recycling.js

function refresh() {
  var SCROLL_DIRECTION_UP = -1;
  var SCROLL_DIRECTION_DOWN = 1;
  var SCROLL_DIRECTION_NONE = 0;
  var direction = SCROLL_DIRECTION_NONE;
  var ratio = 0.5;
  var delta = this.scrollTop - this.scrollPosition;
 
  this.scrollPosition = this.scrollTop;
  
  if (delta < 0) {
    direction = SCROLL_DIRECTION_UP;
  } else if (delta > 0) {
    direction = SCROLL_DIRECTION_DOWN;
  }
 
  var topSpace = this.scrollTop - this.physicalOffset;
  var bottomSpace = (this.physicalSize + this.physicalOffset) - this.scrollTop - this.viewportSize;
  var totalSpace = topSpace + bottomSpace;
  var currentRatio, tileHeight, nth;
  var movingUp = [], recycledTiles= 0;
 
  if (direction === SCROLL_DIRECTION_UP) {
    var isFirstOne = (this.virtualStart === 0);
    currentRatio = topSpace / totalSpace;
 
    if (!isFirstOne && currentRatio < ratio) {
      // move tiles from bottom to top
      // approximate `currentRatio` to `ratio`
      nth = this.physicalEnd;
 
      while (currentRatio < ratio && recycledTiles < this.physicalCount) {
        tileHeight = this.physicalSizes[nth];
        currentRatio += tileHeight / totalSpace;
 
        movingUp.push(nth);
 
        nth = (nth === 0) ? this.physicalCount - 1 : nth - 1;
        recycledTiles++;
      }
      recycledTiles = -recycledTiles;
    }
 
  } else if (direction === SCROLL_DIRECTION_DOWN) {
    var isLastOne = (this.virtualStart + this.physicalCount === this.virtualCount);
    currentRatio = bottomSpace / totalSpace;
 
    if (!isLastOne && currentRatio < ratio) {
      // move tiles from top to bottom
      // approximate `currentRatio` to `ratio`
      nth = this.physicalStart;
 
      while (currentRatio < ratio && recycledTiles < this.physicalCount) {
        tileHeight = this.physicalSizes[nth];
        currentRatio += tileHeight / totalSpace;
 
        this.physicalOffset += tileHeight;
        nth = (nth + 1) % this.physicalCount;
        recycledTiles++;
      }
    }
  }
  
  if (recycledTiles !== 0 || direction === SCROLL_DIRECTION_NONE) {
    this.virtualStart = this.virtualStart + recycledTiles;
    this.physicalStart = this.virtualStart % this.physicalCount;
    this.physicalEnd = (this.physicalStart + this.physicalCount - 1) % this.physicalCount;
 
    this._assignModels();
    
    // measure heights
    this._updateMetrics();
 
    // adjust offset after measuring
    while (movingUp.length) {
      this.physicalOffset -= this.physicalSizes[movingUp.pop()];
    }
 
    this._positionItems();
    this._updateScrollerSize();
  }
}