Скачать или смотреть Understanding Why ngOnChanges Doesn't Fire for Arrays in Angular2

Discover why the `ngOnChanges` lifecycle hook doesn't trigger for arrays in Angular2, and learn the proper way to handle input changes to ensure your child components update correctly.
---
This video is based on the question https://stackoverflow.com/q/74516639/ asked by the user 'user3345721' ( https://stackoverflow.com/u/3345721/ ) and on the answer https://stackoverflow.com/a/74516732/ provided by the user 'Fabian Strathaus' ( https://stackoverflow.com/u/17298437/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Angular2 ngOnChanges not firing when input is array

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Why ngOnChanges Doesn't Fire for Arrays in Angular2

When working with Angular components, developers often encounter the ngOnChanges lifecycle hook, which is intended to inform a component about changes to its input properties. However, it can sometimes be puzzling—especially when these changes involve arrays. If you've found that ngOnChanges isn’t firing when passing an array as an input from a parent component to a child component, this guide is for you. Let's dig into the problem and the solution.

The Problem: ngOnChanges Not Triggering

Imagine you have two components: a parent component and a child component. The parent component passes a simple numeric value as well as an array to the child component. While changes to the numeric value trigger the ngOnChanges lifecycle method in the child component, updates to the array do not.

Example Code:

Here's a brief look at how the parent and child components are set up:

Parent Component:

[[See Video to Reveal this Text or Code Snippet]]

Child Component:

[[See Video to Reveal this Text or Code Snippet]]

Parent Component HTML:

[[See Video to Reveal this Text or Code Snippet]]

The Core Issue

The reason ngOnChanges does not fire when modifying an array is primarily due to how JavaScript (and by extension, TypeScript) handles objects and references.

Primitive Types vs. Objects: In JavaScript, primitive types (like numbers and strings) are stored by value, while objects (including arrays) are stored by reference. When you modify an array using methods such as push, you are mutating the existing object rather than creating a new one.

Since the reference to the arr input property remains unchanged, Angular does not detect the change, and thus ngOnChanges does not trigger.

The Solution: Creating a New Array Reference

To ensure that the ngOnChanges lifecycle method fires when you want to alert the child component of changes to its input, you can create a new reference for the array each time you modify it. This can be achieved by using the spread operator in JavaScript to clone the array with the new value added.

Updated Change Method

Modify the changeArray method as follows:

[[See Video to Reveal this Text or Code Snippet]]

Explanation

By using the spread operator [...], you create a new array that contains all the elements of the old array plus the new element some value. This modifies the reference of arr, which in turn allows Angular to recognize that the input has changed, triggering the ngOnChanges method in the child component.

Conclusion

Understanding the behavior of JavaScript objects, particularly how references work, is crucial in Angular development. By ensuring that you create new references when working with arrays (or any object), you can effectively manage changes and ensure that your components communicate properly.

Tips:

Always strive to immutably update arrays and objects in Angular when you need to trigger lifecycle hooks.

Familiarize yourself with other methods of managing state, like using libraries such as NgRx for more complex applications.

By following these practices, you can help ensure that your Angular components behave as expected, making your applications more robust and reliable.