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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/**
* Virtualized List can efficiently show billions of lines provided
* that all of them have the same height.
*
* Caller is responsible for setting createLineElement(index) function to
* create elements as they are scrolled into the view.
*/
class VirtualizedList extends HTMLElement {
lineHeight = 64;
#lineCount = 0;
get lineCount() {
return this.#lineCount;
}
set lineCount(value) {
this.#lineCount = value;
this.#rebuildVisibleLines();
}
#selectedIndex = 0;
get selectedIndex() {
return this.#selectedIndex;
}
set selectedIndex(value) {
this.#selectedIndex = value;
if (this.#container) {
this.updateLineSelection(true);
}
}
#container;
connectedCallback() {
this.#container = this.ownerDocument.createElement("ul");
this.#container.classList.add("lines-container");
this.appendChild(this.#container);
this.#rebuildVisibleLines();
this.addEventListener("scroll", () => this.#rebuildVisibleLines());
}
requestRefresh() {
this.#container.replaceChildren();
this.#rebuildVisibleLines();
}
updateLineSelection(scrollIntoView) {
const lineElements = this.#container.querySelectorAll(".line");
let selectedElement;
for (let lineElement of lineElements) {
let isSelected = Number(lineElement.dataset.index) === this.selectedIndex;
if (isSelected) {
selectedElement = lineElement;
}
lineElement.classList.toggle("selected", isSelected);
}
if (scrollIntoView) {
if (selectedElement) {
selectedElement.scrollIntoView({ block: "nearest" });
} else {
let selectedTop = this.selectedIndex * this.lineHeight;
if (this.scrollTop > selectedTop) {
this.scrollTop = selectedTop;
} else {
this.scrollTop = selectedTop - this.clientHeight + this.lineHeight;
}
}
}
}
#rebuildVisibleLines() {
if (!this.isConnected || !this.createLineElement) {
return;
}
this.#container.style.height = `${this.lineHeight * this.lineCount}px`;
let firstLineIndex = Math.floor(this.scrollTop / this.lineHeight);
let visibleLineCount = Math.ceil(this.clientHeight / this.lineHeight);
let lastLineIndex = firstLineIndex + visibleLineCount;
let extraLines = Math.ceil(visibleLineCount / 2); // They are present in DOM, but not visible
firstLineIndex = Math.max(0, firstLineIndex - extraLines);
lastLineIndex = Math.min(this.lineCount, lastLineIndex + extraLines);
let previousChild = null;
let visibleLines = new Map();
for (let child of Array.from(this.#container.children)) {
let index = Number(child.dataset.index);
if (index < firstLineIndex || index > lastLineIndex) {
child.remove();
} else {
visibleLines.set(index, child);
}
}
for (let index = firstLineIndex; index <= lastLineIndex; index++) {
let child = visibleLines.get(index);
if (!child) {
child = this.createLineElement(index);
if (!child) {
// Friday fix :-)
//todo: figure out what was on that Friday and how can we fix it
continue;
}
child.style.top = `${index * this.lineHeight}px`;
child.dataset.index = index;
if (previousChild) {
previousChild.after(child);
} else if (this.#container.firstElementChild?.offsetTop > top) {
this.#container.firstElementChild.before(child);
} else {
this.#container.appendChild(child);
}
}
previousChild = child;
}
this.updateLineSelection(false);
}
}
customElements.define("virtualized-list", VirtualizedList);
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