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Salim Chemes
Salim Chemes

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ResizeObserver in Angular

In this post we will review how to implement ResizeObserver in Angular applications

What is ResizeObserver?
Based on the documentation

The ResizeObserver interface reports changes to the dimensions of an Element's content or border box, or the bounding box of an SVGElement.

So, our goal is know when our component is resized.

How can implement ResizeObserver in Angular?
We will use ng-web-apis/resize-observer

The documentation is pretty clear, so let's start with an example to see how this library works.

First thing to do is to add @ng-web-apis/common as dependency

npm i @ng-web-apis/common
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Second is to add

npm i @ng-web-apis/resize-observer
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and finally

npm install --save @types/resize-observer-browser
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Now, let's start adding changes into our app.
Adding ResizeObserverModule into app.module.ts

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { ResizeObserverModule } from '@ng-web-apis/resize-observer';
import { AppComponent } from './app.component';

  declarations: [
  imports: [
    ResizeObserverModule // adding library module
  providers: [],
  bootstrap: [AppComponent]
export class AppModule { }
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Adding a section in app.component.html, to display resize changes.
Note we have waResizeBox="content-box", which is the default value. Possible values are content-box (the default), border-box, and device-pixel-content-box. Check the docs for more details.
waResizeObserver will allow us to react when the component is resized

    <h1 waResizeBox="content-box" (waResizeObserver)="onResize($event)">
        width: {{this.width}}
        height: {{this.height}}
        domRectReadOnly: {{this.domRectReadOnly | json}}
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Creating onResize method in app.component.ts to assign resize event properties and display them in our template

import { Component } from '@angular/core';

  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']

export class AppComponent {
  title = 'resize-observer';
  width: number = 0;
  height: number = 0;
  domRectReadOnly: DOMRectReadOnly | undefined;

  onResize(entry: ResizeObserverEntry[]) {
    this.width = entry[0].contentRect.width
    this.height = entry[0].contentRect.height;
    this.domRectReadOnly = entry[0].contentRect;
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That's it. This is what happens when we resize the browser
Alt Text


  • We have reviewed what ResizeObserver is and how to implement it in Angular
  • ResizeObserver is one of the API's for Angular, take a look into this to find more


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