Quick Fix: Employ the spread operator to create a new array instance after modifying the current one, triggering change detection. Alternatively, define a custom equality function for the signal() to compare arrays.
The Problem:
An Angular developer is facing challenges in correctly using the signal effect to listen to changes in an array of objects. The effect only gets triggered when filtering out an object from the array, but not when pushing or updating an object. The code provided shows the case statements where the effect is getting called and where it’s not. The developer wants to understand why the effect is not getting called as expected and how to fix it.
The Solutions:
Solution 1: Use spread Operator to Update Array
To correctly use Angular signal effect with arrays, the object reference of the signal value must be changed. Signals use Object.is under the hood to check for equality. Modifying an array in place (pushing, sorting, etc) is not considered a “change”.
A simple solution is to use the spread operator when updating the array. This creates a new array object, which triggers the signal effect.
With this change, the effect will be called when an item is inserted into the array.
Solution 2: Fix the syntax error
In the provided code, there is a syntax error in the “INSERT” case. To use the spread operator correctly, the “push” method should be used outside the curly braces, as shown below:
“`
case “INSERT”:
this.houses.update((houses: House[]) => ([…houses, payload.new]));
break;
“`
This corrected code will cause the effect to be triggered when a new object is inserted into the array. Additionally, the linked article on signals can provide further insights into using signals in Angular.
.
Q&A
Why is my array signal effect not triggering when I modify the array in place?
—
Signals use Object.is to check for equality. Modifying an array in place is not considered a change.
How can I trigger the effect when modifying the array?
—
Use the spread operator when updating the array, like this.houses.update(houses => ([...houses, payload.new])).
Video Explanation:
The following video, titled "Manage State with Angular Signals - YouTube", provides additional insights and in-depth exploration related to the topics discussed in this post.
... use signals to manage state, including CRUD operations. Starting in Angular v16, Angular developers can leverage the power of signals to ...
