Manual Change Detection in Angular

Angular's Change Detection mechanism is powerful, but sometimes you need finer control over when and how your application updates its view. This challenge focuses on implementing manual change detection, allowing you to explicitly trigger checks for specific components or the entire application. This skill is crucial for optimizing performance in complex applications or when integrating with third-party libraries that might not play nicely with Angular's default behavior.

Problem Description

Your task is to create an Angular application where you can manually trigger change detection for a specific component and for the entire application.

Requirements:

1. Component-Specific Manual Change Detection:

   • Create a component (e.g., ManualCounterComponent) that displays a counter value.
   • This component should have a button that, when clicked, increments the counter.
   • Crucially, the update to the displayed counter should only happen when a specific "Check Component" button (outside the component, in a parent) is clicked, not automatically after the increment button.
   • You will need to inject ChangeDetectorRef into ManualCounterComponent and use its detectChanges() method.

2. Application-Wide Manual Change Detection:

   • Create a parent component (e.g., AppComponent) that hosts the ManualCounterComponent.
   • The AppComponent should have a button that triggers change detection for the entire application.
   • When this button is clicked, both the ManualCounterComponent's display (if it has been updated internally by its own increment button) and any other potential UI elements in AppComponent should reflect their latest state.
   • You will need to inject NgZone into AppComponent and use its run() method with a no-op function or detectChanges() of the root application's ApplicationRef. For this challenge, focus on using ApplicationRef's tick() method.

3. Visual Feedback:

   • Clearly label the buttons to indicate their purpose ("Increment Counter", "Check Component", "Check Application").
   • The ManualCounterComponent should visually update its counter only after the "Check Component" button is pressed.

Expected Behavior:

• Clicking "Increment Counter" in ManualCounterComponent should increase the internal counter value but not immediately update the displayed value.
• Clicking "Check Component" in AppComponent should update the displayed value of the ManualCounterComponent to reflect the last incremented value.
• Clicking "Check Application" in AppComponent should perform a full change detection cycle for the application, ensuring all bound data is updated. If the ManualCounterComponent's internal counter has been incremented, its display should update as a result of this application-wide check.

Edge Cases:

• Consider what happens if you click "Increment Counter" multiple times before clicking "Check Component". The component should display the final incremented value.
• Ensure that the application-wide check also correctly updates any other bindings in AppComponent (though for simplicity, we'll focus on the ManualCounterComponent).

Examples

Example 1:

• Scenario: User clicks "Increment Counter" twice, then clicks "Check Component".
• Input:
  • ManualCounterComponent's internal counter starts at 0.
  • Click "Increment Counter" -> internal counter becomes 1.
  • Click "Increment Counter" -> internal counter becomes 2.
• Output:
  • The ManualCounterComponent initially displays "Counter: 0".
  • After clicking "Check Component", the ManualCounterComponent displays "Counter: 2".
• Explanation: The counter value was updated internally twice, but the UI did not reflect it. The detectChanges() call on the component's ChangeDetectorRef then forced an update, showing the latest value.

Example 2:

• Scenario: User clicks "Increment Counter" once, then clicks "Check Application".
• Input:
  • ManualCounterComponent's internal counter starts at 0.
  • Click "Increment Counter" -> internal counter becomes 1.
• Output:
  • The ManualCounterComponent initially displays "Counter: 0".
  • After clicking "Check Application", the ManualCounterComponent displays "Counter: 1".
• Explanation: The counter was incremented internally. The ApplicationRef.tick() call initiated a full change detection cycle, which included checking the ManualCounterComponent and updating its display.

Example 3:

• Scenario: User clicks "Check Component" without having clicked "Increment Counter".
• Input:
  • ManualCounterComponent's internal counter is 0.
  • No clicks on "Increment Counter".
• Output:
  • The ManualCounterComponent displays "Counter: 0".
  • Clicking "Check Component" results in no visual change.
• Explanation: detectChanges() will run change detection, but since the internal value hasn't changed, no update is necessary.

Constraints

• Angular version: Latest stable version (e.g., Angular 16+).
• Language: TypeScript.
• The solution should demonstrate understanding of ChangeDetectorRef and ApplicationRef.
• Do not disable Angular's default change detection strategy (e.g., ChangeDetectionStrategy.OnPush for the ManualCounterComponent is acceptable, but the manual triggers must still work).
• The solution should be runnable within a standard Angular CLI project setup.

Notes

• The core idea is to understand when Angular doesn't automatically update the view and how to force it.
• Think about how ChangeDetectorRef is injected into a component.
• Consider how you would access the application's root ApplicationRef for global checks.
• The NgZone's run() method is another way to trigger change detection, but for this challenge, ApplicationRef.tick() is the preferred method for application-wide checks.