ResizeObserverEntry: contentBoxSize property
The contentBoxSize
read-only property of
the ResizeObserverEntry
interface returns an array containing the new
content box size of the observed element when the callback is run.
Value
An array containing objects with the new content box size of the observed element. The array is necessary to support elements that have multiple fragments, which occur in multi-column scenarios. Each object in the array contains two properties:
blockSize
-
The length of the observed element's content box in the block dimension. For boxes with a horizontal
writing-mode
, this is the vertical dimension, or height; if the writing-mode is vertical, this is the horizontal dimension, or width. inlineSize
-
The length of the observed element's content box in the inline dimension. For boxes with a horizontal
writing-mode
, this is the horizontal dimension, or width; if the writing-mode is vertical, this is the vertical dimension, or height.
Note: For more explanation of writing modes and block and inline dimensions, read Handling different text directions.
Examples
The following snippet is taken from the resize-observer-border-radius.html
(see source) example. This example includes a green box, sized as a percentage of the
viewport size. When the viewport size is changed, the box's rounded corners change in
proportion to the size of the box. We could just implement this using
border-radius
with a percentage, but that quickly leads to ugly-looking
elliptical corners; this solution gives you nice square corners that scale with the box
size.
js
const resizeObserver = new ResizeObserver((entries) => {
const calcBorderRadius = (size1, size2) =>
`${Math.min(100, size1 / 10 + size2 / 10)}px`;
for (const entry of entries) {
if (entry.borderBoxSize?.length > 0) {
entry.target.style.borderRadius = calcBorderRadius(
entry.borderBoxSize[0].inlineSize,
entry.borderBoxSize[0].blockSize
);
} else {
entry.target.style.borderRadius = calcBorderRadius(
entry.contentRect.width,
entry.contentRect.height
);
}
}
});
resizeObserver.observe(document.querySelector("div"));
Specifications
Specification |
---|
Resize Observer # dom-resizeobserverentry-contentboxsize |
Browser compatibility
BCD tables only load in the browser