How to Use CSS Transitions and Animations with Angular 2\'s ngIf Directive?

Angular 2 ngIf and CSS Transition/Animation

Angular 2's ngIf directive is commonly used to conditionally render elements based on a given expression. However, when using CSS transitions or animations on these elements, unexpected behaviors may occur.

Consider the following code snippet:

// ...
<div class="note" [ngClass]="{'transition':show}" *ngIf="show">
  <p> Notes</p>
</div>
// ...

In this example, the goal is for a div element to slide in from the right using CSS when show is set to true. The *ngIf directive ensures that the element is only rendered when show is true, but when show is false, the element is completely removed from the DOM.

The issue arises because CSS animations or transitions require the target element to be present in the DOM for the duration of the animation. Since *ngIf removes the element when show is false, any transitions defined in the CSS will not take effect.

Solution:

To address this issue, it is recommended to use the [hidden] attribute instead of ngIf for hiding elements that should be animated.

// ...
<div class="note" [ngClass]="{'transition':show}" [hidden]="!show">
  <p> Notes</p>
</div>
// ...

By using [hidden], the element is still present in the DOM but its visibility is set to "hidden" when show is false. This ensures that the element is available for CSS animations or transitions to be applied.

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