Angular - trigger an event based on a variable change

Issue

I have a list of items that is set in the service and is called in the ngOnInit():

async ngOnInit(){
   this.items = await this.service.getItems();
}

and this list is rendered using the directive *ngfor.

However, now I need to refresh the items list when there is any change in a variable. So I need an event that runs only when a certain variable (lets called it itemCategories) changes its value lets.

How can I achieve this?

Angular version is 12 and I am new to this

Solution

You can use the EventEmitter and @Output() to achieve your goal.
Dropdown menu component:

import { Component,  Output, EventEmitter, NgModule } from '@angular/core';
@Component({
  selector: 'app-dropdown',
  template: `
    <div>
      <select [(ngModel)]="selected" (ngModelChange)="onChange($event)">
        <option *ngFor="let category of categories" [value]="category">{{category}}</option>
      </select>
    </div>`
})
export class DropdownComponent {
  @Output() selectedCategory: EventEmitter<string> = new EventEmitter<string>();
  categories = ["All categories", "A", "B", "C"]; //your categories list
  selected: string = "All categories"; //default value 
  constructor() {
  }

  onChange(newvalue): void {
    this.selectedCategory.emit(newvalue);
  }

}  

Here is the other component that will receive the selected value whenever it changes from the dropdown component

import {Component, NgModule} from '@angular/core'
@Component({
  selector: 'app-main',
  template: `
    <div>
      <app-dropdown (selectedCategory)='onChange($event)'></app-dropdown>
    </div>
  `,
})
export class MainComponent {
  selected: string;
  constructor() {
  }

  onChange(category):void {
    this.selected = category;
    //get your items list based on the selected category
  }
}  

Answered By - Merna Mustafa