Dart – 图书馆

Dart是最初由 Google 开发的开源编程语言。它适用于服务器端和用户端。DartSDK带有其编译器-DartVM和这意味着生成JavaScript相当于一个Dart脚本，以便它可以在这些网站也不支持Dart运行的程序dart2js。

Dart是一种面向对象的语言，与Java Programming非常相似。 Dart被广泛用于创建单页网站和网络应用程序。 dart应用程序的最佳示例是Gmail 。

在本文中，我们将研究dart库以及使用它们的过程。

导入库

要在dart使用库，我们必须先导入它们。导入使库的组件在当前文件中可用。 Dart有一个特殊的关键字import来导入库。

Syntax: import 'dart:sync'

Dart

// Example showing importing a library
  
// Importing built-in library  
import 'dart:math';   
  
void main() {   
   print("Square root of 25 is: ${sqrt(25)}");   
}

输出：

Square root of 25 is: 5

当从另一个包导入库文件时，“package: directive”用于指定该文件的 URI：

import 'package:utilities/utilities.dart';

创建自定义库

自定义库是用户根据需要创建的库。此类库称为用户定义库。 Dart支持创建此类库，我们可以在需要时导入它们。这主要分两步进行：

1. 声明

必须使用关键字“library”声明库名称

library ‘my_lib’  //any random lib name

2. 连接

库可以在同一目录中或从另一个目录中连接。

//within same directory

import ‘library_name’  

//from another directory

import ‘dir/library_name’  

Dart

// Example illustrating creation of a custom library
library basic_calc;    
import 'dart:math';   
    
// library content  
int add(int a,int b){   
   print("Add Method ") ;   
   return a+b;   
}    
int multiplication(int a,int b){   
   print("Multiplication Method") ;   
   return a*b;   
}    
    
int subtraction(int a,int b){   
   print("Subtraction Method") ;   
   return a-b;   
}    
    
int modulus(int a,int b){   
   print("Modulus Method") ;   
   return a%b;   
}

我们创建了一个名为“ basic_calc ”的自定义库。现在我们必须在当前文件中导入我们的库：

Dart

import 'basic.dart';  
  
void main() {  
   var a = 50;   
   var b = 30;   
   var sum = add(n1,n2);   
   var mod = modulus(n1,n2);   
   var mul = multiplication(n1,n2);  
   var div = divide(n1,n2);  
   var sub = subtraction(n1,n2);  
       
       
   print("$n1 + $n2 = $sum");   
   print("$n1 %  $n2= $mod");   
   print("$n1 + $n2 = $mul");   
   print("$n1 - $n2 = $sub");  
       
}

输出：

Add Method
Multiplication Method
Subtraction Method

Modulus Method
50 + 30 = 80
50 % 30= 20
50 * 30 = 1500
50 - 30 = 20

封装

dart为其用户提供的另一个功能是库的封装。封装用于将数据和函数组合成一个称为类的单元。我们可以通过使用_(下划线)，后跟标识符来实现在dart的封装。 _(下划线)符号用于使库的内容完全私有。

注意：与其他 OOP 语言不同， Dart的封装发生在库级别而不是类级别。

Syntax: _identifier

例子：

Dart

// Example showing encapsulation in dart
  
// Define a library naming cake
library cake;
class MainCake{
    
// non-private property
// list of strings
 List randomPieceOfCakes = ['chocolate',
                                    'butterscotch',
                                    'vanilla',
                                    'strawberry'];
  
 // private properties
 String _pieceOfCake1 = "chocolate";
 String pieceOfCake2 = "butterscotch";
}

在这里，我们将导入我们上面创建的蛋糕库：

Dart

import 'cake.dart';
  
void main(){
  MainCake newCake = new MainCake();
    
  // non-private property -  randomPieceOfCakes
  print(newCake.randomPieceOfCakes);
  
  // private property - piece of cake
  // private property error
  print(newCake._pieceOfCake1); 
  
  // non-private private - piece of cake
  print(newCake.pieceOfCake2);
}

作为关键字

我们可以在单个当前文件中导入多个库。但是如果我们创建两个或多个同名的函数，编译器将无法区分两者并提供错误的输出。为了避免这种情况， dart为我们提供了关键字“as”来命名库的别名。

Syntax: import 'my_lib' as prefix  

例子：

Dart

// Example showing the use of as keyword
  
// Defining a lib names greetings
library greetings;    
void sayHi(msg){   
   print("Hello coder. Welcome to ${msg}");  
}

现在我们将定义另一个库

Dart

library hellogreetings;   
void sayHi(msg){   
   print("${msg} has solutions of all your problems");   
}

现在，我们以它们为前缀导入上述库

Dart

import 'greetings.dart';   
import 'hellogreetings.dart' as gret;    
    
// using as prefix avoids function name clashes   
void main(){   
   sayHi("GFG"); 
    
   // To eliminate the name confliction
   gret.sayHi("GFG");     
}

输出：

Hello coder. Welcome to GFG
GFG has solutions of all your problems

以下是Dart的核心库：

TYPE	LIBRARY	DESCRIPTION
Multi-Platform Libraries	dart:async	This library supports asynchronous programming, with classes such as  Future and Stream.
	dart:collection	This library contains classes and utilities that supplement the collection support in dart:core.
	dart:convert	This library contains encoders and decoders for converting between different data representations, including JSON and UTF-8.
	dart:core	This library contains built-in types, collections, and other core functionality for every Dart program.
	dart:developer	This library provides interaction with developer tools such as the debugger and inspector.
	dart:math	This library contains mathematical constants and functions, plus a random number generator.
	dart:typed_data	This library contains lists that efficiently handle fixed sized data files(for example, unsigned 8-byte integers) and SIMD numeric types.
Native platform libraries	dart:io	This library contains file, sockets, HTTP, and other I/O support for non-web applications.
	dart:isolate	This library supports Concurrent programming using isolates independent workers similar to threads.
Web platform libraries	dart:html	This library contains HTML elements and other resources for web-based applications.
	dart:indexed_db	This library supports a Client-side key-value store with support for indexes.
	dart:web_audio	This library supports High-fidelity audio programming in the browser.
	dart:web_gl	This library supports 3D programming in the browser.