Change detection is an integral component of Angular that ensures the user interface is in alignment with the state of the application. ChangeDetectionStrategy is Angular's default detection strategy.Standard, which examines for modifications to all components and their templates on each JavaScript event or timer tick.

Nevertheless, this default strategy may result in superfluous and expensive change detection cycles, particularly in large-scale applications. Alternative strategies, such as OnPush, enter into play at this point. The OnPush strategy optimizes change detection by focusing solely on a component's inputs and references.

Using the OnPush strategy, Angular determines if a component's input properties have changed. If the input values remain the same, Angular assumes that the component's state hasn't changed and skips its change detection entirely. This optimization can significantly improve the performance of your Angular application, especially when used strategically.

import { Component, Input, ChangeDetectionStrategy } from '@angular/core';

@Component({
  selector: 'app-user',
  template: `
    <h2>{{ user.name }}</h2>
    <p>{{ user.email }}</p>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class UserComponent {
  @Input() user: User;
}

In this example, the UserComponent has the OnPush change detection strategy defined. By doing so, we instruct Angular to only perform change detection if the user input property changes.

When using the OnPush strategy, it is essential to ensure that the user input property is immutable. If you update the user object's properties, Angular won't detect the changes, as it relies on reference comparison.

To optimize performance further, you can utilize the ChangeDetectorRef to manually trigger change detection when necessary:

import { Component, Input, ChangeDetectionStrategy, ChangeDetectorRef } from '@angular/core';

@Component({
  selector: 'app-user',
  template: `
    <h2>{{ user.name }}</h2>
    <p>{{ user.email }}</p>
    <button (click)="updateUser()">Update</button>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class UserComponent {
  @Input() user: User;

  constructor(private cdr: ChangeDetectorRef) {}

  updateUser() {
    // Update the user object
    this.cdr.markForCheck(); // Trigger change detection
  }
}

In the above example, we inject the ChangeDetectorRef and call its markForCheck() method to manually trigger change detection when the user clicks the "Update" button.

By understanding and utilizing Angular's change detection strategies, especially the OnPush strategy, you can significantly enhance the performance of your Angular applications. By minimizing unnecessary change detection cycles, your app will run more efficiently and provide a smoother user experience.

Remember to carefully analyze your application's requirements and components' state before choosing a change detection strategy. Applying the OnPush strategy to components that rarely change or have immutable input properties can lead to noticeable performance improvements.