Lifecycle hooks

Angular components come with lifecycle hooks, which get executed at specific times in the component's life. For this purpose, Angular offers different interfaces. Each interface has a method of the same name as the interface name with the prefix ng. Each method is executed when the corresponding lifecycle event occurs. They are also called lifecycle hook methods. Angular calls the lifecycle hook methods in the following sequence after the constructor has been called:

The lifecycle hook method	Purpose and timing
ngOnChanges	This is called whenever one or more data-bound input properties change. This method is called on initial changes (before ngOnInit) and any other subsequent changes. This method has one parameter--an object with keys of type string and values of type SimpleChange. The keys are the component's property names. The SimpleChange object contains current and previous values. A usage example is shown next.
ngOnInit	This is called once, after the first ngOnChanges. Note that the constructor of a component should only be used for dependency injection because data-bound input values are not yet set in the constructor. Everything else should be moved to the ngOnInit hook. A usage example is shown next.
ngDoCheck	This is called during every change detection run. It is a good place for custom logic, which allows us to do a fine-grained check of which property on our object changed.
ngAfterContentInit	This is called once, after Angular puts external content into the component's view. A placeholder for any external content is marked with the ngContent directive (the ng-content tag). A usage example of the ngContent directive is demonstrated afterwards.
ngAfterContentChecked	This is called after Angular checks the content put into the component's view.
ngAfterViewInit	This is called once, after Angular initializes the component's and child's views.
ngAfterViewChecked	This is called after Angular checks the component's views and child views.
ngOnDestroy	This is called just before Angular destroys the component's instance. This happens when you remove the component with built-in structural directives such as ngIf, ngFor, ngSwitch, or when you navigate to another view. This is a good place for cleanup operations such as unsubscribing observables, detaching event handlers, canceling interval timers, and so on.

 

Let's see an example of how to use ngOnInit and ngOnChanges:

import {Component, OnInit, OnChanges, SimpleChange} from '@angular/core';

@Component({
  selector: 'greeting-component',
  template: `<h1>Hello {{text}}</h1>`
})
export class GreetingComponent implements OnInit, OnChanges {
  @Input text: string;

  constructor() { }

  ngOnInit() {
    text = "Angular";
  }
 
  ngOnChanges(changes: {[propertyName: string]: SimpleChange}) {
    console.log(changes.text);
    // changes = {'text': {currentValue: 'World', previousValue: {}}}
    // changes = {'text': {currentValue: 'Angular', 
                  previousValue: 'World'}}
  }
}

Usage in HTML:

<greeting-component [text]="World"></greeting-component>

Let's now see how to use the ngContent directive:

export @Component({
  selector: 'greeting-component',
  template: `<div><ng-content></ng-content> {{text}}</div>`
})
class GreetingComponent {
  @Input text: string;
}

Usage in HTML:

<greeting-component [text]="World"><b>Hello</b></greeting-component>

After the component's initialization, the following hook methods get always executed on every change detection run: ngDoCheck -> ngAfterContentChecked -> ngAfterViewChecked -> ngOnChanges.