block4.js

const timestamp = e.timeStamp;
let visibleLength = this._selectLength(e.nativeEvent.layoutMeasurement);
let contentLength = this._selectLength(e.nativeEvent.contentSize);
let offset = this._selectOffset(e.nativeEvent.contentOffset);
let dOffset = offset - this._scrollMetrics.offset;

// ...
 this._scrollMetrics = {
      contentLength,
      dt,
      dOffset,
      offset,
      timestamp,
      velocity,
      visibleLength,
    };