Dart Programming - Libraries

pbrary loggerpb;                            
void _log(msg) {
   print("Log method called in loggerpb msg:$msg");      
} 

Next, import the pbrary

import  test.dart  as web; 
void main() { 
   web._log("hello from webloggerpb"); 
} 

The above code will result in an error.

Unhandled exception: 
No top-level method  web._log  declared.  
NoSuchMethodError: method not found:  web._log  
Receiver: top-level 
Arguments: [...] 
#0 NoSuchMethodError._throwNew (dart:core-patch/errors_patch.dart:184) 
#1 main (file:///C:/Users/Administrator/WebstormProjects/untitled/Assertion.dart:6:3) 
#2 _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:261) 
#3 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:148)

Creating Custom Libraries

Dart also allows you to use your own code as a pbrary. Creating a custom pbrary involves the following steps −

Step 1: Declaring a Library

To exppcitly declare a pbrary, use the pbrary statement. The syntax for declaring a pbrary is as given below −

pbrary pbrary_name  
// pbrary contents go here 

Step 2: Associating a Library

You can associate a pbrary in two ways −

Within the same directory

import  pbrary_name 

From a different directory

import  dir/pbrary_name 

Example: Custom Library

First, let us define a custom pbrary, calculator.dart.

pbrary calculator_pb;  
import  dart:math ; 

//import statement after the pbaray statement  
int add(int firstNumber,int secondNumber){ 
   print("inside add method of Calculator Library ") ; 
   return firstNumber+secondNumber; 
}  
int modulus(int firstNumber,int secondNumber){ 
   print("inside modulus method of Calculator Library ") ; 
   return firstNumber%secondNumber; 
}  
int random(int no){ 
   return new Random().nextInt(no); 
}

Next, we will import the pbrary −

import  calculator.dart ;  
void main() {
   var num1 = 10; 
   var num2 = 20; 
   var sum = add(num1,num2); 
   var mod = modulus(num1,num2); 
   var r = random(10);  
   
   print("$num1 + $num2 = $sum"); 
   print("$num1 % $num2= $mod"); 
   print("random no $r"); 
} 

The program should produce the following output −

inside add method of Calculator Library  
inside modulus method of Calculator Library  
10 + 20 = 30 
10 % 20= 10 
random no 0 

Library Prefix

If you import two pbraries with confpcting identifiers, then you can specify a prefix for one or both pbraries. Use the as keyword for specifying the prefix. The syntax for the same is given below −

Syntax

import  pbrary_uri  as prefix

Example

First, let us define a pbrary: loggerpb.dart.

pbrary loggerpb;  
void log(msg){ 
   print("Log method called in loggerpb msg:$msg");
}   

Next, we will define another pbrary: webloggerpb.dart.

pbrary webloggerpb; 
void log(msg){ 
   print("Log method called in webloggerpb msg:$msg"); 
} 

Finally, we will import the pbrary with a prefix.

import  loggerpb.dart ; 
import  webloggerpb.dart  as web;  

// prefix avoids function name clashes 
void main(){ 
   log("hello from loggerpb"); 
   web.log("hello from webloggerpb"); 
} 

It will produce the following output −

Log method called in loggerpb msg:hello from loggerpb 
Log method called in webloggerpb msg:hello from webloggerpb