44 lines
1.3 KiB
TypeScript
44 lines
1.3 KiB
TypeScript
export class StickySidebar {
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private lastScrollTop = 0;
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private el: HTMLElement;
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private spacer: HTMLElement;
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private marginTop: number;
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private isTop = false;
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private isBottom = false;
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constructor(el: StickySidebar['el'], spacer: StickySidebar['spacer'], marginTop = 0) {
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this.el = el;
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this.spacer = spacer;
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this.marginTop = marginTop;
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}
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public calc(scrollTop: number) {
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if (scrollTop > this.lastScrollTop) { // downscroll
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const overflow = this.el.clientHeight - window.innerHeight;
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this.el.style.bottom = null;
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this.el.style.top = `${-overflow}px`;
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this.isBottom = (scrollTop + window.innerHeight) >= (this.el.offsetTop + this.el.clientHeight);
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if (this.isTop) {
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this.isTop = false;
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this.spacer.style.marginTop = `${this.lastScrollTop}px`;
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}
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} else { // upscroll
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const overflow = this.el.clientHeight - window.innerHeight;
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this.el.style.top = null;
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this.el.style.bottom = `${-overflow - this.marginTop}px`;
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this.isTop = scrollTop <= this.el.offsetTop;
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if (this.isBottom) {
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this.isBottom = false;
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const overflow = this.el.clientHeight - window.innerHeight;
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this.spacer.style.marginTop = `${this.lastScrollTop - (overflow + this.marginTop)}px`;
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}
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}
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this.lastScrollTop = scrollTop <= 0 ? 0 : scrollTop;
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}
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}
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