virtual-scroll.ts

import { AfterContentInit, ChangeDetectorRef, ContentChild, Directive, DoCheck, ElementRef, Input, IterableChanges, IterableDiffer, IterableDiffers, NgZone, OnDestroy, Renderer, TrackByFn } from '@angular/core';

import { adjustRendered, calcDimensions, estimateHeight, initReadNodes, processRecords, populateNodeData, updateDimensions, updateNodeContext, writeToNodes } from './virtual-util';
import { Config } from '../../config/config';
import { Content, ScrollEvent } from '../content/content';
import { DomController } from '../../platform/dom-controller';
import { isBlank, isFunction, isPresent, assert } from '../../util/util';
import { Platform } from '../../platform/platform';
import { ViewController } from '../../navigation/view-controller';
import { VirtualCell, VirtualData, VirtualNode } from './virtual-util';
import { VirtualItem } from './virtual-item';
import { VirtualFooter } from './virtual-footer';
import { VirtualHeader } from './virtual-header';


/**
 * @name VirtualScroll
 * @description
 * Virtual Scroll displays a virtual, "infinite" list. An array of records
 * is passed to the virtual scroll containing the data to create templates
 * for. The template created for each record, referred to as a cell, can
 * consist of items, headers, and footers.
 *
 * For performance reasons, not every record in the list is rendered at once;
 * instead a small subset of records (enough to fill the viewport) are rendered
 * and reused as the user scrolls.
 *
 * ### The Basics
 *
 * The array of records should be passed to the `virtualScroll` property.
 * The data given to the `virtualScroll` property must be an array. An item
 * template with the `*virtualItem` property is required in the `virtualScroll`.
 * The `virtualScroll` and `*virtualItem` properties can be added to any element.
 *
 * ```html
 * <ion-list [virtualScroll]="items">
 *
 *   <ion-item *virtualItem="let item">
 *     {% raw %}{{ item }}{% endraw %}
 *   </ion-item>
 *
 * </ion-list>
 * ```
 *
 *
 * ### Section Headers and Footers
 *
 * Section headers and footers are optional. They can be dynamically created
 * from developer-defined functions. For example, a large list of contacts
 * usually has a divider for each letter in the alphabet. Developers provide
 * their own custom function to be called on each record. The logic in the
 * custom function should determine whether to create the section template
 * and what data to provide to the template. The custom function should
 * return `null` if a template shouldn't be created.
 *
 * ```html
 * <ion-list [virtualScroll]="items" [headerFn]="myHeaderFn">
 *
 *   <ion-item-divider *virtualHeader="let header">
 *     Header: {% raw %}{{ header }}{% endraw %}
 *   </ion-item-divider>
 *
 *   <ion-item *virtualItem="let item">
 *     Item: {% raw %}{{ item }}{% endraw %}
 *   </ion-item>
 *
 * </ion-list>
 * ```
 *
 * Below is an example of a custom function called on every record. It
 * gets passed the individual record, the record's index number,
 * and the entire array of records. In this example, after every 20
 * records a header will be inserted. So between the 19th and 20th records,
 * between the 39th and 40th, and so on, a `<ion-item-divider>` will
 * be created and the template's data will come from the function's
 * returned data.
 *
 * ```ts
 * myHeaderFn(record, recordIndex, records) {
 *   if (recordIndex % 20 === 0) {
 *     return 'Header ' + recordIndex;
 *   }
 *   return null;
 * }
 * ```
 *
 *
 * ### Approximate Widths and Heights
 *
 * If the height of items in the virtual scroll are not close to the
 * default size of 40px, it is extremely important to provide an value for
 * approxItemHeight height. An exact pixel-perfect size is not necessary,
 * but without an estimate the virtual scroll will not render correctly.
 *
 * The approximate width and height of each template is used to help
 * determine how many cells should be created, and to help calculate
 * the height of the scrollable area. Note that the actual rendered size
 * of each cell comes from the app's CSS, whereas this approximation
 * is only used to help calculate initial dimensions.
 *
 * It's also important to know that Ionic's default item sizes have
 * slightly different heights between platforms, which is perfectly fine.
 *
 *
 * ### Images Within Virtual Scroll
 *
 * HTTP requests, image decoding, and image rendering can cause jank while
 * scrolling. In order to better control images, Ionic provides `<ion-img>`
 * to manage HTTP requests and image rendering. While scrolling through items
 * quickly, `<ion-img>` knows when and when not to make requests, when and
 * when not to render images, and only loads the images that are viewable
 * after scrolling. [Read more about `ion-img`.](../../img/Img/)
 *
 * It's also important for app developers to ensure image sizes are locked in,
 * and after images have fully loaded they do not change size and affect any
 * other element sizes. Simply put, to ensure rendering bugs are not introduced,
 * it's vital that elements within a virtual item does not dynamically change.
 *
 * For virtual scrolling, the natural effects of the `<img>` are not desirable
 * features. We recommend using the `<ion-img>` component over the native
 * `<img>` element because when an `<img>` element is added to the DOM, it
 * immediately makes a HTTP request for the image file. Additionally, `<img>`
 * renders whenever it wants which could be while the user is scrolling. However,
 * `<ion-img>` is governed by the containing `ion-content` and does not render
 * images while scrolling quickly.
 *
 * ```html
 * <ion-list [virtualScroll]="items">
 *
 *   <ion-item *virtualItem="let item">
 *     <ion-avatar item-left>
 *       <ion-img [src]="item.avatarUrl"></ion-img>
 *     </ion-avatar>
 *    {% raw %} {{ item.firstName }} {{ item.lastName }}{% endraw %}
 *   </ion-item>
 *
 * </ion-list>
 * ```
 *
 *
 * ### Custom Components
 *
 * If a custom component is going to be used within Virtual Scroll, it's best
 * to wrap it with a good old `<div>` to ensure the component is rendered
 * correctly. Since each custom component's implementation and internals can be
 * quite different, wrapping within a `<div>` is a safe way to make sure
 * dimensions are measured correctly.
 *
 * ```html
 * <ion-list [virtualScroll]="items">
 *
 *   <div *virtualItem="let item">
 *     <my-custom-item [item]="item">
 *       {% raw %} {{ item }}{% endraw %}
 *     </my-custom-item>
 *   </div>
 *
 * </ion-list>
 * ```
 *
 *
 * ## Virtual Scroll Performance Tips
 *
 * #### iOS Cordova WKWebView
 *
 * When deploying to iOS with Cordova, it's highly recommended to use the
 * [WKWebView plugin](http://blog.ionic.io/cordova-ios-performance-improvements-drop-in-speed-with-wkwebview/)
 * in order to take advantage of iOS's higher performimg webview. Additionally,
 * WKWebView is superior at scrolling efficiently in comparision to the older
 * UIWebView.
 *
 * #### Lock in element dimensions and locations
 *
 * In order for virtual scroll to efficiently size and locate every item, it's
 * very important every element within each virtual item does not dynamically
 * change its dimensions or location. The best way to ensure size and location
 * does not change, it's recommended each virtual item has locked in its size
 * via CSS.
 *
 * #### Use `ion-img` for images
 *
 * When including images within Virtual Scroll, be sure to use
 * [`ion-img`](../img/Img/) rather than the standard `<img>` HTML element.
 * With `ion-img`, images are lazy loaded so only the viewable ones are
 * rendered, and HTTP requests are efficiently controlled while scrolling.
 *
 * #### Set Approximate Widths and Heights
 *
 * As mentioned above, all elements should lock in their dimensions. However,
 * virtual scroll isn't aware of the dimensions until after they have been
 * rendered. For the initial render, virtual scroll still needs to set
 * how many items should be built. With "approx" property inputs, such as
 * `approxItemHeight`, we're able to give virtual sroll an approximate size,
 * therefore allowing virtual scroll to decide how many items should be
 * created.
 *
 * #### Changing dataset should use `virtualTrackBy`
 *
 * It is possible for the identities of elements in the iterator to change
 * while the data does not. This can happen, for example, if the iterator
 * produced from an RPC to the server, and that RPC is re-run. Even if the
 * "data" hasn't changed, the second response will produce objects with
 * different identities, and Ionic will tear down the entire DOM and rebuild
 * it. This is an expensive operation and should be avoided if possible.
 *
 * #### Efficient headers and footer functions
 *
 * Each virtual item must stay extremely efficient, but one way to really
 * kill its performance is to perform any DOM operations within section header
 * and footer functions. These functions are called for every record in the
 * dataset, so please make sure they're performant.
 *
 */
@Directive({
  selector: '[virtualScroll]'
})
export class VirtualScroll implements DoCheck, AfterContentInit, OnDestroy {

  _differ: IterableDiffer<any>;
  _scrollSub: any;
  _scrollEndSub: any;
  _resizeSub: any;
  _init: boolean = false;
  _lastEle: boolean = false;
  _hdrFn: Function;
  _ftrFn: Function;
  _records: any[] = [];
  _cells: VirtualCell[] = [];
  _nodes: VirtualNode[] = [];
  _vHeight: number = 0;
  _lastCheck: number = 0;
  _data: VirtualData = {
    scrollTop: 0,
  };
  _queue: number = SCROLL_QUEUE_NO_CHANGES;
  _recordSize: number = 0;

  @ContentChild(VirtualItem) _itmTmp: VirtualItem;
  @ContentChild(VirtualHeader) _hdrTmp: VirtualHeader;
  @ContentChild(VirtualFooter) _ftrTmp: VirtualFooter;


  /**
   * @input {array} The data that builds the templates within the virtual scroll.
   * This is the same data that you'd pass to `*ngFor`. It's important to note
   * that when this data has changed, then the entire virtual scroll is reset,
   * which is an expensive operation and should be avoided if possible.
   */
  @Input()
  set virtualScroll(val: any) {
    this._records = val;
    if (isBlank(this._differ) && isPresent(val)) {
      this._differ = this._iterableDiffers.find(val).create(this.virtualTrackBy);
    }
  }

  /**
   * @input {number} The buffer ratio is used to decide how many cells
   * should get created when initially rendered. The number is a
   * multiplier against the viewable area's height. For example, if it
   * takes `20` cells to fill up the height of the viewable area, then
   * with a buffer ratio of `3` it will create `60` cells that are
   * available for reuse while scrolling. For better performance, it's
   * better to have more cells than what are required to fill the
   * viewable area. Default is `3`.
   */
  @Input() bufferRatio: number = 3;

  /**
   * @input {string} The approximate width of each item template's cell.
   * This dimension is used to help determine how many cells should
   * be created when initialized, and to help calculate the height of
   * the scrollable area. This value can use either `px` or `%` units.
   * Note that the actual rendered size of each cell comes from the
   * app's CSS, whereas this approximation is used to help calculate
   * initial dimensions before the item has been rendered. Default is
   * `100%`.
   */
  @Input() approxItemWidth: string = '100%';

  /**
   * @input {string} It is important to provide this
   * if virtual item height will be significantly larger than the default
   * The approximate height of each virtual item template's cell.
   * This dimension is used to help determine how many cells should
   * be created when initialized, and to help calculate the height of
   * the scrollable area. This height value can only use `px` units.
   * Note that the actual rendered size of each cell comes from the
   * app's CSS, whereas this approximation is used to help calculate
   * initial dimensions before the item has been rendered. Default is
   * `40px`.
   */
  @Input() approxItemHeight: string;

  /**
   * @input {string} The approximate width of each header template's cell.
   * This dimension is used to help determine how many cells should
   * be created when initialized, and to help calculate the height of
   * the scrollable area. This value can use either `px` or `%` units.
   * Note that the actual rendered size of each cell comes from the
   * app's CSS, whereas this approximation is used to help calculate
   * initial dimensions. Default is `100%`.
   */
  @Input() approxHeaderWidth: string = '100%';

  /**
   * @input {string} The approximate height of each header template's cell.
   * This dimension is used to help determine how many cells should
   * be created when initialized, and to help calculate the height of
   * the scrollable area. This height value can only use `px` units.
   * Note that the actual rendered size of each cell comes from the
   * app's CSS, whereas this approximation is used to help calculate
   * initial dimensions before the item has been rendered. Default is `40px`.
   */
  @Input() approxHeaderHeight: string = '40px';

  /**
   * @input {string} The approximate width of each footer template's cell.
   * This dimension is used to help determine how many cells should
   * be created when initialized, and to help calculate the height of
   * the scrollable area. This value can use either `px` or `%` units.
   * Note that the actual rendered size of each cell comes from the
   * app's CSS, whereas this approximation is used to help calculate
   * initial dimensions before the item has been rendered. Default is `100%`.
   */
  @Input() approxFooterWidth: string = '100%';

  /**
   * @input {string} The approximate height of each footer template's cell.
   * This dimension is used to help determine how many cells should
   * be created when initialized, and to help calculate the height of
   * the scrollable area. This height value can only use `px` units.
   * Note that the actual rendered size of each cell comes from the
   * app's CSS, whereas this approximation is used to help calculate
   * initial dimensions before the item has been rendered. Default is `40px`.
   */
  @Input() approxFooterHeight: string = '40px';

  /**
   * @input {function} Section headers and the data used within its given
   * template can be dynamically created by passing a function to `headerFn`.
   * For example, a large list of contacts usually has dividers between each
   * letter in the alphabet. App's can provide their own custom `headerFn`
   * which is called with each record within the dataset. The logic within
   * the header function can decide if the header template should be used,
   * and what data to give to the header template. The function must return
   * `null` if a header cell shouldn't be created.
   */
  @Input() set headerFn(val: Function) {
    if (isFunction(val)) {
      this._hdrFn = val.bind((this._ctrl._cmp) || this);
    }
  }

  /**
   * @input {function} Section footers and the data used within its given
   * template can be dynamically created by passing a function to `footerFn`.
   * The logic within the footer function can decide if the footer template
   * should be used, and what data to give to the footer template. The function
   * must return `null` if a footer cell shouldn't be created.
   */
  @Input() set footerFn(val: Function) {
    if (isFunction(val)) {
      this._ftrFn = val.bind((this._ctrl._cmp) || this);
    }
  }

  /**
   * @input {function} Same as `ngForTrackBy` which can be used on `ngFor`.
   */
  @Input() virtualTrackBy: TrackByFn;


  constructor(
    private _iterableDiffers: IterableDiffers,
    private _elementRef: ElementRef,
    private _renderer: Renderer,
    private _zone: NgZone,
    private _cd: ChangeDetectorRef,
    private _content: Content,
    private _plt: Platform,
    private _ctrl: ViewController,
    private _config: Config,
    private _dom: DomController
  ) {
    // hide the virtual scroll element with opacity so we don't
    // see jank as it loads up, but we're still able to read
    // dimensions because it's still rendered and only opacity hidden
    this.setElementClass('virtual-loading', true);

    // wait for the content to be rendered and has readable dimensions
    const readSub = _ctrl.readReady.subscribe(() => {
      readSub.unsubscribe();
      this.readUpdate(true);
    });

    // wait for the content to be writable
    const writeSub = _ctrl.writeReady.subscribe(() => {
      writeSub.unsubscribe();
      this._init = true;
      this.writeUpdate(true);
      this._listeners();
    });
  }

  /**
   * @hidden
   */
  ngDoCheck() {
    // only continue if we've already initialized
    if (!this._init) {
      return;
    }

    // and if there actually are changes
    const changes = this._changes();
    if (!isPresent(changes)) {
      return;
    }

    let needClean = false;
    if (changes) {
      changes.forEachOperation((item, _, cindex) => {
        if (item.previousIndex != null || (cindex < this._recordSize)) {
          needClean = true;
        }
      });
    } else {
      needClean = true;
    }
    this._recordSize = this._records.length;

    this.readUpdate(needClean);
    this.writeUpdate(needClean);
  }

  readUpdate(needClean: boolean) {
    if (needClean) {
      // reset everything
      console.debug(`virtual-scroll, readUpdate: slow path`);
      this._cells.length = 0;
      this._nodes.length = 0;
      this._itmTmp.viewContainer.clear();

      // ******** DOM READ ****************
      this.calcDimensions();
    } else {
      console.debug(`virtual-scroll, readUpdate: fast path`);
    }
  }

  writeUpdate(needClean: boolean) {
    console.debug(`virtual-scroll, writeUpdate`);
    const data = this._data;
    const stopAtHeight = (data.scrollTop + data.renderHeight);
    data.scrollDiff = SCROLL_DIFFERENCE_MINIMUM + 1;

    processRecords(stopAtHeight,
      this._records,
      this._cells,
      this._hdrFn,
      this._ftrFn,
      this._data);

    // ******** DOM WRITE ****************
    this.renderVirtual(needClean);
  }

  private calcDimensions() {
    calcDimensions(this._data, this._elementRef.nativeElement,
      this.approxItemWidth, this.approxItemHeight,
      this.approxHeaderWidth, this.approxHeaderHeight,
      this.approxFooterWidth, this.approxFooterHeight,
      this.bufferRatio);
  }

  private _changes(): IterableChanges<any> {
    if (isPresent(this._records) && isPresent(this._differ)) {
      return this._differ.diff(this._records);
    }
    return null;
  }

  /**
   * @hidden
   * DOM WRITE
   */
  renderVirtual(needClean: boolean) {
    this._plt.raf(() => {
      const nodes = this._nodes;
      const cells = this._cells;
      const data = this._data;
      const records = this._records;

      if (needClean) {
        // ******** DOM WRITE ****************
        updateDimensions(this._plt, nodes, cells, data, true);
        data.topCell = 0;
        data.bottomCell = (cells.length - 1);
      }

      adjustRendered(cells, data);

      this._zone.run(() => {
        populateNodeData(
          data.topCell, data.bottomCell,
          data.viewWidth, true,
          cells, records, nodes,
          this._itmTmp.viewContainer,
          this._itmTmp.templateRef,
          this._hdrTmp && this._hdrTmp.templateRef,
          this._ftrTmp && this._ftrTmp.templateRef, needClean,
        );
      });

      if (needClean) {
        this._cd.detectChanges();
      }

      // at this point, this fn was called from within another
      // requestAnimationFrame, so the next dom reads/writes within the next frame
      // wait a frame before trying to read and calculate the dimensions

      // ******** DOM READ ****************
      this._dom.read(() => initReadNodes(this._plt, nodes, cells, data));

      this._dom.write(() => {
        // update the bound context for each node
        updateNodeContext(nodes, cells, data);

        // ******** DOM WRITE ****************
        this._stepChangeDetection();
        // ******** DOM WRITE ****************
        this._stepDOMWrite();
        // ******** DOM WRITE ****************
        this._content.imgsUpdate();

        // First time load
        if (!this._lastEle) {
          // add an element at the end so :last-child css doesn't get messed up
          // ******** DOM WRITE ****************
          var ele = this._elementRef.nativeElement;
          var lastEle: HTMLElement = this._renderer.createElement(ele, 'div');
          lastEle.className = 'virtual-last';
          this._lastEle = true;

          // ******** DOM WRITE ****************
          this.setElementClass('virtual-scroll', true);

          // ******** DOM WRITE ****************
          this.setElementClass('virtual-loading', false);
        }
        assert(this._queue === SCROLL_QUEUE_NO_CHANGES, 'queue value should be NO_CHANGES');
      });
    });
  }

  /**
   * @private
   */
  resize() {
    // only continue if we've already initialized
    if (!this._init) {
      return;
    }

    console.debug('virtual-list: resized window');
    this.calcDimensions();
    this.writeUpdate(false);
  }

  /**
   * @hidden
   */
  private _stepDOMWrite() {
    const cells = this._cells;
    const nodes = this._nodes;
    const recordsLength = this._records.length;

    // ******** DOM WRITE ****************
    writeToNodes(this._plt, nodes, cells, recordsLength);

    // ******** DOM WRITE ****************
    this._setHeight(
      estimateHeight(recordsLength, cells[cells.length - 1], this._vHeight, 0.25)
    );

    // we're done here, good work
    this._queue = SCROLL_QUEUE_NO_CHANGES;
  }

  /**
   * @private
   */
  private _stepChangeDetection() {
    // we need to do some change detection in this frame
    // we've got work painting do, let's throw it in the
    // domWrite callback so everyone plays nice
    // ******** DOM WRITE ****************
    const nodes = this._nodes;
    for (var i = 0; i < nodes.length; i++) {
      if (nodes[i].hasChanges) {
        (<any>nodes[i].view).detectChanges();
      }
    }

    // on the next frame we need write to the dom nodes manually
    this._queue = SCROLL_QUEUE_DOM_WRITE;
  }

  /**
   * @private
   */
  private _stepNoChanges() {
    const data = this._data;

    // let's see if we've scroll far enough to require another check
    const diff = data.scrollDiff = (data.scrollTop - this._lastCheck);
    if (Math.abs(diff) < SCROLL_DIFFERENCE_MINIMUM) {
      return;
    }

    const cells = this._cells;
    const nodes = this._nodes;
    const records = this._records;

    // don't bother updating if the scrollTop hasn't changed much
    this._lastCheck = data.scrollTop;

    if (diff > 0) {
      // load data we may not have processed yet
      var stopAtHeight = (data.scrollTop + data.renderHeight);

      processRecords(stopAtHeight, records, cells,
                      this._hdrFn, this._ftrFn, data);
    }

    // ******** DOM READ ****************
    updateDimensions(this._plt, nodes, cells, data, false);

    adjustRendered(cells, data);

    var hasChanges = populateNodeData(
      data.topCell, data.bottomCell,
      data.viewWidth, diff > 0,
      cells, records, nodes,
      this._itmTmp.viewContainer,
      this._itmTmp.templateRef,
      this._hdrTmp && this._hdrTmp.templateRef,
      this._ftrTmp && this._ftrTmp.templateRef, false
    );

    if (hasChanges) {
      // queue making updates in the next frame
      this._queue = SCROLL_QUEUE_CHANGE_DETECTION;

      // update the bound context for each node
      updateNodeContext(nodes, cells, data);
    }
  }

  /**
   * @private
   */
  scrollUpdate(ev: ScrollEvent) {
    // set the scroll top from the scroll event
    this._data.scrollTop = ev.scrollTop;

    // there is a queue system so that we can
    // spread out the work over multiple frames
    const queue = this._queue;
    if (queue === SCROLL_QUEUE_NO_CHANGES) {
      // no dom writes or change detection to take care of
      this._stepNoChanges();
    } else if (queue === SCROLL_QUEUE_CHANGE_DETECTION) {
      this._dom.write(() => this._stepChangeDetection());
    } else {
      assert(queue === SCROLL_QUEUE_DOM_WRITE, 'queue value unexpected');
      // there are DOM writes we need to take care of in this frame
      this._dom.write(() => this._stepDOMWrite());
    }
  }

  /**
   * @hidden
   * DOM WRITE
   */
  scrollEnd(ev: ScrollEvent) {
    // ******** DOM READ ****************
    updateDimensions(this._plt, this._nodes, this._cells, this._data, false);
    adjustRendered(this._cells, this._data);

    // ******** DOM WRITE ***************
    this._dom.write(() => {
      // update the bound context for each node
      updateNodeContext(this._nodes, this._cells, this._data);

      // ******** DOM WRITE ***************
      this._stepChangeDetection();
      // ******** DOM WRITE ****************
      this._stepDOMWrite();
    });
  }

  /**
   * NO DOM
   */
  private _listeners() {
    assert(!this._scrollSub, '_listeners was already called');
    if (!this._scrollSub) {
      if (this._config.getBoolean('virtualScrollEventAssist')) {
        // use JS scrolling for iOS UIWebView
        // goal is to completely remove this when iOS
        // fully supports scroll events
        // listen to JS scroll events
        this._content.enableJsScroll();
      }

      this._resizeSub = this._plt.resize.subscribe(this.resize.bind(this));
      this._scrollSub = this._content.ionScroll.subscribe(this.scrollUpdate.bind(this));
      this._scrollEndSub = this._content.ionScrollEnd.subscribe(this.scrollEnd.bind(this));
    }
  }

  /**
   * DOM WRITE
   */
  private _setHeight(newVirtualHeight: number) {
    if (newVirtualHeight !== this._vHeight) {
      // ******** DOM WRITE ****************
      this._renderer.setElementStyle(this._elementRef.nativeElement, 'height', newVirtualHeight > 0 ? newVirtualHeight + 'px' : '');

      this._vHeight = newVirtualHeight;
      console.debug('VirtualScroll, height', newVirtualHeight);
    }
  }

  /**
   * @hidden
   */
  ngAfterContentInit() {
    assert(this._itmTmp, 'virtualItem required within virtualScroll');

    if (!this.approxItemHeight) {
      this.approxItemHeight = '40px';
      console.warn('Virtual Scroll: Please provide an "approxItemHeight" input to ensure proper virtual scroll rendering');
    }
  }

  setElementClass(className: string, add: boolean) {
    this._renderer.setElementClass(this._elementRef.nativeElement, className, add);
  }

  /**
   * @hidden
   */
  ngOnDestroy() {
    this._resizeSub && this._resizeSub.unsubscribe();
    this._scrollSub && this._scrollSub.unsubscribe();
    this._scrollEndSub && this._scrollEndSub.unsubscribe();
    this._resizeSub = this._scrollEndSub = this._scrollSub = null;
    this._hdrFn = this._ftrFn = this._records = this._cells = this._nodes = this._data = null;
  }
}

const SCROLL_DIFFERENCE_MINIMUM = 40;
const SCROLL_QUEUE_NO_CHANGES = 1;
const SCROLL_QUEUE_CHANGE_DETECTION = 2;
const SCROLL_QUEUE_DOM_WRITE = 3;