Angular Animate.leave Bug: Styles Not Saved!

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Introduction

Hey Angular developers! Today, we're diving deep into a peculiar bug that some of you might have encountered while working with Angular animations, specifically when using animate.leave. This issue revolves around the fact that styles are not being preserved until the very end of the transition, leading to some unexpected and, frankly, quite annoying visual glitches. Let's break down the problem, explore a minimal reproduction, and discuss potential implications.

Which @angular/* package(s) are the source of the bug?

The core @angular/* package seems to be the culprit here. This means that the fundamental animation functionalities within Angular are where the issue lies.

Is this a regression?

Yes, this appears to be a regression. This means that it worked as expected in a previous version but is now misbehaving in the current version.

Description

When using animate.leave, styles are not preserved until the end of the animation. So, what does this mean in practice? Imagine you have a component that you want to animate out of the view using animate.leave. You set up your animation, expecting that the component's final styles will be maintained until the animation is complete. However, the element expands in size, which is likely not the expected behavior, which is the styles are not preserved as expected during the animation, causing visual inconsistencies. This can lead to a jarring user experience, especially when dealing with complex layouts and transitions.

In the reproduction example, clicking the "Toggle" button should trigger the component's closing animation (using grid-template-rows). The animation plays, but the element expands in size. This is likely not the expected behavior.

Example Scenario

Consider a scenario where you're using grid-template-rows to animate the closing of a component. You'd expect the component to smoothly shrink and disappear. However, because the styles aren't preserved, the component might suddenly expand before animating out, creating an unpleasant visual effect. This is just one example, and the issue can manifest in various ways depending on the specific styles and animations you're using.

Please provide a link to a minimal reproduction of the bug

You can find a minimal reproduction of this bug on StackBlitz:

https://stackblitz.com/edit/stackblitz-starters-cuyyzgnx

This StackBlitz project should give you a clear and concise example of the issue in action. Feel free to fork it, modify it, and experiment with different animation configurations to better understand the problem.

Please provide the exception or error you saw

There isn't a traditional error or exception thrown here. The problem is more of a visual anomaly rather than a code-breaking issue. This can make it harder to detect and debug, as you might not immediately realize that something is wrong until you see the unexpected behavior.

Please provide the environment you discovered this bug in (run ng version)

Angular CLI: 20.3.5
Node: 20.19.1
Package Manager: npm 10.8.2
OS: linux x64
    

Angular: 20.3.4
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, router

Package                      Version
------------------------------------
@angular-devkit/architect    0.2003.5
@angular-devkit/core         20.3.5
@angular-devkit/schematics   20.3.5
@angular/build               20.3.5
@angular/cli                 20.3.5
@schematics/angular          20.3.5
rxjs                         7.8.2
typescript                   5.8.2
zone.js                      0.15.0

Environment Details

  • Angular CLI: 20.3.5
  • Node: 20.19.1
  • npm: 10.8.2
  • OS: Linux x64
  • Angular Version: 20.3.4 (including animations, common, compiler, core, etc.)

This information is crucial because it helps narrow down the specific versions where the bug is present. If you're experiencing this issue, comparing your environment to the one listed above can help determine if you're running a version known to have this problem.

Anything else?

This issue is likely related to the fixed bug https://github.com/angular/angular/issues/64209

Potential Connection

It's worth noting that this bug might be related to a previously fixed issue (https://github.com/angular/angular/issues/64209). While that issue was supposedly resolved, it's possible that the fix didn't fully address all edge cases, or that a similar problem has resurfaced in a newer version. Keeping an eye on related issues and discussions can provide valuable insights and potential workarounds.

Understanding Angular Animations

Before we dive deeper, let's take a moment to understand how Angular animations work. Angular animations are built on top of the Web Animations API and provide a powerful way to create smooth and engaging user interfaces. The @angular/animations module allows you to define animation triggers, states, and transitions using metadata. These animations can be triggered based on changes in component state, such as when an element is added or removed from the DOM.

Key Concepts in Angular Animations

  • Triggers: Animation triggers are directives that listen for changes in an expression and initiate animations based on those changes.
  • States: Animation states define the different visual states that an element can be in. For example, a component might have states like 'open' and 'closed'.
  • Transitions: Animation transitions define how an element animates from one state to another. You can specify the duration, easing, and styles for each transition.
  • animate: The animate function is used to define the animation timeline, specifying the styles that should be applied over a given duration.
  • style: The style function is used to define the styles that should be applied at a specific point in the animation timeline.

Diving Deeper into the animate.leave Bug

The animate.leave function is specifically designed to handle animations when an element is being removed from the DOM. It allows you to define how the element should animate out of the view, providing a smooth transition rather than an abrupt disappearance. However, the bug we're discussing today disrupts this process, causing the styles to not be preserved until the end of the animation.

Why This Bug Matters

This bug can have a significant impact on the user experience, especially in applications that rely heavily on animations. Imagine a complex dashboard with multiple components that animate in and out of view. If the animate.leave function doesn't work as expected, the transitions can become jarring and unprofessional, detracting from the overall user experience. Moreover, this bug can make it more difficult to create consistent and predictable animations, as you might need to implement workarounds to ensure that the styles are preserved.

Potential Workarounds

While we wait for an official fix from the Angular team, there are a few potential workarounds that you can try:

  1. Using setTimeout: You can use setTimeout to delay the removal of the element from the DOM until after the animation has completed. This can give the illusion that the styles are being preserved, but it's not a perfect solution and can introduce timing issues.
  2. Applying Styles Directly: Instead of relying on animate.leave to handle the styles, you can apply the final styles directly to the element before triggering the animation. This can ensure that the styles are preserved, but it might require more manual intervention.
  3. Creating Custom Animations: You can create custom animations using the Web Animations API directly, bypassing the animate.leave function altogether. This gives you more control over the animation process, but it also requires more code and effort.

Conclusion

The animate.leave bug in Angular is a frustrating issue that can lead to unexpected visual glitches and a subpar user experience. While there's no official fix yet, understanding the problem and exploring potential workarounds can help you mitigate its impact. Keep an eye on the Angular issue tracker for updates and fixes, and don't hesitate to share your experiences and solutions with the community. Together, we can help make Angular animations more reliable and predictable. Happy coding, folks!