@angular/core APIs

Implements Angular’s core functionality, low-level services, and utilities.

Some of the core classes/interfaces/functions are as follows :

1. EventEmitter

It is a Class, In Angular, a component can emit an event using @Output and EventEmitter. Both are parts of the @angular/core.
It emit custom events synchronously or asynchronously, and register handlers for those events by subscribing to an instance.

appchild.component.ts

import { Component, EventEmitter, Output } from '@angular/core';
@Component({
    selector: 'app-child',
    template: `<button class='btn btn-primary' (click)="valueChanged()">Click me</button> `
})
export class AppChildComponent {
    @Output() valueChange = new EventEmitter();
    Counter = 0;
    valueChanged() { // You can give any function name
        this.counter = this.counter + 1;
        this.valueChange.emit(this.counter);
    }
}

to emit values in the parent component.

2. OnDestroy

It is an interface, it is a lifecycle hook that is called when a directive, pipe, or service is destroyed. Use for any custom cleanup that needs to occur when the instance is destroyed.

// its own Definition

interface OnDestroy {
  ngOnDestroy(): void
}

// Usage of ngOnDestroy()

@Component({selector: 'my-cmp', template: `...`})
class MyComponent implements OnDestroy {
  ngOnDestroy() {
    // ...
  }
}

3. OnInit

It is an interface, it is a lifecycle hook that is called after Angular has initialized all data-bound properties of a directive. Define an ngOnInit() method to handle any additional initialization tasks.
It is invoked only once when the directive is instantiated.

// its own Definition

interface OnInit {
  ngOnInit(): void
}

// Usage of ngOnInit()

@Component({selector: 'my-cmp', template: `...`})
class MyComponent implements OnInit {
  ngOnInit() {
    // ...
  }
}

4. ViewChild

It is one of the decorators, it is a Property decorator that configures a view query. The change detector looks for the first element or the directive matching the selector in the view DOM. If the view DOM changes, and a new child matches the selector, the property is updated.

// EXAMPLE

import {Component, Directive, Input, ViewChild} from '@angular/core';

@Directive({selector: 'pane'})
export class Pane {
  @Input() id !: string;
}

@Component({
  selector: 'example-app',
  template: `
    <pane id="1" *ngIf="shouldShow"></pane>
    <pane id="2" *ngIf="!shouldShow"></pane>

    <button (click)="toggle()">Toggle</button>

    <div>Selected: {{selectedPane}}</div>
  `,
})
export class ViewChildComp {
  @ViewChild(Pane, {static: false})
  set pane(v: Pane) {
    setTimeout(() => { this.selectedPane = v.id; }, 0);
  }
  selectedPane: string = '';
  shouldShow = true;
  toggle() { this.shouldShow = !this.shouldShow; }
}

Summary

In this tutorial we learnt about the angular core APIs , few classes, interfaces and functions.
Hope you like it !

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