Using Angular 2 to assign a function to the ngClass directive within the template

Question

Using Angular 2 to assign a function to the ngClass directive within the template

I've been searching for a solution to this issue, but so far nothing has worked for me.

When I try to pass a function to the ngStyle directive, I encounter the following error:

The expression 'getClass()' in ProductView has changed after it was checked.

This is how my template looks like:

<div class="itemContainer">
    <div class="well imageHolder" [ngClass]="getClass()">
        <img [src]="'img/thumbs/' + item.images[0]" class="productImage" id="productImage">
    </div>
</div>

I'm not sure what could resolve this issue or if it's even possible to do so at the moment. I have also noticed that this error occurs with ngStyle as well.

Any assistance would be greatly appreciated.

angular typescript

Answer 1

Answer №1

If you're looking to assign a class to an element dynamically, a simple solution is to create a function that returns the desired class name.

For example, in your HTML code:

<div [ngClass]="getClassByValue('a')"> My Div</div>

In your TypeScript file, define the function like so:

  getClassByValue(value: string) {
    switch (value) {
      case "a": return "class-a";
      case "b": return "class-b";
    }
  }

This will result in the element having the class name "class-a".

I hope this explanation is helpful to you!

Answer 2

If you're looking to assign a class to an element dynamically, a simple solution is to create a function that returns the desired class name.

For example, in your HTML code:

<div [ngClass]="getClassByValue('a')"> My Div</div>

In your TypeScript file, define the function like so:

  getClassByValue(value: string) {
    switch (value) {
      case "a": return "class-a";
      case "b": return "class-b";
    }
  }

This will result in the element having the class name "class-a".

I hope this explanation is helpful to you!

Answer 3

Answer №2

When working with property bindings in Angular, the syntax is as follows:

[someProperty]="an Angular template expression"

.

In your particular scenario, the template expression happens to be a function instead of a component property. While this is acceptable, it's necessary to adhere to the "Expression Guidelines" outlined in the Template Syntax development guide. According to these guidelines, expressions must be "idempotent." This essentially means that if the

expression returns a string or a number, calling it twice consecutively should produce the same string or number each time. If the expression returns an object (such as a Date or Array), it should return the same object reference when called twice in succession.

In the absence of your actual getClass() function code, we can assume that it might be breaching the idempotent rule by possibly returning a new array or object every time it is called.

In default development mode, Angular performs change detection twice, thereby identifying any idempotence violations.

To rectify this issue, ensure that your function returns the same array or object reference while allowing for modifications within the array contents or object properties/values. For instance,

export class MyComponent {
   anArray = [];
   getClass() {
      // manipulate anArray without reassigning, and then return it
      return this.anArray;
   }
}

Answer 4

When working with property bindings in Angular, the syntax is as follows:

[someProperty]="an Angular template expression"

.

In your particular scenario, the template expression happens to be a function instead of a component property. While this is acceptable, it's necessary to adhere to the "Expression Guidelines" outlined in the Template Syntax development guide. According to these guidelines, expressions must be "idempotent." This essentially means that if the

expression returns a string or a number, calling it twice consecutively should produce the same string or number each time. If the expression returns an object (such as a Date or Array), it should return the same object reference when called twice in succession.

In the absence of your actual getClass() function code, we can assume that it might be breaching the idempotent rule by possibly returning a new array or object every time it is called.

In default development mode, Angular performs change detection twice, thereby identifying any idempotence violations.

To rectify this issue, ensure that your function returns the same array or object reference while allowing for modifications within the array contents or object properties/values. For instance,

export class MyComponent {
   anArray = [];
   getClass() {
      // manipulate anArray without reassigning, and then return it
      return this.anArray;
   }
}

Answer 5

Answer №3

In order to avoid issues, it is recommended not to perform this action (even though it may have worked in angular 1.x :) ) The validation check will only be present if you enable production mode:

https://angular.io/docs/js/latest/api/core/enableProdMode-function.html

My assumption (disclaimer: it's just an assumption ...) is that you should assign values to literals/variables that accurately represent the current state. Variables do not lead to side effects, unlike methods. This requirement seems to be enforced to prevent frequent digest loops.

Here is the recommended approach:

<div class="itemContainer">
    <div class="well imageHolder" [ngClass]="{ 'classA': variableA, 'classB': variableB }">
        <img [src]="'img/thumbs/' + item.images[0]" class="productImage" id="productImage">
    </div>
</div>

Instead of executing code during each digest loop to determine the class assignment (potentially triggering additional digest loops by modifying other elements), set the variables when necessary.

Answer 6

In order to avoid issues, it is recommended not to perform this action (even though it may have worked in angular 1.x :) ) The validation check will only be present if you enable production mode:

https://angular.io/docs/js/latest/api/core/enableProdMode-function.html

My assumption (disclaimer: it's just an assumption ...) is that you should assign values to literals/variables that accurately represent the current state. Variables do not lead to side effects, unlike methods. This requirement seems to be enforced to prevent frequent digest loops.

Here is the recommended approach:

<div class="itemContainer">
    <div class="well imageHolder" [ngClass]="{ 'classA': variableA, 'classB': variableB }">
        <img [src]="'img/thumbs/' + item.images[0]" class="productImage" id="productImage">
    </div>
</div>

Instead of executing code during each digest loop to determine the class assignment (potentially triggering additional digest loops by modifying other elements), set the variables when necessary.

Using Angular 2 to assign a function to the ngClass directive within the template

Answer №1

Answer №2

Answer №3

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