共计 5482 个字符，预计需要花费 14 分钟才能阅读完成。
SharedPreferences 存储
缓存大量的键值对信息（比方记录用户是否浏览了布告，或是简略的计数），能够应用 SharedPreferences。
SharedPreferences 会以原生平台相干的机制，为简略的键值对数据提供长久化存储，即在 iOS 上应用 NSUserDefaults，在 Android 应用 SharedPreferences。
SharedPreferences 的应用形式非常简单不便。不过须要留神的是，以键值对的形式只能存储根本类型的数据，比方 int、double、bool 和 string。
1. pubspec.yaml 引入
shared_preferences: 0.5.12+4
2. 代码
import 'dart:developer';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
void main() {runApp(MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(primarySwatch: Colors.blue,),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
// 波及到耗时的文件读写，以异步的形式对这些操作进行包装
// 读取 SharedPreferences 中 key 为 counter 的值
Future<int> _loadCounter() async {SharedPreferences prefs = await SharedPreferences.getInstance();
int counter = (prefs.getInt('counter') ?? 0);
return counter;
}
// 递增写入 SharedPreferences 中 key 为 counter 的值
Future<void> _incrementCounter() async {SharedPreferences prefs = await SharedPreferences.getInstance();
int counter = (prefs.getInt('counter') ?? 0) + 1;
print("以后值:" + counter.toString());
prefs.setInt('counter', counter); //setter（setInt）办法会同步更新内存中的键值对，而后将数据保留至磁盘，因而无需再调用更新办法强制刷新缓存
}
_fresh() {// _incrementCounter();
_incrementCounter().then((_) => {_loadCounter().then((value) {setState(() {_counter = value;});
print("before:$value");
})
});
}
// void _incrementCounter() {// setState(() {
// _counter++;
// });
// }
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
// Here we take the value from the MyHomePage object that was created by
// the App.build method, and use it to set our appbar title.
title: Text(widget.title),
),
body: Center(
// Center is a layout widget. It takes a single child and positions it
// in the middle of the parent.
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('You have pushed the button this many times:',),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _fresh,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
数据库存储
如果须要长久化大量格式化后的数据，并且这些数据还会以较高的频率更新，为了思考进一步的扩展性，通常会选用 sqlite 数据库来应答这样的场景。
与文件和 SharedPreferences 相比，数据库在数据读写上能够提供更快、更灵便的解决方案。
1. pubspec.yaml 引入
sqflite: 1.3.2+1
path_provider: ^1.6.24
2. 代码
import 'package:flutter/material.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';
import 'package:path_provider/path_provider.dart';
import 'dart:io';
void main() {runApp(MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(primarySwatch: Colors.blue,),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class Student {
String id;
String name;
int score;
// 构造方法
Student({
this.id,
this.name,
this.score,
});
// 用于将 JSON 字典转换成类对象的工厂类办法
factory Student.fromJson(Map<String, dynamic> parsedJson) {
return Student(id: parsedJson['id'],
name: parsedJson['name'],
score: parsedJson['score'],
);
}
Map<String, dynamic> toJson() {
return {
'id': id,
'name': name,
'score': score,
};
}
}
class _MyHomePageState extends State<MyHomePage> {var student1 = Student(id: '123', name: '张三', score: 110);
query_database(id) {Future<Student> user = query_by_id(id);
user.then((t) {print("单用户查问胜利");
print('the sroce is :' + t.score.toString());
student1.score = t.score;
});
}
// 插入数据
Future<void> insertStudent(Student std) async {
final Database db = await database;
await db.insert(
'students',
std.toJson(),
// 插入抵触策略，新的替换旧的
conflictAlgorithm: ConflictAlgorithm.replace,
);
}
Future<Student> query_by_id(String id) async {
final Database db = await database;
List<Map> maps = await db.query('students',
columns: ['id', 'name', 'score'], where: 'id = ?', whereArgs: [id]);
if (maps.length > 0) {print("依据 id 查到了数据");
return Student.fromJson(maps.first);
}
print("依据 id 没有查到数据");
return null;
}
// 初始化数据库办法
initDB() async {Directory documentsDirectory = await getApplicationDocumentsDirectory();
String path = join(documentsDirectory.path, "students_database.db");
return await openDatabase(path, version: 1, onOpen: (db) {},
onCreate: (Database db, int version) async {await db.execute('''CREATE TABLE students(id TEXT PRIMARY KEY, name TEXT, score INTEGER)''');
});
}
Database _database;
Future<Database> get database async {if (_database != null) {print(_database);
return _database;
}
_database = await initDB();
print("创立新数据库");
return _database;
}
void _incrementCounter() async {
student1.score += 1;
await insertStudent(student1);
setState(() {});
}
Future<List<Student>> students() async {
final Database db = await database;
final List<Map<String, dynamic>> maps = await db.query('students');
return List.generate(maps.length, (i) => Student.fromJson(maps[i]));
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('You have pushed the button this many times:',),
Text(student1.score.toString(),
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
@override
void dispose() async {
// TODO: implement dispose
super.dispose();
final Database db = await database;
db.close();}
}Flutter 写的 app, 须要源码能够私信~~
- 简繁火星字体转换
- 哄女友神器
- 号码测吉凶
- 电视节目直播表
最好的笔记软件
正文完