Flutter 错误：_TypeError（类型“Null”不是“FutureOr<Database>”类型的子类型）

Posted 2023-03-12

【中文标题】Flutter 错误：_TypeError（类型“Null”不是“FutureOr<Database>”类型的子类型）

【英文标题】：Flutter Error: _TypeError (type 'Null' is not a subtype of type 'FutureOr<Database>')

【发布时间】：2021-09-15 09:29:13

【问题描述】：

在 assets 文件夹中打开 sqlite 数据库。但我得到这个错误。 我试过'return _db ?? = 等待 initDb(); ' 但这是行不通的。 我不明白为什么我得到这个错误.. 请帮帮我..

import 'dart:async';
import 'dart:io';
import 'dart:typed_data';
import 'package:bankingapp/models/histories.dart';
import 'package:flutter/services.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';

class DbHelper 
  static Database? _db;

  Future<Database> get db async 
    return _db = await initDb(); // <-- error is here
  

  initDb() async 
    var dbDir = await getDatabasesPath();
    var dbPath = join(dbDir, "app.db");

// Delete any existing database:
    await deleteDatabase(dbPath);

// Create the writable database file from the bundled demo database file:
    ByteData data = await rootBundle.load("assets/database/bankingapp.db");
    List<int> bytes =
        data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
    await File(dbPath).writeAsBytes(bytes);

    var db = await openDatabase(dbPath);

    /*
    var dbFolder = await getDatabasesPath();
    String path = join(dbFolder, 'app.db');
    
    return await openDatabase(path);*/
  

  Future<List<Histories>> getHistories() async 
    var dbClient = await db;
    var result = await dbClient.rawQuery("SELECT * FROM Histories");
    return result.map((data) => Histories.fromMap(data)).toList();
  

【问题讨论】：

你检查过this吗？

是的，但我什么都不懂：/

【参考方案1】：

此代码将帮助您。

Future<Database> initDb() async 
    var dbFolder = await getDatabasesPath();
    String path = join(dbFolder, 'app.db');

    // Delete any existing database:
    await deleteDatabase(path);

    // Create the writable database file from the bundled demo database file:
    try 
      await Directory(dirname(path)).create(recursive: true);
     catch (_) 
    ByteData data =
        await rootBundle.load(join("assets/database", "bankingapp.db"));
    List<int> bytes =
        data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
    await new File(path).writeAsBytes(bytes, flush: true);

    //open the database
    return await openDatabase(path);