react-native/Libraries/Experimental/ViewabilityHelper.js

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/**
* Copyright (c) 2013-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';
/**
* A row is said to be in a "viewable" state when either of the following
* is true:
* - Occupying >= viewablePercentThreshold of the viewport
* - Entirely visible on screen
*/
const ViewabilityHelper = {
computeViewableRows(
viewablePercentThreshold: number,
rowFrames: {[key: string]: Object},
data: Array<{rowKey: string, rowData: any}>,
scrollOffsetY: number,
viewportHeight: number
): Array<number> {
const viewableRows = [];
let firstVisible = -1;
for (let idx = 0; idx < data.length; idx++) {
const frame = rowFrames[data[idx].rowKey];
if (!frame) {
continue;
}
const top = frame.y - scrollOffsetY;
const bottom = top + frame.height;
if ((top < viewportHeight) && (bottom > 0)) {
firstVisible = idx;
if (_isViewable(
viewablePercentThreshold,
top,
bottom,
viewportHeight
)) {
viewableRows.push(idx);
}
} else if (firstVisible >= 0) {
break;
}
}
return viewableRows;
},
};
function _isViewable(
viewablePercentThreshold: number,
top: number,
bottom: number,
viewportHeight: number
): bool {
return _isEntirelyVisible(top, bottom, viewportHeight) ||
_getPercentOccupied(top, bottom, viewportHeight) >=
viewablePercentThreshold;
}
function _getPercentOccupied(
top: number,
bottom: number,
viewportHeight: number
): number {
let visibleHeight = Math.min(bottom, viewportHeight) - Math.max(top, 0);
visibleHeight = Math.max(0, visibleHeight);
return Math.max(0, visibleHeight * 100 / viewportHeight);
}
function _isEntirelyVisible(
top: number,
bottom: number,
viewportHeight: number
): bool {
return top >= 0 && bottom <= viewportHeight && bottom > top;
}
module.exports = ViewabilityHelper;