Angular skipSelf Injection Challenge

Angular's dependency injection system is powerful, but sometimes you need to inject a service that is provided at a higher level in the component tree, not directly by the current component. This challenge focuses on implementing and understanding the skipSelf token option in Angular, a crucial feature for controlling dependency resolution.

Problem Description

Your task is to create an Angular application structure with multiple components, where a service is provided at a higher ancestor component. You will then implement a child component that needs to inject this service but explicitly exclude any instance of the service that might be provided by itself.

What needs to be achieved: Create a scenario where a service is provided by a parent component and injected into a child component using skipSelf.

Key requirements:

1. Define a simple Angular service (e.g., LoggerService).
2. Create a parent component that provides an instance of LoggerService in its providers array.
3. Create a child component that attempts to inject LoggerService.
4. Configure the injection in the child component to use skipSelf so that it bypasses any potential self-provided instance and looks for an ancestor provider.
5. Demonstrate that the skipSelf injection correctly retrieves the service provided by the parent.

Expected behavior: When the child component is instantiated, it should successfully receive the LoggerService instance provided by its parent component. If the child component also provided LoggerService in its own providers array, the skipSelf injection should ignore that self-provided instance.

Edge cases to consider:

• What happens if the service is not provided by any ancestor (including the parent)?
• What happens if the child component itself provides the service?

Examples

Example 1:

Component Hierarchy:
AppRoot -> ParentComponent -> ChildComponent

LoggerService is provided in ParentComponent.providers.
ChildComponent injects LoggerService using { skipSelf: true }.

Input: (Conceptual - the user would set up the component hierarchy and providers as described)

• LoggerService class definition.
• ParentComponent with providers: [{ provide: LoggerService, useClass: LoggerService }].
• ChildComponent with constructor(@Inject(LoggerService, { skipSelf: true }) private logger: LoggerService).

Output: The logger property in ChildComponent will hold the instance of LoggerService provided by ParentComponent. Calling a method like logger.log('Hello') will log the message to the console (assuming LoggerService has a log method that does this).

Explanation: The skipSelf: true option tells Angular's injector to skip checking the current injector (the one associated with ChildComponent) and proceed to the parent injector. Since LoggerService is provided by ParentComponent, its injector finds and returns the service.

Example 2:

Component Hierarchy:
AppRoot -> ParentComponent -> IntermediateComponent -> ChildComponent

LoggerService is provided in ParentComponent.providers.
IntermediateComponent also provides LoggerService in its providers.
ChildComponent injects LoggerService using { skipSelf: true }.

Input: (Conceptual)

• LoggerService class definition.
• ParentComponent with providers: [{ provide: LoggerService, useClass: LoggerService }].
• IntermediateComponent with providers: [{ provide: LoggerService, useValue: { log: () => console.log('Log from Intermediate') } }].
• ChildComponent with constructor(@Inject(LoggerService, { skipSelf: true }) private logger: LoggerService).

Output: The logger property in ChildComponent will hold the instance of LoggerService provided by ParentComponent. The instance provided by IntermediateComponent will be ignored due to skipSelf: true. Calling logger.log('Test') will result in Log from Parent being outputted (assuming a basic LoggerService implementation).

Explanation: skipSelf: true first checks IntermediateComponent's injector. It would find a LoggerService there, but skipSelf instructs it to ignore this. It then moves to ParentComponent's injector, which does provide LoggerService, and returns that instance.

Constraints

• The solution must be written in TypeScript.
• Angular version compatibility should be considered for standard DI features.
• The LoggerService should be a simple class with at least one method (e.g., log(message: string): void).
• The component hierarchy should be at least three levels deep (parent -> child).

Notes

• Consider how you would typically provide a service in Angular.
• Think about the role of injectors in Angular's DI.
• The @Inject() decorator is essential for passing options like skipSelf.
• You can create a simple LoggerService that just logs to the console to easily verify that the correct instance is being injected.
• Successfully demonstrating that skipSelf bypasses a self-provided instance is a key part of this challenge.