Detect when input value changed in directive


I'm trying to detect when the <strong>value</strong> of an input changed in a directive. I have the following directive:

import { ElementRef, Directive, Renderer} from '@angular/core'; @Directive({ selector: '[number]', host: {"(input)": 'onInputChange($event)'} }) export class Number { constructor(private element: ElementRef, private renderer: Renderer){ } onInputChange(event){ console.log('test'); } }

The problem in this directive is that it detects only when there is an input and not when the value changes programatically. I use reacive form and sometimes I set the value with the patchValue() function. How can I do so the change function gets triggered?


You need to make an input property of input and then use the ngOnChanges hook to tell when the input property changes.

<pre class="lang-js prettyprint-override">@Directive({ selector: '[number]' }) export class NumberDirective implements OnChanges { @Input() public number: any; @Input() public input: any; ngOnChanges(changes: SimpleChanges){ if(changes.input){ console.log('input changed'); } } }

<a href="http://plnkr.co/edit/Ygd9CDWg2OxG6n88zRqD" rel="nofollow">Plunkr</a>


You can also use HostListener. For more information about HostListener, you can go through <a href="https://angular.io/guide/attribute-directives" rel="nofollow">this link</a>. Here is the code.

import {Directive, ElementRef, HostListener} from '@angular/core'; @Directive({ selector: '[number]' }) export class NumberDirective { @Input() public number: any; @Input() public input: any; constructor(private el: ElementRef) {} @HostListener('change') ngOnChanges() { console.log('test'); } }


There is a better way to use this result, used for example in the *ngIf Angular <a href="https://github.com/angular/angular/blob/1a44a0b4a87767236f6eea07deca5e4d8789572f/packages/common/src/directives/ng_if.ts#L113" rel="nofollow">source code</a>.

You can combine an @Input() with a setter. When the input changes, the setter is called again.

@Input() set numberOfWheels(wheels: number) { if(wheels === 2) { console.log("It's a bike!"); } else if(wheels === 4) { console.log("It's a car!"); } else { console.log("I don't know what it is :("); } }

You can save the previous value in the Directive property in order to use it later and compare it with the new value:

private previousValue: any = null; @Input() set myInputName(value: any) { console.log(`Previous value is: ${this.previousValue}`); console.log(`New value is: ${value}`); this.previousValue = value; }


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