如何从 sqflite 表中获取数据并将其显示为文本小部件内的 %

Question

import 'dart:io';

import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path_provider/path_provider.dart';

class DatabaseHelper {

  static final _databaseName = "MyDatabase.db";
  static final _databaseVersion = 1;

  static final table = 'my_table';

  static final columnId = '_id';
  static final columnName = 'name';
  static final columnAge = 'age';

  // make this a singleton class
  DatabaseHelper._privateConstructor();
  static final DatabaseHelper instance = DatabaseHelper._privateConstructor();

  // only have a single app-wide reference to the database
  static Database _database;
  Future<Database> get database async {
    if (_database != null) return _database;
    // lazily instantiate the db the first time it is accessed
    _database = await _initDatabase();
    return _database;
  }

  // this opens the database (and creates it if it doesn't exist)
  _initDatabase() async {
    Directory documentsDirectory = await getApplicationDocumentsDirectory();
    String path = join(documentsDirectory.path, _databaseName);
    return await openDatabase(path,
        version: _databaseVersion,
        onCreate: _onCreate);
  }

  // SQL code to create the database table
  Future _onCreate(Database db, int version) async {
    await db.execute('''
          CREATE TABLE $table (
            $columnId INTEGER PRIMARY KEY,
            $columnName TEXT NOT NULL,
            $columnAge INTEGER NOT NULL
          )
          ''');
  }

  // Helper methods

  // Inserts a row in the database where each key in the Map is a column name
  // and the value is the column value. The return value is the id of the
  // inserted row.
  Future<int> insert(Map<String, dynamic> row) async {
    Database db = await instance.database;
    return await db.insert(table, row);
  }

  // All of the rows are returned as a list of maps, where each map is
  // a key-value list of columns.
  Future<List<Map<String, dynamic>>> queryAllRows() async {
    Database db = await instance.database;
    return await db.query(table);
  }

  // All of the methods (insert, query, update, delete) can also be done using
  // raw SQL commands. This method uses a raw query to give the row count.
  Future<double> queryRowCount() async {
    Database db = await instance.database;
    List<Map<String, dynamic>> x = await db.rawQuery('SELECT COUNT(*) FROM $table');
    int rowCount = Sqflite.firstIntValue(x);
    return rowCount.toDouble();
  }

  // We are assuming here that the id column in the map is set. The other
  // column values will be used to update the row.
  Future<int> update(Map<String, dynamic> row) async {
    Database db = await instance.database;
    int id = row[columnId];
    return await db.update(table, row, where: '$columnId = ?', whereArgs: [id]);
  }

  // Deletes the row specified by the id. The number of affected rows is
  // returned. This should be 1 as long as the row exists.
  Future<int> delete(int id) async {
    Database db = await instance.database;
    return await db.delete(table, where: '$columnId = ?', whereArgs: [id]);
  }

  Future<List<Map<String, dynamic>>> queryOmnivore() async {
    Database db = await instance.database;
    return await db.query(table, where: '$columnName = ?', whereArgs: ['omnivore']);
  }

  Future<List<Map<String, dynamic>>> queryPescatarian() async {
    Database db = await instance.database;
    return await db.query(table, where: '$columnName = ?', whereArgs: ['pescatarian']);
  }

  Future<List<Map<String, dynamic>>> queryVegetarian() async {
    Database db = await instance.database;
    return await db.query(table, where: '$columnName = ?', whereArgs: ['vegetarian']);
  }

  Future<int> queryVegetarianCount() async {
    var vegList = await queryVegetarian();
    int count = vegList.length;
    return count;
  }

  Future<double> queryOmnivoreCount() async {
    var omniList = await queryOmnivore();
    int omniCount = omniList.length;
    return omniCount.toDouble();
  }

  Future<double> calcOmnivorePercentage() async {
    var x = await queryOmnivoreCount();
    var y = await queryRowCount();
    double omniPercentage = (x / y) * 100;
    return omniPercentage;
  }
}

大家好！

我希望有人可以帮助我吗？！

我试图弄清楚如何从我创建的 sqflite 表中取出数据，执行计算，将其表示为其他值的百分比，并将其显示在应用程序的文本小部件中。

我实际上已经设法使用以下代码在控制台中打印结果：

void omnivorePercentageQ() async {
    final omni = await dbHelper.calcOmnivorePercentage();
    print('omnivore percentage: ${omni.toStringAsFixed(1)}');
  } 

但我不知道如何让它显示在应用程序本身的文本小部件中。

任何想法将不胜感激！

谢谢，

杰森

Answer 1

您离答案不远了，并且已经掌握了所需计算的值。如我所见，您不需要将任何参数传递给函数，因此我建议使用 futurebuilder：

return FutureBuilder(
    future: dbHelper.calcOmnivorePercentage(),
    builder: (context, AsyncSnapshot<double> snapshot) {
      if (snapshot.hasData) {
        return Center( child: Text('${snapshot.data.toStringAsFixed(1)}'),);
      }else
      return Center(
        child: CupertinoActivityIndicator(),
      );
    });

Future Builder 类https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html 用于管理依赖于期货的小部件，因为您的计算和数据库查询是异步的，您可以检查其状态（如在 snapshot.hasData 中的小部件内部）。该条件检查未来是否已经完成，否则显示一个指标。希望对你有帮助