Custom Change Detection Strategy in Angular

Angular's built-in change detection is powerful, but sometimes you need more fine-grained control over when and how your components update. This challenge asks you to implement a custom change detection strategy that bypasses the default mechanism for specific components, allowing you to optimize performance by manually triggering updates only when necessary.

Problem Description

Your task is to create a custom change detection strategy in Angular that can be applied to individual components. This strategy should prevent the component and its descendants from being checked automatically by Angular's zone.js or default change detection runners. Instead, you will be responsible for manually triggering change detection for these components when you deem it appropriate.

Key Requirements:

1. Custom ChangeDetectionStrategy: Create a new ChangeDetectionStrategy that does not rely on the default mechanisms.
2. Component Implementation: Demonstrate how to apply this custom strategy to an Angular component.
3. Manual Trigger: Implement a method within your custom strategy (or a way to access it) that allows for manual invocation of change detection.
4. Demonstration: Show that the component does not update automatically when its input properties change if the custom strategy is applied and the manual trigger is not called.
5. Successful Update: Show that the component does update when the manual trigger is called after an input property has changed.

Expected Behavior:

• A component using the default ChangeDetectionStrategy.Default or ChangeDetectionStrategy.OnPush will update automatically when its inputs change (for Default) or when references change (for OnPush).
• A component using your custom strategy should not update automatically when its inputs change.
• Only by explicitly calling your manual trigger mechanism should the component update.

Edge Cases:

• Consider how to handle asynchronous operations and their impact on your custom strategy.
• Ensure your manual trigger correctly updates the view.

Examples

Example 1:

• Scenario: A simple DisplayCounterComponent that receives a count input.
• Input:
  • AppComponent renders DisplayCounterComponent with ChangeDetectionStrategy.Default.
  • AppComponent has a button to increment its internal counter.
  • AppComponent passes its counter to DisplayCounterComponent.
• Expected Output: When the button in AppComponent is clicked, the DisplayCounterComponent updates its displayed count automatically.

Example 2:

• Scenario: The same DisplayCounterComponent but now configured with your custom ChangeDetectionStrategy.
• Input:
  • AppComponent renders DisplayCounterComponent with your custom strategy.
  • AppComponent has a button to increment its internal counter.
  • AppComponent passes its counter to DisplayCounterComponent.
• Initial State: DisplayCounterComponent displays the initial count.
• Action: Click the increment button in AppComponent.
• Expected Output (before manual trigger): The DisplayCounterComponent does not update its displayed count.
• Action: Call the manual change detection trigger for DisplayCounterComponent.
• Expected Output (after manual trigger): The DisplayCounterComponent now updates and displays the new count.

Example 3: Handling Different Triggering Mechanisms

• Scenario: A DataFetchComponent that fetches data.
• Input:
  • DataFetchComponent uses your custom strategy.
  • It has an internal fetchData() method that simulates an API call and updates its internal data property.
• Initial State: DataFetchComponent shows a "Loading..." message.
• Action: Call fetchData() within DataFetchComponent. The simulated API call completes, and the data property is updated internally.
• Expected Output (before manual trigger): The DataFetchComponent still shows "Loading..." and does not display the fetched data.
• Action: Manually trigger change detection for DataFetchComponent.
• Expected Output (after manual trigger): The DataFetchComponent now displays the fetched data.

Constraints

• The solution must be written in TypeScript.
• You should leverage Angular's ChangeDetectorRef and potentially custom ɵdetectChanges or similar internal APIs (though be mindful that these are internal and subject to change). A more robust approach might involve creating a custom RendererFactory or interacting with ApplicationRef.
• The custom strategy should be easily applicable to any component using the changeDetection property in the @Component decorator.
• The manual trigger should be accessible and straightforward to use from the parent component or within the component itself.

Notes

This challenge delves into the internal workings of Angular's change detection. While direct manipulation of internal APIs (ɵdetectChanges) is one way to achieve this, consider if there are more officially supported (though perhaps less direct) ways to achieve a similar outcome by understanding how ChangeDetectorRef works. Think about how ChangeDetectorRef.detectChanges() and ChangeDetectorRef.markForCheck() interact with different strategies. The core idea is to disable the automatic checks and enable a manual override. You might need to create a custom ɵDefaultIterableDifferFactory or similar to truly bypass default behavior, or more pragmatically, find a way to hook into the ChangeDetectorRef lifecycle.