react-native/Libraries/Lists/ViewabilityHelper.js

/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ViewabilityHelper
 * @flow
 */
'use strict';

const invariant = require('fbjs/lib/invariant');

export type ViewToken = {item: any, key: string, index: ?number, isViewable: boolean, section?: any};

export type ViewabilityConfig = {|
  /**
   * Minimum amount of time (in milliseconds) that an item must be physically viewable before the
   * viewability callback will be fired. A high number means that scrolling through content without
   * stopping will not mark the content as viewable.
   */
  minimumViewTime?: number,

  /**
   * Percent of viewport that must be covered for a partially occluded item to count as
   * "viewable", 0-100. Fully visible items are always considered viewable. A value of 0 means
   * that a single pixel in the viewport makes the item viewable, and a value of 100 means that
   * an item must be either entirely visible or cover the entire viewport to count as viewable.
   */
  viewAreaCoveragePercentThreshold?: number,

  /**
   * Similar to `viewAreaPercentThreshold`, but considers the percent of the item that is visible,
   * rather than the fraction of the viewable area it covers.
   */
  itemVisiblePercentThreshold?: number,

  /**
   * Nothing is considered viewable until the user scrolls or `recordInteraction` is called after
   * render.
   */
  waitForInteraction?: boolean,
|};

/**
* A Utility class for calculating viewable items based on current metrics like scroll position and
* layout.
*
* An item is said to be in a "viewable" state when any of the following
* is true for longer than `minimumViewTime` milliseconds (after an interaction if `waitForInteraction`
* is true):
*
* - Occupying >= `viewAreaCoveragePercentThreshold` of the view area XOR fraction of the item
*   visible in the view area >= `itemVisiblePercentThreshold`.
* - Entirely visible on screen
*/
class ViewabilityHelper {
  _config: ViewabilityConfig;
  _hasInteracted: boolean = false;
  _lastUpdateTime: number = 0;
  _timers: Set<number> = new Set();
  _viewableIndices: Array<number> = [];
  _viewableItems: Map<string, ViewToken> = new Map();

  constructor(config: ViewabilityConfig = {viewAreaCoveragePercentThreshold: 0}) {
    this._config = config;
  }

  /**
   * Cleanup, e.g. on unmount. Clears any pending timers.
   */
  dispose() {
    this._timers.forEach(clearTimeout);
  }

  /**
   * Determines which items are viewable based on the current metrics and config.
   */
  computeViewableItems(
    itemCount: number,
    scrollOffset: number,
    viewportHeight: number,
    getFrameMetrics: (index: number) => ?{length: number, offset: number},
    renderRange?: {first: number, last: number}, // Optional optimization to reduce the scan size
  ): Array<number> {
    const {itemVisiblePercentThreshold, viewAreaCoveragePercentThreshold} = this._config;
    const viewAreaMode = viewAreaCoveragePercentThreshold != null;
    const viewablePercentThreshold = viewAreaMode ?
      viewAreaCoveragePercentThreshold :
      itemVisiblePercentThreshold;
    invariant(
      viewablePercentThreshold != null &&
      (itemVisiblePercentThreshold != null) !== (viewAreaCoveragePercentThreshold != null),
      'Must set exactly one of itemVisiblePercentThreshold or viewAreaCoveragePercentThreshold',
    );
    const viewableIndices = [];
    if (itemCount === 0) {
      return viewableIndices;
    }
    let firstVisible = -1;
    const {first, last} = renderRange || {first: 0, last: itemCount - 1};
    invariant(
      last < itemCount,
      'Invalid render range ' + JSON.stringify({renderRange, itemCount})
    );
    for (let idx = first; idx <= last; idx++) {
      const metrics = getFrameMetrics(idx);
      if (!metrics) {
        continue;
      }
      const top = metrics.offset - scrollOffset;
      const bottom = top + metrics.length;
      if ((top < viewportHeight) && (bottom > 0)) {
        firstVisible = idx;
        if (_isViewable(
          viewAreaMode,
          viewablePercentThreshold,
          top,
          bottom,
          viewportHeight,
          metrics.length,
        )) {
          viewableIndices.push(idx);
        }
      } else if (firstVisible >= 0) {
        break;
      }
    }
    return viewableIndices;
  }

  /**
   * Figures out which items are viewable and how that has changed from before and calls
   * `onViewableItemsChanged` as appropriate.
   */
  onUpdate(
    itemCount: number,
    scrollOffset: number,
    viewportHeight: number,
    getFrameMetrics: (index: number) => ?{length: number, offset: number},
    createViewToken: (index: number, isViewable: boolean) => ViewToken,
    onViewableItemsChanged: ({viewableItems: Array<ViewToken>, changed: Array<ViewToken>}) => void,
    renderRange?: {first: number, last: number}, // Optional optimization to reduce the scan size
  ): void {
    const updateTime = Date.now();
    if (this._lastUpdateTime === 0 && itemCount > 0 && getFrameMetrics(0)) {
      // Only count updates after the first item is rendered and has a frame.
      this._lastUpdateTime = updateTime;
    }
    const updateElapsed = this._lastUpdateTime ? updateTime - this._lastUpdateTime : 0;
    if (this._config.waitForInteraction && !this._hasInteracted) {
      return;
    }
    let viewableIndices = [];
    if (itemCount) {
      viewableIndices = this.computeViewableItems(
        itemCount,
        scrollOffset,
        viewportHeight,
        getFrameMetrics,
        renderRange,
      );
    }
    if (this._viewableIndices.length === viewableIndices.length &&
        this._viewableIndices.every((v, ii) => v === viewableIndices[ii])) {
      // We might get a lot of scroll events where visibility doesn't change and we don't want to do
      // extra work in those cases.
      return;
    }
    this._viewableIndices = viewableIndices;
    this._lastUpdateTime = updateTime;
    if (this._config.minimumViewTime && updateElapsed < this._config.minimumViewTime) {
      const handle = setTimeout(
        () => {
          this._timers.delete(handle);
          this._onUpdateSync(viewableIndices, onViewableItemsChanged, createViewToken);
        },
        this._config.minimumViewTime,
      );
      this._timers.add(handle);
    } else {
      this._onUpdateSync(viewableIndices, onViewableItemsChanged, createViewToken);
    }
  }

  /**
   * Records that an interaction has happened even if there has been no scroll.
   */
  recordInteraction() {
    this._hasInteracted = true;
  }

  _onUpdateSync(viewableIndicesToCheck, onViewableItemsChanged, createViewToken) {
    // Filter out indices that have gone out of view since this call was scheduled.
    viewableIndicesToCheck = viewableIndicesToCheck.filter(
      (ii) => this._viewableIndices.includes(ii)
    );
    const prevItems = this._viewableItems;
    const nextItems = new Map(
      viewableIndicesToCheck.map(ii => {
        const viewable = createViewToken(ii, true);
        return [viewable.key, viewable];
      })
    );

    const changed = [];
    for (const [key, viewable] of nextItems) {
      if (!prevItems.has(key)) {
        changed.push(viewable);
      }
    }
    for (const [key, viewable] of prevItems) {
      if (!nextItems.has(key)) {
        changed.push({...viewable, isViewable: false});
      }
    }
    if (changed.length > 0) {
      this._viewableItems = nextItems;
      onViewableItemsChanged({viewableItems: Array.from(nextItems.values()), changed});
    }
  }
}


function _isViewable(
  viewAreaMode: boolean,
  viewablePercentThreshold: number,
  top: number,
  bottom: number,
  viewportHeight: number,
  itemLength: number,
): bool {
  if (_isEntirelyVisible(top, bottom, viewportHeight)) {
    return true;
  } else {
    const pixels = _getPixelsVisible(top, bottom, viewportHeight);
    const percent = 100 * (viewAreaMode ? pixels / viewportHeight : pixels / itemLength);
    return percent >= viewablePercentThreshold;
  }
}

function _getPixelsVisible(
  top: number,
  bottom: number,
  viewportHeight: number
): number {
  const visibleHeight = Math.min(bottom, viewportHeight) - Math.max(top, 0);
  return Math.max(0, visibleHeight);
}

function _isEntirelyVisible(
  top: number,
  bottom: number,
  viewportHeight: number
): bool {
  return top >= 0 && bottom <= viewportHeight && bottom > top;
}

module.exports = ViewabilityHelper;