Dart: Libraries

Dart, an open-source programming language initially developed by Google, is designed for bother server-side and client-side applications.

Its software development kit(SDK) includes the Dart VM compiler and a utility called dart2js, which converts Dart code into JavaScript, enabling its execution on platforms that do directly support Dart.

Dart is an Object-oriented language and is quite similar to that of Java Programming. Dart is extensively used to create single-page websites and web-applications. The best example of a dart application is Gmail.

Importing Libraries:

For using libraries in the dart, we must import them first. Importing makes the components of a library available in the current file. Dart has a special keyword import for importing libraries.

Syntax: import 'dart:sync'

// Example showing importing a library 

// Importing built-in library   
import 'dart:math';    

void main() {    
   print("Square root of 25 is: ${sqrt(25)}");    
}

Output:

Square root of 25 is: 5

When importing a library file from another package, “package: directive” is used to specify the URI of that file:

import 'package:utilities/utilities.dart';

Creating Custom Libraries

Custom libraries are the libraries that the user creates according to his need.

Such libraries are known as user-defined libraries. Dart supports the creation of such libraries, and we can import them when needed.

This takes place in basically two steps:

1. Declaration

Library name must be declared using the keyword “library”

library ‘my_lib’ //any random lib name

2. Connection

A library can be connected within the same directory or from another directory.

//within same directory

import ‘library_name’

//from another directory

import ‘dir/library_name’

// 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;    
}

We have created a custom library named “basic_calc“. Now we have to import our lib in a current file:

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");   

}

Output:

Add Method
Multiplication Method
Subtraction Method

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

Encapsulation

Another feature provided by dart to its users is the encapsulation of libraries. Encapsulation is used for combining data and functions into a single unit called class. We can achieve encapsulation in dart by using the (underscore), followed by the identifier. The (underscore) symbol is used to make the library’s content completely private.

NOTE: Encapsulation in Dart takes place at library level instead of class-level, unlike other OOP languages.

Syntax: _identifier

Example:

// Example showing encapsulation in dart 

// Define a library naming cake 
library cake; 
class MainCake{ 

// non-private property 
// list of strings 
 List<String> randomPieceOfCakes = ['chocolate', 
                                    'butterscotch', 
                                    'vanilla', 
                                    'strawberry']; 

 // private properties 
 String _pieceOfCake1 = "chocolate"; 
 String pieceOfCake2 = "butterscotch"; 
}

Here we will import the cake library that we created above:

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); 
}

As Keyword

We can import multiple libraries within a single current file. But if we create two or more functions with the same name, the compiler would be unable to differentiate between the two and will provide the wrong output. To avoid this, dart provides us keyword “as” for naming the alias of a library.

Syntax: import 'my_lib' as prefix

Example:

// Example showing the use of as keyword 

// Defining a lib names greetings 
library greetings;     
void sayHi(msg){    
   print("Hello coder. Welcome to ${msg}");   
}

Now we will define another library

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

Now, we import the above libraries with them as the prefix

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");      
}

Output:

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

Below are the core libraries of Dart: