(),
        where: '$colId = ?', whereArgs: [product.id]);
    return result;
  }

  Future<int> deleteProduct(int id) async {
    var db = await databaseHelper.database;
    int result =
        await db.rawDelete('DELETE FROM $productTable WHERE $colId = $id');
    return result;
  }

  Future<int?> getCount() async {
    Database db = await databaseHelper.database;
    List<Map<String, dynamic>> x =
        await db.rawQuery('SELECT COUNT (*) from $productTable');
    int? result = Sqflite.firstIntValue(x);
    return result;
  }

  Future<List<Product>> getProductList() async {
    var productMapList = await getProductMapList();
    int count = productMapList.length;

    var productList = <Product>[];

    for (int i = 0; i < count; i++) {
      //print('yes --${productMapList[i]}');
      productList.add(Product.fromJson(productMapList[i]));
    }

    return productList;
  }
}

得到了什么

我从这行得到一个错误

Directory directory = await getApplicationDocumentsDirectory();

到目前为止做了什么

• 已删除应用并重新安装
• 已降级 sqflite 和 path_provider 插件依赖项
• 使缓存无效并重新启动
• 飘飘然
• 在我的代码中添加了 hive_flutter

Future<void> main() async {
          Get.put(ProductController());
          WidgetsFlutterBinding.ensureInitialized();
          await Hive.initFlutter();
          runApp(MyApp());
        }

我的依赖

intl: ^0.17.0
sqflite: ^2.0.0+4
path_provider: ^2.0.5
hive_flutter: ^1.1.0

我显然不知道做错了什么，但我真的需要帮助来解决它

在我忘记之前，这是我的颤振医生-v

[✓] Flutter (Channel stable, 2.5.3, on macOS 11.4 20F71 darwin-x64, locale en-US)
    • Flutter version 2.5.3 at /Users/apple/Developer/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 18116933e7 (7 days ago), 2021-10-15 10:46:35 -0700
    • Engine revision d3ea636dc5
    • Dart version 2.14.4

[!] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at /Users/apple/Library/Android/sdk
    • Platform android-30, build-tools 30.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)
    ! Some Android licenses not accepted.  To resolve this, run: flutter doctor --android-licenses

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.5, Build version 12E262
    • CocoaPods version 1.10.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2020.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      ???? https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      ???? https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)

[✓] Connected device (2 available)
    • iPhone 12 Pro Max (mobile) • 3D408340-B566-4B0C-A9C3-161B148ECC4A • ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-5 (simulator)
    • Chrome (web)               • chrome                               • web-javascript • Google Chrome 94.0.4606.81

! Doctor found issues in 1 category.

Answer 1

您是否添加了存储权限并在运行时请求它们？