Why Do CSS Transitions Sometimes Skip or Fail to Trigger Callbacks?

CSS Transition Skipped or Callback Not Invoked: Understanding the Cause

In CSS, transitions provide an elegant way to smoothly animate property changes over time. However, certain situations can arise where transitions fail to execute or the associated callbacks remain uncalled.

One such scenario occurs when assigning new location values to a projectile element during game development. Despite having replaced jQuery calls with pure JS, the transition appeared to be skipped entirely, and no callback was triggered.

The Mystery of the Missing Transition

Curiously, wrapping the CSS position change within a 1ms setTimeout somehow remedied the issue, although occasionally the transition would still bypass. This inconsistency raised the question: why did the setTimeout help, and why did the callback sometimes fail?

The Role of Computed Styles

The key to understanding this behavior lies in CSS computed styles. When you set new styles via JavaScript, the browser may not immediately apply them. Instead, the computed style of the element remains unchanged, leading to a discrepancy between the inline styles and the computed values.

In the case of the projectile element, although the inline style specified left and top values, the computed style initially retained the default values of 0px.

Impact on Transitions

Transitions rely on computed styles to determine the starting and ending states of the animation. When the initial computed style differs from the inline style you're trying to transition from, the transition essentially has no effect and appears to skip.

The Solution: Forcing a Reflow

To resolve this issue and ensure that transitions always execute correctly, it's necessary to force the browser to perform a reflow, which recalculates the computed styles.

One way to achieve this is by accessing a property that requires up-to-date styles, such as offsetHeight. In the code provided, animdiv.offsetHeight triggers the reflow before the new left and top values are assigned.

Conclusion

By understanding the issue with computed styles, you can ensure that CSS transitions operate as expected and that callbacks are invoked reliably. Remember to force a reflow before transitioning elements if you observe irregular behavior.

The above is the detailed content of Why Do CSS Transitions Sometimes Skip or Fail to Trigger Callbacks?. For more information, please follow other related articles on the PHP Chinese website!