Introduction

In this lesson, you will learn about the following directives:

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

In addition to introducing the above directives, you will also learn to restrict the visibility of a library’s APIs.

Revisiting the setup

Let’s revisit the sample libraries lib1 and lib2created in the previous lesson for demonstration.

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

lib1

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

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

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

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

File ‘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 takes a number and returns true if it is even or vice versa.
• Odd number API (bool isNumberOdd(int num)): This API takes a number and returns true if it is odd or vice versa.

File ‘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;