Angular Singleton Service Implementation

Creating a singleton service in Angular ensures that only one instance of a service exists throughout the application's lifecycle. This is particularly useful for services that manage shared resources, global configurations, or maintain state that should be consistent across different components. This challenge will guide you through implementing a robust and testable singleton service in Angular using TypeScript.

Problem Description

You are tasked with creating an Angular service that functions as a singleton. This means that regardless of how many times the service is injected into different components or modules, only a single instance of the service will ever be created. The service should provide a method to retrieve a counter value and a method to increment the counter. The service should be designed to be easily testable and maintainable.

Key Requirements:

• Singleton Instance: Only one instance of the service should be created.
• Counter Management: The service must have a private counter variable initialized to 0.
• getCounter() Method: A public method that returns the current value of the counter.
• incrementCounter() Method: A public method that increments the counter by 1.
• Testability: The service should be designed in a way that allows for easy unit testing (e.g., using dependency injection for any external dependencies).

Expected Behavior:

1. When the service is injected into multiple components, all components should share the same instance.
2. Calling incrementCounter() in one component should affect the counter value visible in all other components.
3. getCounter() should return the current, updated counter value.

Edge Cases to Consider:

• Multiple Injections: Ensure the singleton behavior holds true when the service is injected multiple times within the same component or across different components.
• Lazy Loading: Consider how the service behaves if it's lazily loaded in a module. The singleton pattern should still be maintained.

Examples

Example 1:

Input: Inject the service into ComponentA and ComponentB. ComponentA calls incrementCounter() 5 times.
Output: ComponentB.getCounter() returns 5.
Explanation: Since it's a singleton, both components share the same instance, and the counter is incremented globally.

Example 2:

Input: Inject the service into ComponentA, ComponentB, and ComponentC. ComponentA calls getCounter(), then incrementCounter() twice, then getCounter() again. ComponentB calls getCounter().
Output: ComponentA.getCounter() returns 2, ComponentB.getCounter() returns 2.
Explanation: The counter is incremented in ComponentA, and both components see the updated value.

Example 3: (Edge Case)

Input: The service is injected into two different modules, one eagerly loaded and one lazily loaded.
Output: Both modules share the same singleton instance, and changes made in one module are reflected in the other.
Explanation: The singleton pattern must be maintained regardless of loading strategy.

Constraints

• The service must be written in TypeScript.
• The service should be injectable using Angular's dependency injection system.
• The counter variable must be private.
• The service should not rely on any external libraries beyond Angular itself.
• The service should be designed with testability in mind.

Notes

• Consider using a private constructor to prevent direct instantiation of the service.
• The provideIn property in the @Injectable decorator is a key element for ensuring singleton behavior in Angular. Explore its usage.
• Think about how to test the service effectively, ensuring that only one instance is created and that the counter is updated correctly.
• While there are other ways to implement a singleton, Angular's dependency injection system provides a clean and recommended approach.