Can I still use ngChanges with signals?

Question

With the traditional @Input() decorator, one can use the ngChanges to do something if the value change. In this case, I'm using a component in the ng-bootstrap modal. The modal contains a form. When the item to edit is selected, it's passed to the component that's in the modal.

@Input() inputVar!: SomeType;

ngOnChanges(changes: SimpleChanges): void {
  const chInputVar = changes['inputVar'];

  //Do something when the value changes...
}

Now, I'd like to use the signal input:

inputVar = input<SomeType>();
//How to act when the value changes?

Answer 1

ngOnChanges fires for all input changes. Instead go for effect which fires only when the signals inside the effect.

@Component({
  selector: 'app-child',
  standalone: true,
  template: ``,
})
export class Child {
  inputVar = input<number>();

  constructor() {
    effect(() => {
      const value = this.inputVar();
      console.log('signal change', value);
      // Perform the signal change logic here!
    });
  }
}

Stackblitz Demo

---

If you must use ngOnChanges then below is a working example for the same.

import { Component, effect, input, SimpleChanges } from '@angular/core';
import { bootstrapApplication } from '@angular/platform-browser';
import 'zone.js';

@Component({
  selector: 'app-child',
  standalone: true,
  template: ``,
})
export class Child {
  inputVar = input<number>();

  constructor() {}

  ngOnChanges(changes: SimpleChanges) {
    console.log(changes);
    const inputVar = changes?.['inputVar'];
    if (inputVar?.currentValue !== inputVar?.previousValue) {
      console.log('change', inputVar);
    }
  }
}

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [Child],
  template: `
    <app-child [inputVar]="value" />
  `,
})
export class App {
  value = 0;

  ngOnInit() {
    setInterval(() => {
      this.value = Math.random();
    }, 1000);
  }
}

bootstrapApplication(App);

Stackblitz Demo