Introduction

This article is an introduction to using libraries in Dart/Flutter. A library is a reusable module for frequently used programs/code. It helps to write modular code base. In Dart, each app is a library. This article is divided into two parts.

Part-1: This part covers how to use Dart libraries including following:

• Using Prefix for library
• Importing specific APIs
• Defer/delay loading library

Part-2: The second part covers understanding following directives:

• The part directive
• The library directive
• The export directive

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Setup

Let's create two libraries lib1 and lib2 for demonstration.

lib1

This library has APIs to calculate sum and difference of two integers.

• Addition API: The int addition(int a, int b) calculates sum of two given integers and returns an integer.

• Subtraction API: The int subtraction(int a, int b) calculates difference of two given integers and returns an integer.

• int _add(int a, int b): It is an internal function/method that does perform the real addition. This method is used by addition api. Identifiers with underscore (_) prefix are only available inside library.

SourceCode: Please refer to lib1.dart

  int addition(int a, int b) => _add(a, b);

  int subtraction(int a, int b) => a - b;

  int _add(int a, int b) => a + b;

lib2

This library has APIs to check if the given number is even or odd.

• Even number API (bool isNumberOdd(int num)): This api take a number and returns true if its even or vice versa.
• Odd number API (bool isNumberOdd(int num)): This api take a number and returns true if its odd or vice versa.

SourceCode: Please refer to lib2.dart

  bool isNumberOdd(int num) => num.isOdd;

  bool isNumberEven(int num) => num.isEven;

  //Added to demonstrate handle conflicting name in two different libraries
  int addition(int a, int b) => a + b + 2;

Using Prefix for library

Let's use lib1.dart to calculate addition of two numbers 5 and 2, and use lib2.dart to check if number 5 is even or odd. For lib2.dart, a prefix check is used. This is used to access lib2's APIs thereafter. All lib2's API's can be accessed using check alias.

The lib_prefix.dart:

SourceCode: Please refer to lib_prefix.dart

import 'lib1.dart';
import 'lib2.dart' as check; //Using prefix for lib2

void main() {
  int num1 = 5;
  int num2 = 2;

  int sum = addition(num1, num2);
  print("Sum of $num1 and $num2 is $sum");

  //Using check to access the API
  print("is number even? ${check.isNumberEven(num2)}");
}

Output:

Sum of 5 and 2 is 7
is number even? true

Handling Conflicts

Using prefix will only call method from respective library when more than one libraries have similar named apis.

The lib_conflict.dart:

SourceCode: Please refer to lib_conflict.dart

import 'lib1.dart';
import 'lib2.dart' as check;

void main() {
  int num1 = 5;
  int num2 = 2;

  int sum = addition(num1, num2);
  print("Sum of $num1 and $num2 is $sum");

  //Calls addition() from lib2
  sum = check.addition(num1, num2);
  print("Sum of $num1 and $num2 is $sum");
}

Output:

Sum of 5 and 2 is 7
Sum of 5 and 2 is 9

Importing specific APIs

Dart libraries allow to import only specific apis using show and hide keywords.

show:

The show is used when a specific API needs to be made visible/accessible. For example, in lib1.dart, I want to access only subtraction(...) API.

The lib_show.dart:

SourceCode: Please refer to lib_show.dart

import 'lib1.dart' show subtraction;

void main() {
  int num1 = 5;
  int num2 = 2;

  //Compile-time error because addition is no longer visible
  //int sum = addition(num1, num2);

  int difference = subtraction(num1, num2);
  print("Difference of $num1 and $num2 is $difference");
}

Output:

Difference of 5 and 2 is 3

hide:

The hide is used when everything but a specific API is made accessible/visible.

The lib_hide.dart:

SourceCode: Please refer to lib_hide.dart

import 'lib1.dart' hide subtraction;

void main() {
  int num1 = 5;
  int num2 = 2;

  int sum = addition(num1, num2);
  print("Sum of $num1 and $num2 is $sum");

  //Compile-time error because subtraction() API is hidden
  //int difference = subtraction(num1, num2);
  //print("Difference of $num1 and $num2 is $difference");
}

Output:

Sum of 5 and 2 is 7

Defer/delay loading library

A deferred library is loaded when it's actually used/needed. It's declared using deferred as <name> in import statement. The library uses loadLibrary() method to invoke itself. The loadLibrary() method returns a Future. That's why library invocation needs to be done in a async block.

The lib_deferred.dart:

SourceCode: Please refer to lib_deferred.dart

import 'lib1.dart' deferred as lib1;

void main() {
  int num1 = 5;
  int num2 = 2;

  delayedInvocation(num1, num2);
}

Future delayedInvocation(int num1, int num2) async {
  //Loads lib1 here
  await lib1.loadLibrary();
  int sum = lib1.addition(num1, num2);
  print("Sum of $num1 and $num2 is $sum");
}

Output:

Sum of 5 and 2 is 7

Summary

In this article, we saw few ways to use Dart libraries. We learned how to use prefix to refer to libraries in import statements, importing only specific libraries, and delaying loading libraries until they are needed.

That's it for this article. Check out the Dart Vocabulary Series for other Dart stuff.

Source Code

Please checkout the source code at Github here

References

• Official Dart Language Tour

Happy Darting :)

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