How to listen for value changes from class property TypeScript - Angular

Question

In AngularJS, we can listen variable change using $watch , $digest ... which is no longer possible with the new versions of Angular (5, 6).

In Angular, this behaviour is now part of the component lifecycle.

I checked on the official documention, articles and especially on Angular 5 change detection on mutable objects , to find out how to listen to a variable (class property) change in a TypeScript class / Angular

What is proposed today is :

 import { OnChanges, SimpleChanges, DoCheck } from '@angular/core'; @Component({ selector: 'my-comp', templateUrl: 'my-comp.html', styleUrls: ['my-comp.css'], inputs:['input1', 'input2'] }) export class MyClass implements OnChanges, DoCheck, OnInit{ //I can track changes for this properties @Input() input1:string; @Input() input2:string; //Properties what I want to track ! myProperty_1: boolean myProperty_2: ['A', 'B', 'C']; myProperty_3: MysObject; constructor() { } ngOnInit() { } //Solution 1 - fired when Angular detects changes to the @Input properties ngOnChanges(changes: SimpleChanges) { //Action for change } //Solution 2 - Where Angular fails to detect the changes to the input property //the DoCheck allows us to implement our custom change detection ngDoCheck() { //Action for change } } 

This is only true for @Input() property !

If I want to track changes of my component's own properties ( myProperty_1 , myProperty_2 or myProperty_3 ), this will not work.

Can someone help me to solve this problematic ? Preferably a solution that is compatible with Angular 5

Answer 1

You can still check component's field members value change by KeyValueDiffers via DoCheck lifehook.

import { DoCheck, KeyValueDiffers, KeyValueDiffer } from '@angular/core';

differ: KeyValueDiffer<string, any>;
constructor(private differs: KeyValueDiffers) {
  this.differ = this.differs.find({}).create();
}

ngDoCheck() {
  const change = this.differ.diff(this);
  if (change) {
    change.forEachChangedItem(item => {
      console.log('item changed', item);
    });
  }
}

see demo .

Answer 2

I think the nicest solution to your issue is to use a decorator that replaces the original field with a property automatically, then on the setter you can create a SimpleChanges object similar to the one created by angular in order to use the same notification callback as for angular (alternatively you could create a different interface for these notifications, but the same principle applies)

import { OnChanges, SimpleChanges, DoCheck, SimpleChange } from '@angular/core';

function Watch() : PropertyDecorator & MethodDecorator{
    function isOnChanges(val: OnChanges): val is OnChanges{
        return !!(val as OnChanges).ngOnChanges
    }
    return (target : any, key: string | symbol, propDesc?: PropertyDescriptor) => {
        let privateKey = "_" + key.toString();
        let isNotFirstChangePrivateKey = "_" + key.toString() + 'IsNotFirstChange';
        propDesc = propDesc || {
            configurable: true,
            enumerable: true,
        };
        propDesc.get = propDesc.get || (function (this: any) { return this[privateKey] });

        const originalSetter = propDesc.set || (function (this: any, val: any) { this[privateKey] = val });

        propDesc.set = function (this: any, val: any) {
            let oldValue = this[key];
            if(val != oldValue) {
                originalSetter.call(this, val);
                let isNotFirstChange = this[isNotFirstChangePrivateKey];
                this[isNotFirstChangePrivateKey] = true;
                if(isOnChanges(this)) {
                    var changes: SimpleChanges = {
                        [key]: new SimpleChange(oldValue, val, !isNotFirstChange)
                    }
                    this.ngOnChanges(changes);
                }
            }
        }
        return propDesc;
    }
}

// Usage
export class MyClass implements OnChanges {


    //Properties what I want to track !
    @Watch()
    myProperty_1: boolean  =  true
    @Watch()
    myProperty_2 =  ['A', 'B', 'C'];
    @Watch()
    myProperty_3 = {};

    constructor() { }
    ngOnChanges(changes: SimpleChanges) {
        console.log(changes);
    }
}

var myInatsnce = new MyClass(); // outputs original field setting with firstChange == true
myInatsnce.myProperty_2 = ["F"]; // will be notified on subsequent changes with firstChange == false

Answer 3

as said you can use

public set myProperty_2(value: type): void {
 if(value) {
  //doMyCheck
 }

 this._myProperty_2 = value;
}

and then if you need to retrieve it

public get myProperty_2(): type {
  return this._myProperty_2;
}

in that way you can do all the checks that you want while setting/ getting your variables such this methods will fire every time you set/get the myProperty_2 property.

small demo: https://stackblitz.com/edit/angular-n72qlu

Answer 4

I think I came into the way to listen to DOM changes that you can get any changes that do to your element, I really hope these hints and tips will help you to fix your problem, following the following simple step:

First , you need to reference your element like this:

in HTML:

<section id="homepage-elements" #someElement>
....
</section>

And in your TS file of that component:

@ViewChild('someElement')
public someElement: ElementRef;

Second , you need to create an observer to listen to that element changes, you need to make your component ts file to implements AfterViewInit, OnDestroy , then implement that ngAfterViewInit() there ( OnDestroy has a job later):

private changes: MutationObserver;
ngAfterViewInit(): void {
  console.debug(this.someElement.nativeElement);

  // This is just to demo 
  setInterval(() => {
    // Note: Renderer2 service you to inject with constructor, but this is just for demo so it is not really part of the answer
    this.renderer.setAttribute(this.someElement.nativeElement, 'my_custom', 'secondNow_' + (new Date().getSeconds()));
  }, 5000);

  // Here is the Mutation Observer for that element works
  this.changes = new MutationObserver((mutations: MutationRecord[]) => {
      mutations.forEach((mutation: MutationRecord) => {
        console.debug('Mutation record fired', mutation);
        console.debug(`Attribute '${mutation.attributeName}' changed to value `, mutation.target.attributes[mutation.attributeName].value);
      });
    }
  );

  // Here we start observer to work with that element
  this.changes.observe(this.someElement.nativeElement, {
    attributes: true,
    childList: true,
    characterData: true
  });
}

You will see the console will work with any changes on that element:

This is another example here that you will see 2 mutation records fired and for the class that changed:

// This is just to demo
setTimeout(() => {
   // Note: Renderer2 service you to inject with constructor, but this is just for demo so it is not really part of the answer
  this.renderer.addClass(this.someElement.nativeElement, 'newClass' + (new Date().getSeconds()));
  this.renderer.addClass(this.someElement.nativeElement, 'newClass' + (new Date().getSeconds() + 1));
}, 5000);

// Here is the Mutation Observer for that element works
this.changes = new MutationObserver((mutations: MutationRecord[]) => {
    mutations.forEach((mutation: MutationRecord) => {
      console.debug('Mutation record fired', mutation);
      if (mutation.attributeName == 'class') {
        console.debug(`Class changed, current class list`, mutation.target.classList);
      }
    });
  }
);

Console log:

And just housekeeping stuff, OnDestroy :

ngOnDestroy(): void {
  this.changes.disconnect();
}

Finally, you can look into this Reference: Listening to DOM Changes Using MutationObserver in Angular

Answer 5

You can import ChangeDetectorRef

 constructor(private cd: ChangeDetectorRef) {
          // detect changes on the current component
            // this.cd is an injected ChangeDetector instance
            this.cd.detectChanges();

            // or run change detection for the all app
            // this.appRef is an ApplicationRef instance
            this.appRef.tick();
}