Angular Self-Injection Challenge

In Angular, components and services can be injected with dependencies to leverage existing functionality. However, sometimes a component needs to inject itself or an instance of itself into another component or service. This challenge focuses on implementing and understanding how to achieve this "self-injection" pattern in Angular.

Problem Description

Your task is to create an Angular application where a parent component can inject an instance of itself into a child component. The child component should then be able to access and utilize methods or properties of the parent component through this injected instance.

Key Requirements:

1. Parent Component: Create a parent component (ParentComponent) with at least one public method and one public property.
2. Child Component: Create a child component (ChildComponent) that needs to receive an instance of ParentComponent.
3. Injection Mechanism: Implement a way for ParentComponent to provide itself as an injectable dependency to ChildComponent.
4. Usage in Child: ChildComponent must demonstrate that it has successfully received the ParentComponent instance by calling its public method or accessing its public property.
5. Template Integration: Both components should be rendered in the application's main template.

Expected Behavior:

When the application loads, the ChildComponent should correctly display information or trigger an action that originates from the ParentComponent instance injected into it.

Edge Cases to Consider:

• What happens if ChildComponent is rendered without a ParentComponent in its DOM hierarchy? (While not strictly required for this challenge, it's good to think about for real-world scenarios).
• Ensuring the correct instance of ParentComponent is injected, especially if multiple instances exist.

Examples

Example 1:

Parent Component (Conceptual):

// parent.component.ts
import { Component } from '@angular/core';

@Component({
  selector: 'app-parent',
  template: `
    <div>
      <h2>Parent Component</h2>
      <p>Parent Data: {{ parentData }}</p>
      <button (click)="incrementData()">Increment from Parent</button>
      <app-child></app-child>
    </div>
  `,
})
export class ParentComponent {
  parentData: number = 0;

  incrementData(): void {
    this.parentData++;
    console.log('Parent data incremented:', this.parentData);
  }
}

Child Component (Conceptual):

// child.component.ts
import { Component, Injector } from '@angular/core';
import { ParentComponent } from '../parent/parent.component'; // Assuming path

@Component({
  selector: 'app-child',
  template: `
    <div>
      <h3>Child Component</h3>
      <p>Accessing Parent Data: {{ parentInstance?.parentData }}</p>
      <button (click)="callParentMethod()">Call Parent Method</button>
    </div>
  `,
})
export class ChildComponent {
  parentInstance: ParentComponent | undefined;

  constructor(private injector: Injector) {}

  ngOnInit() {
    // Attempt to get the parent instance
    this.parentInstance = this.injector.get(ParentComponent);
  }

  callParentMethod() {
    if (this.parentInstance) {
      this.parentInstance.incrementData();
    } else {
      console.error('Parent instance not found!');
    }
  }
}

App Template (Conceptual):

<!-- app.component.html -->
<app-parent></app-parent>

Expected Output (in browser console and UI):

• When the app loads, the ChildComponent will display the initial parentData (0).
• Clicking "Increment from Parent" in the ParentComponent will update its parentData and the ChildComponent will reflect this update.
• Clicking "Call Parent Method" in the ChildComponent will trigger incrementData() on the ParentComponent, updating parentData and consequently refreshing the UI in both components.

Explanation:

The ChildComponent uses the Injector to retrieve an instance of ParentComponent that is available in its component tree. It then uses this retrieved instance to display the parent's data and call its methods.

Constraints

• Angular version: Latest stable version (e.g., Angular 16+).
• Language: TypeScript.
• No external libraries beyond Angular core.
• The solution should be achievable within a single Angular module.

Notes

• Consider how Angular's dependency injection system resolves tokens. What token would you use to get an instance of a component itself?
• Think about the lifecycle of components and when the injection should occur. ngOnInit is a good candidate.
• The Injector is a powerful tool for obtaining instances of services or components.
• There are multiple ways to achieve this. Explore the Injector service as a primary approach.