Question
Answer and Explanation
The "ResizeObserver loop completed with undelivered notifications" error typically occurs in web browsers when the ResizeObserver API is used incorrectly or when the browser cannot efficiently deliver all resize notifications in a timely manner. Here’s a breakdown of the common reasons:
1. Infinite Loops due to ResizeObserver:
- If the callback function you provide to the ResizeObserver changes the size of an element being observed, it can trigger another resize observation, creating an infinite loop. For example, if you’re adjusting an element's width within the observer callback, it can lead to a continuous resize-and-adjust cycle.
2. Rapid or Frequent Resizes:
- If the browser window or element is resized very rapidly, the ResizeObserver may not be able to deliver all notifications immediately, leading to undelivered notifications. The browser might then throttle or cancel some of these notifications to prevent performance issues.
3. Layout Thrashing:
- When you modify the DOM (Document Object Model) in a way that forces the browser to recalculate layout multiple times within the same callback cycle, it can overwhelm the browser’s layout engine. This leads to a performance bottleneck and may cause some notifications to be dropped.
4. Complex Layout Calculations:
- If the elements being observed are involved in complex layout calculations, each resize notification can trigger expensive recalculations, potentially leading to delays. In such cases, browsers might not be able to keep up and therefore skip certain notifications.
5. Bug in Browser or Extension:
- Although less common, bugs in the browser itself or certain browser extensions could also cause this warning. It's important to test across different browsers and in a controlled environment to rule this out.
Example of an Incorrect Implementation (Leading to the Issue):
Assume an element with the id `resizableElement` is being observed:
const resizableElement = document.getElementById('resizableElement');
const resizeObserver = new ResizeObserver(entries => {
for (let entry of entries) {
// Incorrectly trying to adjust the element’s size within the callback,
// leading to a loop:
entry.target.style.width = entry.contentRect.width + 10 + 'px';
}
});
resizeObserver.observe(resizableElement);
In the code above, modifying the width in the callback will trigger more resize notifications in an endless loop because the size of the observed element has changed within the callback.
How to Avoid this Error:
- Avoid Modifying Observed Elements in Callback: Avoid directly changing the size or layout properties of the element being observed within the ResizeObserver callback. If you need to change something based on the resize, make the changes to another element.
- Debounce Resize Actions: If your callback needs to perform intensive tasks based on resizes, implement a debouncing mechanism. This allows the callback to execute only after a certain period of inactivity or a specific interval has passed.
- Optimize DOM Manipulations: Batch DOM changes and minimize the number of layout recalculations. Use CSS transitions and animations where possible instead of changing element properties dynamically in JS.
- Test Across Browsers: Test your code across multiple browsers and check for any unexpected behavior.
By addressing these points, you can typically resolve the "ResizeObserver loop completed with undelivered notifications" warning and ensure the efficient operation of your website or application.