Life Cycle of Angular Components: Various Methods Explained

Launched in 2009 by Google, Angular is a JavaScript development framework with the Typescript programming language. It is one of the most popular design frameworks for app development. The best part about the Angular framework is that it is an open-source application. Developers use the Angular framework to create mobile and web applications and visualize data. For Angular development, developers have to follow an extensive process called the Angular lifecycle. 

This blog helps you understand the Angular lifecycle and its various hook methods.

What is Angular Lifecycle?

The cycle of events required to run the Angular components is called the Angular lifecycle. The Angular components are the building blocks of an application necessary for the UI. In simple terms, Angular components are blocks responsible for controlling a patch of the screen in an Angular application. The components comprise various properties along with a selector, template, and style. Components also require metadata for their processing. 

Angular goes through different change detection steps to track changes in data-bound properties during the lifecycle. An Angular lifecycle starts when the Angular initiates a component class. It then defines the component’s view and the view of child components. Towards the end, Angular destroys the components that are no longer in use. It also calls various hook methods after it instantiates a component.

The various Angular component lifecycle hooks serve as functionality in the Angular framework. Each hook method allows the developers to run different codes during a particular event.

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Lifecycle of Angular Components

All Angular components have a different lifecycle divided into eight different stages. These stages are called lifecycle hook events that can be used separately at the time of application. Before the lifecycle starts, we execute the constructor of the component class, and then the various hook events take place. The hook events of the Angular lifecycle are divided into two main categories. One phase is linked to the original component whereas, the second phase relates to the children of that particular component. 

Let us understand the various stages of the angular lifecycle:

• ngOnChanges:- The ngOnChanges() event occurs when we change the value of an input control within the component, and the bound property of the component alters. In simple terms, this hook method takes place whenever the Angular sets or resets the in-bound properties of the component. During this method, a ‘SimpleCharge’ object is received for the current and previous property values.
  This hook method receives an altered data map of the current and previous property values. The data map is wrapped as SimpleCharge. It is pertinent to remember that this event takes place very often. Therefore, you have to be extremely careful while carrying out any changes during the event as it can significantly impact the performance. You have to use this hook method only if the component has bound inputs and you change the bound properties of the component. Therefore, it is usually treated as a call-back method.
• ngOnInit:- This event is used to initialize the data in a component. ngOnInit() initializes the component or the directive once the data-bound properties are displayed by Angular and the input properties are set by the component. The primary purpose of ngOnInit() is to perform complex initializations. The hook method first uses the component to source the initial data or local variables into simple values. It is then used to set up the component once Angular sets the input properties. The ngOnInit() event is usually called after the ngOnChanges if the component has bound properties. However, if the component does not contain any bound properties, we can directly execute the ngOnInit() event. You must also remember that this hook is called only once.
• ngDoCheck:- This hook event occurs each time we check the properties of an input method. ngDoCheck event is an integral part of each change detection cycle and can be used along with your own logic check. This event facilitates the detection of custom changes or algorithms in the component. ngDoCheck book method allows detection of those changes that Angular cannot detect. You can invoke ngDoCheck after ngOnChanges for every change detection run. It can be triggered after ngOnInit on the first run, even if no changes have been made to the bound properties of the component.
   
• ngAfterContentInit:- This hook stage or lifecycle method is similar to ngOnInit as it occurs only once during the component’s lifetime. You can trigger this event a single time when you have to check the component’s bindings for the first time. We execute this event after Angular performs any content projection activity within the component. In simple words, ngAfterContentInit is executed when there is a projection of external content by Angular into the view of the component or the view within which a directive is present. ngAfterContentInit is invoked Angular immediately after executing the ngDoCheck method. This lifecycle method is related to initializations of the child component.
   
• ngAfterContentChecked:- This hook stage is subsequent action to the ngDoCheck method. As the name suggests, ngAfterContentChecked method is executed after Angular detects the changes in the external content that is projected within the component. This event takes place after every change detection cycle. It is called after ngAfterContentInit() and every subsequent ngDoCheck(). Like ngAfterContentInit, ngAfterContentChecked is also associated with child component initializations.
   
• ngAfterViewInit:- The ngAfterViewInit is triggered after the initialization of the component’s view is completed. It is only called once after the ngAfterContentChecked() lifecycle method. This hook method is used only for components and not child components.

• ngAfterViewChecked:- This lifecycle is invoked once Angular finishes the detection or checking view of the component or the child component. Even though the ngAfterViewChecked method is called after the ngAfterViewInit method, it is executed after every subsequent ngAfterContentChecked() lifecycle stage. This hook method is related only to the angular components.
   
• ngOnDestroy:- The ngOnDestroy() hook method is executed before Angular destroys the components that are no longer required. This lifecycle method detaches the events and helps avoid any memory leaks. It also unsubscribes the observables. We call this event only once before removing the component. This hook method serves as a cleanup before Angular destroys the component.

Interfaces in Angular Lifecycle

Angular lifecycles are usually defined through the component class. However, interfaces can also be used to define Angular component class as the various hook methods already have an associated Typescript interface. We usually understand an interface as a characteristic that helps identify various methods and related properties. Even though unnecessary, it is highly recommended to implement interfaces in the Angular lifecycle. The purpose of each interface is to define the prototype for hook methods. Interfaces are named with the prefix ‘ng’.

Conclusion

Angular components are based on typescript programming. Therefore, it is necessary to know the basics of typescript programming before implementing angular lifecycle hook methods. It is also essential to learn the fundamentals of Angular app design, the processes of lifecycle methods, and the know-how of which hook methods are to be used. 

If you are planning to pursue a career in Angular app development, you must have an in-depth knowledge of the entire process and the functions of different hook methods. Furthermore, as Angular is a javascript development framework, you must learn the Java programming language. upGrad’s Executive PG Program in Software Development is an excellent option to learn about Angular app development from scratch.