firefish/packages/client/src/scripts/scroll.ts

type ScrollBehavior = "auto" | "smooth" | "instant";

export function getScrollContainer(el: HTMLElement | null): HTMLElement | null {
	if (el == null) return null;
	const overflow = window.getComputedStyle(el).getPropertyValue("overflow-y");
	if (overflow === "scroll" || overflow === "auto") {
		if (el.tagName === "HTML") {
			return null;
		}
		return el;
	} else {
		return getScrollContainer(el.parentElement);
	}
}

export function getScrollPosition(el: Element | null): number {
	const container = getScrollContainer(el);
	return container == null ? window.scrollY : container.scrollTop;
}

export function isTopVisible(el: Element | null): boolean {
	const scrollTop = getScrollPosition(el);
	const topPosition = el.offsetTop; // TODO: container内でのelの相対位置を取得できればより正確になる

	return scrollTop <= topPosition;
}

export function isBottomVisible(
	el: HTMLElement,
	tolerance = 1,
	container = getScrollContainer(el),
) {
	if (container)
		return (
			el.scrollHeight <=
			container.clientHeight + Math.abs(container.scrollTop) + tolerance
		);
	return el.scrollHeight <= window.innerHeight + window.scrollY + tolerance;
}

export function onScrollTop(el: Element, cb) {
	const container = getScrollContainer(el) || window;
	const onScroll = (ev) => {
		if (!document.body.contains(el)) return;
		if (isTopVisible(el)) {
			cb();
			container.removeEventListener("scroll", onScroll);
		}
	};
	container.addEventListener("scroll", onScroll, { passive: true });
}

export function onScrollBottom(el: Element, cb) {
	const container = getScrollContainer(el) || window;
	const onScroll = (ev) => {
		if (!document.body.contains(el)) return;
		const pos = getScrollPosition(el);
		if (pos + el.clientHeight > el.scrollHeight - 1) {
			cb();
			container.removeEventListener("scroll", onScroll);
		}
	};
	container.addEventListener("scroll", onScroll, { passive: true });
}

export function scroll(
	el: Element,
	options: {
		top?: number;
		left?: number;
		behavior?: ScrollBehavior;
	},
) {
	const container = getScrollContainer(el);
	if (container == null) {
		window.scroll(options);
	} else {
		container.scroll(options);
	}
}

export function scrollToTop(
	el: Element,
	options: { behavior?: ScrollBehavior } = {},
) {
	scroll(el, { top: 0, ...options });
}

export function scrollToBottom(
	el: Element,
	options: { behavior?: ScrollBehavior } = {},
) {
	scroll(el, { top: 99999, ...options }); // TODO: ちゃんと計算する
}

export function isBottom(el: Element, asobi = 0) {
	const container = getScrollContainer(el);
	const current = container
		? el.scrollTop + el.offsetHeight
		: window.scrollY + window.innerHeight;
	const max = container ? el.scrollHeight : document.body.offsetHeight;
	return current >= max - asobi;
}