Detecting if Component is "Updated" in Angular

Overview

Rendering components in an Angular application is an essential task, and it’s important to be able to know when a component has finished rendering. Although the Angular framework provides a number of methods to detect when a component is rendered, a few extra steps are required to ensure that a component is fully rendered before it can be used.

This document will discuss the methods available to detect when a component is fully rendered in an Angular application. It will cover the different ways of detecting when a component is rendered, and how to use these methods to ensure that a component is fully rendered before it is used. Finally, this document will discuss the best practices for using these detection methods in an Angular application and the final call which best suits the use-case.

First Things First

The closest thing that angular provides for detecting whether a component is rendered are the following lifecycle hooks:

• AfterViewInit 🔗 The first step in detecting when a component is fully rendered is to use the ngAfterViewInit() hook. This hook is called after a component's view, and its child views, have been created. It can be used to detect when a component's view has been initialized and is ready to be used.

• AfterViewChecked 🔗 In addition, the ngAfterViewChecked() hook can be used to detect when a component has been changed and re-rendered. This hook is called after every change detection cycle and can be used to detect when a component has finished rendering and is ready to be used.

• AfterContentInit 🔗

  Finally, the ngAfterContentInit() hook can be used to detect when a component's content has been initialized and is ready to use. This hook is called after a component's content, including any projected content, has been initialized and is ready to be used.

Caveats

By using the ngAfterViewInit(), ngAfterViewChecked(), and ngAfterContentInit() hooks, we can only detect when a component is initialized. These hooks are triggered after a component's view, content, and child views are created or have changed, respectively.

Its like knowing the start of a process, of which, you want to know the end.

Approach That Might Actually Work

1. Using testability api whenStable().
2. Using zone.js to look for unresolved macrotasks.
3. Router: NavigationEnd.
4. Mutation Observer.

Mutation Observer: Expensive but Effective

At last I decided to go with Mutation Observer.

Mutation Observer is an API provided by browsers that can be used to watch for changes in the structure of the DOM. It can be effectively used to detect when a component has finished rendering, by watching for changes in the component's DOM and firing a callback when the DOM stops changing.

However, it's important to note that Mutation Observer can be quite expensive in terms of performance. It's recommended to use it sparingly and disconnect the observer as soon as it's not needed to avoid any potential performance issues.

Here's the demo code for how it can be used to detect DOM mutations:

const config = { attributes: true, childList: true, subtree: true }

// Callback function to execute when mutations are observed
const callback = function (mutationsList, observer) {
  for (let mutation of mutationsList) {
    if (mutation.type === 'childList') {
      console.log('A child node has been added or removed.')
    } else if (mutation.type === 'attributes') {
      console.log('The ' + mutation.attributeName + ' attribute was modified.')
    }
  }
}

// Create an observer instance linked to the callback function
const observer = new MutationObserver(callback)

// Start observing the target node for configured mutations
observer.observe(targetNode, config)

// Later, you can stop observing
observer.disconnect()

Conclusion

This article explored different methods to detect when a component is fully rendered in an Angular application. We discussed using Angular's lifecycle hooks, the testability API whenStable(), zone.js, router navigation, and the Mutation Observer API. While these methods have their caveats, a careful combination and understanding of them can help you with your specific use case. For me, it was the Mutation Observer API.