Detect when a property of an input of a component changed in angular

Question

I have an angular component which has an input Person

export class Person {
  public name = 'Initial Name';
}

export class PersonComponent implements OnInit, OnChanges {
  @Input() public person: Person;

  ngOnChanges(changes: SimpleChanges): void {
    console.log('changed');
  }
}

In the parent component of this component, if I replace the Person object which is given as the input to the child component, ngOnChanges() gets fired in the child component.

But, if I only change the name of the Person object from the parent component, ngOnChanges() doesn't get fired in the child component (anyway, if I bind the person name to an html element in the child template, it gets updated).

Is there any way to get notified in the child component when a property of an input is changed?

Christian · Accepted Answer · 2018-07-15 16:31:41Z

11

No, the ngOnChanges hook will only fire when the reference changes. You can easily achieve what you're going for by doing this when changing the name property in the parent component:

this.person = {...this.person, name: 'new value'};

answered Jul 15, 2018 at 16:31

Christian

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5 Comments

Lahiru Chandima Over a year ago

My actual object is much complex than this and I only posted it this way for simplicity. It would be great if there is another way to achieve this.

maxime1992 Over a year ago

Doesn't really matter how deep is your object. Immutability is the way to go here :)

maxime1992 Over a year ago

Unless you want to pass a given property as a separated input which would probably not be a good idea

Christian Over a year ago

@LahiruChandima, ...this.person is the spread operator. It'll copy all of the object's properties, and it'll be overriden by anything to the right, in this case the name property. So all of the person's properties will remain the same except for the name

Reactgular Over a year ago

This is only a partial answer. It solves one of the problems of mutating data, but ignores the bigger issue of why you should use immutable data with Angular. It's a very broad topic ranging from freezing objects to DOM rendering performance. If you've already designed your Angular application to use mutating data then you've painted yourself into a corner already but probably don't know it yet. Next will come questions about state management, mixing methods and properties on frozen data, etc.. etc.. Stop now, Google immutable Javascript and learn about OnPush else you'll have more issues.

user184994 · Accepted Answer · 2018-07-15 17:06:53Z

4

You could always create an Observable on the Person that emits whenever a property changes.

import { Subject } from 'rxjs';

export class Person {

  private _changes = new Subject<any>();
  private _name = 'Initial Name';

  // subscribe to this one
  public changes = this._changes.asObservable();

  set name(newName: string) {
    this._name = newName;
    this._changes.next({'name': newName});
  }

  get name() {
    return this._name;
  }

}

This way, you can subscribe to person.changes, which will emit a value every time name is changed

When you write person.name = 'Fred', the setter will be invoked, which in turn will emit the new value from the Observable.

answered Jul 15, 2018 at 17:06

user184994

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Comments

Eliseo · Accepted Answer · 2018-07-15 19:25:54Z

If you use a getter, each time you give a value the setter is executed. So a work-around can be have two variables

  @Input() person;
  @Input() 
  set changePerson(value)
  {
    console.log(this.person);
  }

So, in your parent you can write

  <app-child [person]="person" [changePerson]="changePerson"></app-child>

  this.person.name="new name";
  this.changePerson=true;

Another option is using a EventEmitter in a service,each time you change a

    //Our service
    @Injectable({
      providedIn: 'root',
    })
    export class ChangeService {
      changed: EventEmitter<any>=new EventEmitter();
      emit(value:any)
      {
          this.changed.emit(value);
      }
    }

//Our parent
constructor (private changeService:ChangeService){}
  ngOnInit()
  {
    setTimeout(()=>{
      this.changeService.emit(this.person)

    })
    setTimeout(()=>{
      this.person.name="George";
      this.changeService.emit(this.person);
    },5000);
  }

//Our child
  person:any;
  ngOnInit() {
    this.changeService.changed.subscribe(res=>{
      this.person=res;
      console.log(res);
    })
  }

Robert Leeuwerink · Accepted Answer · 2018-07-16 15:32:40Z

-2

Why do you need your child to know about the change? in your child you should only change things related to the child and bubble them up to the parent. With Observables it is easier for the child to still display the information. I recommend using @Output and then changing the object in the parent

answered Jul 16, 2018 at 15:32

Robert Leeuwerink

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