Implementing a Scoped Service in Angular

Angular services are typically singletons, meaning a single instance is shared across the entire application. However, there are scenarios where you need a service instance that is scoped to a specific component or a subset of components. This challenge focuses on creating a service that behaves as a scoped service, effectively providing a new instance when injected into a different component.

Problem Description

You need to implement a ScopedService in Angular that provides a unique instance for each component that injects it. This means that changes made to the service instance within one component should not affect the instance used by other components. The service should maintain a counter that increments each time a new instance is created and provide a method to retrieve this counter value.

Key Requirements:

• Unique Instances: Each component injecting ScopedService should receive a distinct instance.
• Counter: The service must maintain a private counter that tracks the number of instances created.
• Getter Method: Provide a public method getCounter() that returns the current value of the counter.
• Immutability: The counter itself should not be directly accessible from outside the service.

Expected Behavior:

When a component injects ScopedService, the counter should increment. Subsequent injections into different components should further increment the counter. Changes to properties of one instance should not be reflected in other instances.

Edge Cases to Consider:

• Multiple injections within the same component should still result in a single instance for that component.
• The counter should start at 0 when the application initializes.

Examples

Example 1:

// Component A
constructor(private serviceA: ScopedService) {
  serviceA.getCounter(); // Returns 1
  serviceA.data = "Component A Data";
  serviceA.getCounter(); // Returns 1 (still)
}

// Component B
constructor(private serviceB: ScopedService) {
  serviceB.getCounter(); // Returns 2
  serviceB.data = "Component B Data";
  serviceB.getCounter(); // Returns 2 (still)
}

Explanation: Component A and Component B each receive a unique instance of ScopedService. The counter increments with each new injection. Changes to serviceA.data do not affect serviceB.data.

Example 2:

// Component C
constructor(private serviceC: ScopedService) {
  serviceC.getCounter(); // Returns 3
}

// Another instance of Component C
constructor(private serviceD: ScopedService) {
  serviceD.getCounter(); // Returns 4
}

Explanation: Multiple instances of the same component also receive unique instances of the service, incrementing the counter.

Constraints

• The service must be implemented using TypeScript and Angular's dependency injection system.
• The counter must be a private member of the service class.
• The getCounter() method must be public.
• The service should not rely on external libraries beyond Angular itself.
• The service should be lightweight and efficient. Avoid unnecessary complexity.

Notes

Consider using a factory function or a custom provider to achieve the scoped behavior. The key is to ensure that a new instance is created each time the service is injected, rather than relying on the default singleton behavior of Angular's dependency injection. Think about how Angular's provide function can be leveraged to control the service's scope.