Method measureScrollOffset

src/cdk/scrolling/scrollable.ts:153–197

* Measures the scroll offset relative to the specified edge of the viewport. This method can be * used instead of directly checking scrollLeft or scrollTop, since browsers are not consistent * about what scrollLeft means in RTL. The values returned by this method are normalized such that *

(from: 'top' | 'left' | 'right' | 'bottom' | 'start' | 'end')

   * @param from The edge to measure from.
   */
  measureScrollOffset(from: 'top' | 'left' | 'right' | 'bottom' | 'start' | 'end'): number {
    const LEFT = 'left';
    const RIGHT = 'right';
    const el = this.elementRef.nativeElement;
    if (from == 'top') {
      return el.scrollTop;
    }
    if (from == 'bottom') {
      return el.scrollHeight - el.clientHeight - el.scrollTop;
    }

    // Rewrite start & end as left or right offsets.
    const isRtl = this.dir && this.dir.value == 'rtl';
    if (from == 'start') {
      from = isRtl ? RIGHT : LEFT;
    } else if (from == 'end') {
      from = isRtl ? LEFT : RIGHT;
    }

    if (isRtl && getRtlScrollAxisType() == RtlScrollAxisType.INVERTED) {
      // For INVERTED, scrollLeft is (scrollWidth - clientWidth) when scrolled all the way left and
      // 0 when scrolled all the way right.
      if (from == LEFT) {
        return el.scrollWidth - el.clientWidth - el.scrollLeft;
      } else {
        return el.scrollLeft;
      }
    } else if (isRtl && getRtlScrollAxisType() == RtlScrollAxisType.NEGATED) {
      // For NEGATED, scrollLeft is -(scrollWidth - clientWidth) when scrolled all the way left and
      // 0 when scrolled all the way right.
      if (from == LEFT) {
        return el.scrollLeft + el.scrollWidth - el.clientWidth;
      } else {
        return -el.scrollLeft;
      }
    } else {
      // For NORMAL, as well as non-RTL contexts, scrollLeft is 0 when scrolled all the way left and
      // (scrollWidth - clientWidth) when scrolled all the way right.
      if (from == LEFT) {
        return el.scrollLeft;
      } else {
        return el.scrollWidth - el.clientWidth - el.scrollLeft;
      }
    }
  }
}