所谓的库，就是一些功能的集合，通过封装，我们可以在项目中通过引用来使用。

库在Dart中分为几种

自定义的库

创建

把类、接口、方法等内容单独抽离当如Dart文件

使用

import './xxx'

./person.dart

class Person {
  String name;
  int age;
  Person(this.name, this.age);
  showName() {
    return this.name;
  }
  showAge() {
    return this.age;
  }
}
show() {
  return 1;
}

./1.dart

import "./Person.dart";
void main() {
  var p1 = Person('张三', 18);
  print(p1.name); //张三
  print(p1.showAge()); //18
  show(); //1
}

系统库

系统定义的库，这里面有两种：
（1）一种核心库：内置库（该库会被自动导入每个Dart程序）包含：

控制台打印：

print

数字

.parse 、.toString()

字符串正则

.contains 、.statsWith 、.endsWith 、…

集合List、Set、map

.add 、.addAll 、.sort

编码和解码完整合法的URI encodeFull 、decodeFull()
编码和解码 URI 组件 encodeComponent() 、decodeComponent()
URI 解析构建 parse() 、Uri()

DateTime

获取当前时间 DateTime.now()
创建DateTime对象 DateTime(2020, 10, 10)
返回现在距离1970.1.1日的毫秒数 DateTime().millisecondsSinceEpoch

（2）其它系统库
这个是需要手动导入的。
dart:async - 异步编程从dart2.1开始不用引入（async、await）
dart:math
…

引入 import "dart:math"

import "dart:math";
// import "dart:async"; //从Dart 2.1 开始不用引入
void main() {
  // 数学方法
  print(max(10, 20)); //20
  print(min(10, 20)); //10
  // 异步async
  show2() {
    return 2;
  }
  show() async {
    var s1 = await show2();
    print(s1);
  }
  show(); //2
}

main函数也可以写成异步

import "dart:math";
show2() {
  return 2;
}
show() async {
  var s1 = await show2();
  print(s1);
}
void main() async {
  var a = await show(); //2
}

第三方的库

第三方库 pub包中的库
第三方地址：

https://pub.dev/packages
https://pub.flutter-io.cn/packages
https://pub.dartlang.org/flutter/

1、需要在项目目录下创建 pubspec.yaml 文件
2、在pubspec.yaml文件中配置名称、描述、依赖等信息
3、运行 pub get 获取包下载到本地
4、项目中引用库 import "package://http://xxx"

导入库时，可以使用`as`关键字来给库起别名，避免命名空间冲突。

import 'package:lib1/lib1.dart';
import 'package:lib2/lib2.dart' as lib2;
// 使用lib1中的Element
Element element1 = new Element();
// 使用lib2中的Element
lib2.Element element2 = new lib2.Element();

//Person.dart
class Person {}
show() {
  return 1;
}
//1.dart
import "./Person.dart";
import "./Person.dart" as aa;
void main() {
  print(aa.show());    //1
}

使用`show`和`hide`关键字控制库中成员的可见性

// 仅导入foo，屏蔽库中其他成员
import 'package:lib1/lib1.dart' show foo;
// 屏蔽foo，库中其他成员都可见
import 'package:lib2/lib2.dart' hide foo;

为了减少 APP 的启动时间，加载很少使用的功能，我们还可以延迟导入库。使用 deferred as关键字延迟导入

import 'package:deferred/hello.dart' deferred as hello;
// 当需要使用时，再通过库标识符调用 loadLibrary函数加载
hello.loadLibrary();

创建pubspec.yaml

name: demo
description: play
dependencies:
  http: ^0.12.2

运行命令 pub get 或者 flutter pub get

引用http库示例：

import 'dart:convert' as convert;
import 'package:http/http.dart' as http;
void main(List<String> arguments) async {
  // This example uses the Google Books API to search for books about http.
  // https://developers.google.com/books/docs/overview
  var url = 'https://www.googleapis.com/books/v1/volumes?q={http}';
  // Await the http get response, then decode the json-formatted response.
  var response = await http.get(url);
  if (response.statusCode == 200) {
    var jsonResponse = convert.jsonDecode(response.body);
    var itemCount = jsonResponse['totalItems'];
    print('Number of books about http: $itemCount.');
  } else {
    print('Request failed with status: ${response.statusCode}.');
  }
}

引入本地库

使用

dependencies:
  flutter:
    sdk: flutter
  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2
#本地插件引用 注意缩进格式
  utils:
    path: plugins/utils

import 'package:flutter/material.dart';
import "package:utils/Base.dart";
//... color: Color(Base.getRandomColor()),

引入本地项目文件（绝对路径方式）

./pubspec.yaml

name: myApp

./lib/MyHomePage.dart

import "package:myApp/utils/Base.dart";    //以绝对路径引入本地项目里文件
//... color: Color(Base.getRandomColor()),

环境变量

flutter镜像

flutter 国内镜像配置：
Linux 或 Mac：
export PUB_HOSTED_URL=https://pub.flutter-io.cn
export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn

Windows：
新增两个环境变量——用户变量即可
PUB_HOSTED_URL ： https://pub.flutter-io.cn
FLUTTER_STORAGE_BASE_URL ： https://storage.flutter-io.cn

为 pub 命令配置环境变量

https://dart.cn/tools/pub/environment-variables

Environment variables allow you to customize pub to suit your needs.
PUB_CACHESome of pub’s dependencies are downloaded to the pub cache. By default, this directory is located under .pub-cache in your home directory (on Mac and Linux), or in %LOCALAPPDATA%\Pub\Cache (on Windows). (The precise location of the cache may vary depending on the Windows version.) You can use the PUB_CACHE environment variable to specify another location. For more information, see The system package cache.PUB_HOSTED_URLPub downloads dependencies from the pub.dev site. To specify the location of a particular mirror server, use the PUB_HOSTED_URL environment variable. For example:

PUB_HOSTED_URL = http://user:password@177.0.0.1:9999

Note: If you are attempting to use pub get behind a corporate firewall and it fails, please see pub get fails from behind a corporate firewall for information on how to set up the proxy environment variables for your platform.

库