Fix new message do not appear sometimes (#185)

Signed-off-by: Ajay Bura <ajbura@gmail.com>

src/app/organisms/room/TimelineScroll.js

@@ -0,0 +1,136 @@
+import { getScrollInfo } from '../../../util/common';
+
+class TimelineScroll {
+  constructor(target) {
+    if (target === null) {
+      throw new Error('Can not initialize TimelineScroll, target HTMLElement in null');
+    }
+    this.scroll = target;
+
+    this.backwards = false;
+    this.inTopHalf = false;
+
+    this.isScrollable = false;
+    this.top = 0;
+    this.bottom = 0;
+    this.height = 0;
+    this.viewHeight = 0;
+
+    this.topMsg = null;
+    this.bottomMsg = null;
+    this.diff = 0;
+  }
+
+  scrollToBottom() {
+    const scrollInfo = getScrollInfo(this.scroll);
+    const maxScrollTop = scrollInfo.height - scrollInfo.viewHeight;
+
+    this._scrollTo(scrollInfo, maxScrollTop);
+  }
+
+  // use previous calc by this._updateTopBottomMsg() & this._calcDiff.
+  tryRestoringScroll() {
+    const scrollInfo = getScrollInfo(this.scroll);
+
+    let scrollTop = 0;
+    const ot = this.inTopHalf ? this.topMsg?.offsetTop : this.bottomMsg?.offsetTop;
+    if (!ot) scrollTop = Math.round(this.height - this.viewHeight);
+    else scrollTop = ot - this.diff;
+
+    this._scrollTo(scrollInfo, scrollTop);
+  }
+
+  scrollToIndex(index, offset = 0) {
+    const scrollInfo = getScrollInfo(this.scroll);
+    const msgs = this.scroll.lastElementChild.lastElementChild.children;
+    const offsetTop = msgs[index]?.offsetTop;
+
+    if (offsetTop === undefined) return;
+    // if msg is already in visible are we don't need to scroll to that
+    if (offsetTop > scrollInfo.top && offsetTop < (scrollInfo.top + scrollInfo.viewHeight)) return;
+    const to = offsetTop - offset;
+
+    this._scrollTo(scrollInfo, to);
+  }
+
+  _scrollTo(scrollInfo, scrollTop) {
+    this.scroll.scrollTop = scrollTop;
+
+    // browser emit 'onscroll' event only if the 'element.scrollTop' value changes.
+    // so here we flag that the upcoming 'onscroll' event is
+    // emitted as side effect of assigning 'this.scroll.scrollTop' above
+    // only if it's changes.
+    // by doing so we prevent this._updateCalc() from calc again.
+    if (scrollTop !== this.top) {
+      this.scrolledByCode = true;
+    }
+    const sInfo = { ...scrollInfo };
+
+    const maxScrollTop = scrollInfo.height - scrollInfo.viewHeight;
+
+    sInfo.top = (scrollTop > maxScrollTop) ? maxScrollTop : scrollTop;
+    this._updateCalc(sInfo);
+  }
+
+  // we maintain reference of top and bottom messages
+  // to restore the scroll position when
+  // messages gets removed from either end and added to other.
+  _updateTopBottomMsg() {
+    const msgs = this.scroll.lastElementChild.lastElementChild.children;
+    const lMsgIndex = msgs.length - 1;
+
+    // TODO: classname 'ph-msg' prevent this class from being used
+    const PLACEHOLDER_COUNT = 2;
+    this.topMsg = msgs[0]?.className === 'ph-msg'
+      ? msgs[PLACEHOLDER_COUNT]
+      : msgs[0];
+    this.bottomMsg = msgs[lMsgIndex]?.className === 'ph-msg'
+      ? msgs[lMsgIndex - PLACEHOLDER_COUNT]
+      : msgs[lMsgIndex];
+  }
+
+  // we calculate the difference between first/last message and current scrollTop.
+  // if we are going above we calc diff between first and scrollTop
+  // else otherwise.
+  // NOTE: This will help to restore the scroll when msgs get's removed
+  // from one end and added to other end
+  _calcDiff(scrollInfo) {
+    if (!this.topMsg || !this.bottomMsg) return 0;
+    if (this.inTopHalf) {
+      return this.topMsg.offsetTop - scrollInfo.top;
+    }
+    return this.bottomMsg.offsetTop - scrollInfo.top;
+  }
+
+  _updateCalc(scrollInfo) {
+    const halfViewHeight = Math.round(scrollInfo.viewHeight / 2);
+    const scrollMiddle = scrollInfo.top + halfViewHeight;
+    const lastMiddle = this.top + halfViewHeight;
+
+    this.backwards = scrollMiddle < lastMiddle;
+    this.inTopHalf = scrollMiddle < scrollInfo.height / 2;
+
+    this.isScrollable = scrollInfo.isScrollable;
+    this.top = scrollInfo.top;
+    this.bottom = scrollInfo.height - (scrollInfo.top + scrollInfo.viewHeight);
+    this.height = scrollInfo.height;
+    this.viewHeight = scrollInfo.viewHeight;
+
+    this._updateTopBottomMsg();
+    this.diff = this._calcDiff(scrollInfo);
+  }
+
+  calcScroll() {
+    if (this.scrolledByCode) {
+      this.scrolledByCode = false;
+      return undefined;
+    }
+
+    const scrollInfo = getScrollInfo(this.scroll);
+    this._updateCalc(scrollInfo);
+
+    return this.backwards;
+  }
+}
+
+export default TimelineScroll;