Kilt and Code
Kilt and Code

There are often scenarios in Angular where we need to communicate between components, specifically to send data from a child component to it’s parent component. Consider this Angular 12 app for managing a to do list:

Angular To Do App

When saving a new to do to the list, I’m using the child component (the dialog window) to send the new to do to an API to be stored in a database. In addition to this, I need to communicate to the parent component (the to do list) that the new to do was saved and add it to the list:

Angular To Do App - Save Dialog

I’ll begin by demonstrating the current Angular app (currently without an EventEmitter) and explain how we can implement an EventEmitter to send data between the child and parent components.

app.component.ts (without EventEmitter)

In app.component.ts (the parent component) I’m retrieving the list of to dos from an API and assigning them to an array:

import { Component, OnInit } from '@angular/core';
import { Title } from '@angular/platform-browser';
import { ToDoListService } from './services/to-do-list.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {
  public title = 'To-Do List';
  public toDoList: string[] = [];

  constructor(
    private titleService: Title,
    private toDoListService: ToDoListService) {}

  ngOnInit(): void {
    this.titleService.setTitle(this.title);
    this.toDoList = this.toDoListService.getToDoList();
  }
}

app.component.html (without EventEmitter)

In app.component.html I’m looping through the array of to dos and referencing SaveNewToDoComponent (the child component) that is being used as a dialog window to save a new to do:

<nav class="navbar navbar-expand-lg navbar-dark bg-primary">
    ...
</div>

<div class="container mt-4">
  <div class="row">
    <div class="col-md-12">
      <div class="card">
        <div class="card-body">
          <h5 class="card-title">To Do List</h5>
          <p class="card-text"><small>View all of the to-dos or save a new to-do to the list.</small></p>
          <button type="button" class="btn btn-primary mt-1 mb-4" data-toggle="modal" data-target="#toDoModal">
            Save New To Do
          </button>
          <ul class="list-group">
            <li class="list-group-item list-group-item-action" *ngFor="let toDo of toDoList"></li>
          </ul>
        </div>
      </div>
    </div>
  </div>
</div>

<app-save-new-to-do></app-save-new-to-do>

save-new-to-do.ts (without EventEmitter)

In save-new-to-do.ts (the child component) I’m retrieving the new to do from the form and sending it to an API to be stored in a database:

import { Component, EventEmitter, OnInit, Output } from '@angular/core';
import { FormBuilder } from '@angular/forms';
import { ToDoListService } from 'src/app/services/to-do-list.service';

@Component({
  selector: 'app-save-new-to-do',
  templateUrl: './save-new-to-do.component.html',
  styleUrls: ['./save-new-to-do.component.css']
})
export class SaveNewToDoComponent implements OnInit {
  formGroup: any;

  constructor(
    private formBuilder: FormBuilder,
    private toDoListService: ToDoListService) { }

  ngOnInit(): void {
    this.formGroup = this.formBuilder.group({
      toDo: ''
    });
  }
  
  public saveNewToDo(formData: any) {
    let toDo = formData['toDo'];
    this.toDoListService.saveNewToDo(toDo);
  }
}

Adding EventEmitter to the Angular App

I’ll add an EventEmitter by adding an output decorator to the SaveNewToDoComponent class:

@Output() saveToDoEvent = new EventEmitter<string>();

Also in the SaveNewToDoComponent class, I’ll emit an event containing the new to do once the to do has been sent to the API:

this.saveToDoEvent.emit(toDo);

save-new-to-do.ts (with EventEmitter)

Below is the entire component save-new-to-do.ts using an EventEmitter:

import { Component, EventEmitter, OnInit, Output } from '@angular/core';
import { FormBuilder } from '@angular/forms';
import { ToDoListService } from 'src/app/services/to-do-list.service';

@Component({
  selector: 'app-save-new-to-do',
  templateUrl: './save-new-to-do.component.html',
  styleUrls: ['./save-new-to-do.component.css']
})
export class SaveNewToDoComponent implements OnInit {
  formGroup: any;
  @Output() saveToDoEvent = new EventEmitter<string>();

  constructor(
    private formBuilder: FormBuilder,
    private toDoListService: ToDoListService) { }

  ngOnInit(): void {
    this.formGroup = this.formBuilder.group({
      toDo: ''
    });
  }
  
  public saveNewToDo(formData: any) {
    let toDo = formData['toDo'];
    this.toDoListService.saveNewToDo(toDo);
    this.saveToDoEvent.emit(toDo);
  }
}

Within app.component.html, I’ll use event binding to communicate the event occurring in the child component and send the new to do to the parent component by executing the new method ‘appendToDo’, passing in the new to do as a parameter:

<app-save-new-to-do (saveToDoEvent)="appendToDo($event)"></app-save-new-to-do>

Finally, I’ll add the new method ‘appendToDo’ to the parent component to push the new to do to the array:

public appendToDo(toDo: string) {
    if (toDo?.length > 0) {
        this.toDoList.push(toDo);
    }
}

Now that I’ve added an EventEmitter into the Angular app, the list will be automatically updated with the new to do:

Angular To-Do App - Last Step

GitHub Repository

Thank you for reading this article! The full solution used in this guide is available at github.com/kiltandcode/angular-event-emitter-example.