Angular Component Lifecycle Hooks Explained

Angular is an amazing tool in the list of the greatest frameworks for creating interactive web applications with powerful tools for developers. Another fundamental concept found in Angular is the component-based model of the application, where the whole application is broken down into different components.

Like any other component in Angular, the Angular charts have a life cycle that is a series of changes that happen to the component during its existence until it is destroyed. Angular offers a list of hooks that can be bound to the component in order to trigger functions at specific stages of this lifecycle and hence gain ultimate control of the component’s life cycle.

These are just lifecycle hooks, and Angular aficionados should familiarize themselves with them as they are instrumental to the enhancement of your applications’ performance as well as capabilities.

What are Angular Component Lifecycle Hooks?

As with any other application framework, there are lifecycle hooks in Angular that are triggered during a certain step in the lifecycle of a component. These hooks allow the developer to do certain actions during invocation, during updates, or before the destruction of the component. With these hooks in place, the developer is able to control tasks such as data retrieval, UI refresh, or resource release, so as to guarantee that the component functions as intended. 

 Angular identifies eight lifecycle hooks, each one having a specific role that it plays. All of these hooks can be performed using the methods in the component class, where you are able to code something to be performed when any lifecycle event occurs. It is, therefore, important to know when and when not to apply these hooks in order to develop strong Angular applications.

Eight Lifecycle Hooks in Angular

• ngOnChanges

The ngOnChanges hook is the first and one of the most important hooks to be called when it comes to an Angular component. It can be triggered by the change of one or more data-bound input properties. This hook takes a SimpleChanges object that has information on the previous and current values of the inputs. For this reason, the ngOnChanges function is well-suited when it comes to responding to changes in the input properties, such as recalculating values or updating the interface.

ngOnChanges(changes: SimpleChanges) {
  for (let propName in changes) {
    let change = changes[propName];
    console.log(`Property ${propName} changed from ${change.previousValue} to ${change.currentValue}`);
  }
}

• ngOnInit

The ngOnInit hook is invoked only once after the first time that the hook of type OnChanges is executed and is often used for initializing a component. This is where you would put code that should only be executed once,, like when loading data from a server or defining the state of the component. 

ngOnInit() {
  console.log('Component initialized');
  this.loadData();
}

Unlike ngOnChanges, ngOnInit does not get called multiple times; it runs only once when the component is initialized.

• ngDoCheck

ngDoCheck is a hook that can be used to run a developer’s own change detection logic. Angular has a built-in mechanism for detecting change, but sometimes you need to detect and do some action that Angular does not detect. This hook is invoked during every change detection cycle and can be used to check for specific conditions and respond accordingly.

ngDoCheck() {
  console.log('Change detection triggered');
}

This hook is powerful but should be used with caution, as it can impact performance if overused.

• ngAfterContentInit

The ngAfterContentInit hook is invoked after Angular has pumped its content into the app component. This is most noticeable, for instance, when employing one of the key Angular mechanisms of content designation, such as ng-content. This hook is used to accomplish any initialization related to the projected content.

ngAfterContentInit() {
  console.log('Content projected');
}

Since this hook is invoked only once after the content is projected, it is suitable for the initialization of any content-reliant logic.

• ngAfterContentChecked

ngAfterContentChecked is fired up if the projected content changes have been checked by Angular. This hook is invoked each time change detection is processed, and it is helpful when it comes to response to the changes within projected content not picked up by Angular change detection.

ngAfterContentChecked() {
  console.log('Projected content checked');
}

This hook can prove especially helpful in supporting changes made to dynamic content.

• ngAfterViewInit

The ngAfterViewInit hook is always called after all of the view components have been initialized by Angular. This hook is typically used to accomplish operations that would include interacting with child components or DOM elements in the view.

ngAfterViewInit() {
  console.log('View initialized');
  this.initializeChildComponents();
}

Use this hook when you need to work with the component’s view or its children after they have been initialized.

• ngAfterViewChecked

It is used to detect when Angular’s change detection has checked the view for changes. This particular hook is called each time after the change detection cycle is completed, and it can be useful for operations with the component view.

ngAfterViewChecked() {
  console.log('View checked for changes');
}

Like the ngDoCheck hook, this hook should be used sparingly because it can lead to performance problems.

• ngOnDestroy

The ngOnDestroy hook is invoked before the component is destroyed. It is the last hook in the component’s life and is employed to release resources, for instance, cancel out the observables, remove event listeners, or cancel out timers.

ngOnDestroy() {
  console.log('Component is being destroyed');
  this.cleanupResources();
}

Using ngOnDestroy correctly means that your application does not leak memory or other resources that could cause multiple problems on your application.

Conclusion

Angular has defined various lifecycle hooks that will give developers a means of handling events of a component’s lifecycle. If you know these hooks, you can finely tune your components for maximum performance, control resources well, and give users precisely what they will expect. All hooks are used to accomplish different tasks and enable you to interject in the correct phase of the component’s existence. Whether it is for the initialization of data, for handling changes, or for the cleanup of resources, the lifecycle hooks of Angular provide enough leeway to build scalable and sustainable applications.

For more detailed information on Angular and the use of lifecycle hooks, you can take an Angular JS Course, which gives more detailed insight into lifecycle hooks, along with practical exercises and tutorials from experts.

FAQs

What are lifecycle hooks in Angular?

Lifecycle hooks are specifically defined methods of an Angular component that execute for a definite phase of the component’s lifecycle. These hooks allow the execution of code in response to events in the lifecycle of a component, such as creation, update, or destruction.

 What are lifecycle hooks in Angular W3schools?

W3Schools defines Angular lifecycle hooks as functions that inform and also allow the manipulation of various phases in the existence of a component. Such hooks include ngOnInit, The ngOnChanges, The ngOnDestroy, among others, which enable the developers to leverage on particular lifecycle hooks.

 What are lifecycle hooks in Angular interview questions?

Common interview questions about Angular lifecycle hooks might include:

• “What is the purpose of the ngOnInit hook?”
• “How do ngAfterViewInit and ngAfterContentInit differ?”
• “Can you explain the use of ngOnDestroy and why it’s important?”

 How many lifecycle methods are there in Angular?

Angular provides eight lifecycle hooks: ngOnChanges, ngOnInit, ngDoCheck, ngAfterContentInit, ngAfterContentChecked, ngAfterViewInit, ngAfterViewChecked, and ngOnDestroy. Each of these hooks serves a specific purpose in managing the component’s lifecycle.