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index.ts
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index.ts
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let props: (keyof DOMRect)[] = [
"bottom",
"height",
"left",
"right",
"top",
"width",
];
let rectChanged = (a: DOMRect = {} as DOMRect, b: DOMRect = {} as DOMRect) =>
props.some((prop) => a[prop] !== b[prop]);
let observedNodes = new Map<Element, RectProps>();
let rafId: number;
let run = () => {
const changedStates: RectProps[] = [];
observedNodes.forEach((state, node) => {
let newRect = node.getBoundingClientRect();
if (rectChanged(newRect, state.rect)) {
state.rect = newRect;
changedStates.push(state);
}
});
changedStates.forEach((state) => {
state.callbacks.forEach((cb) => cb(state.rect));
});
rafId = window.requestAnimationFrame(run);
};
export default function observeRect(
node: Element,
cb: (rect: DOMRect) => void
) {
return {
observe() {
let wasEmpty = observedNodes.size === 0;
if (observedNodes.has(node)) {
observedNodes.get(node)!.callbacks.push(cb);
} else {
observedNodes.set(node, {
rect: undefined,
hasRectChanged: false,
callbacks: [cb],
});
}
if (wasEmpty) run();
},
unobserve() {
let state = observedNodes.get(node);
if (state) {
// Remove the callback
const index = state.callbacks.indexOf(cb);
if (index >= 0) state.callbacks.splice(index, 1);
// Remove the node reference
if (!state.callbacks.length) observedNodes.delete(node);
// Stop the loop
if (!observedNodes.size) cancelAnimationFrame(rafId);
}
},
};
}
export type PartialRect = Partial<DOMRect>;
export type RectProps = {
rect: DOMRect | undefined;
hasRectChanged: boolean;
callbacks: Function[];
};