react-native-easy-guesture-responder/src/TouchHistoryMath.js

A powerful and easy to use function for React Native guesture responders. Supporting both iOS and Android. Free and made possible along with costly maintenance and updates by [Lue Hang](https://www.facebook.com/lue.hang) (the author).

"use strict";

var TouchHistoryMath = {
  /**
   * This code is optimized and not intended to look beautiful. This allows
   * computing of touch centroids that have moved after `touchesChangedAfter`
   * timeStamp. You can compute the current centroid involving all touches
   * moves after `touchesChangedAfter`, or you can compute the previous
   * centroid of all touches that were moved after `touchesChangedAfter`.
   *
   * @param {TouchHistoryMath} touchHistory Standard Responder touch track
   * data.
   * @param {number} touchesChangedAfter timeStamp after which moved touches
   * are considered "actively moving" - not just "active".
   * @param {boolean} isXAxis Consider `x` dimension vs. `y` dimension.
   * @param {boolean} ofCurrent Compute current centroid for actively moving
   * touches vs. previous centroid of now actively moving touches.
   * @return {number} value of centroid in specified dimension.
   */
    centroidDimension: function (
        touchHistory,
        touchesChangedAfter,
        isXAxis,
        ofCurrent
    ) {
        var touchBank = touchHistory.touchBank;
        var total = 0;
        var count = 0;

        var oneTouchData = touchHistory.numberActiveTouches === 1 ?
            touchHistory.touchBank[touchHistory.indexOfSingleActiveTouch] :
            null;

        if (oneTouchData !== null) {
            if (
                oneTouchData.touchActive &&
                oneTouchData.currentTimeStamp > touchesChangedAfter
            ) {
                total += ofCurrent && isXAxis ?
                    oneTouchData.currentPageX :
                    ofCurrent && !isXAxis ?
                    oneTouchData.currentPageY :
                    !ofCurrent && isXAxis ?
                    oneTouchData.previousPageX :
                    oneTouchData.previousPageY;
                count = 1;
            }
        } else {
            for (var i = 0; i < touchBank.length; i++) {
                var touchTrack = touchBank[i];
                if (
                    touchTrack !== null &&
                    touchTrack !== undefined &&
                    touchTrack.touchActive &&
                    touchTrack.currentTimeStamp >= touchesChangedAfter
                ) {
                    var toAdd; // Yuck, program temporarily in invalid state.
                    if (ofCurrent && isXAxis) {
                        toAdd = touchTrack.currentPageX;
                    } else if (ofCurrent && !isXAxis) {
                        toAdd = touchTrack.currentPageY;
                    } else if (!ofCurrent && isXAxis) {
                        toAdd = touchTrack.previousPageX;
                    } else {
                        toAdd = touchTrack.previousPageY;
                    }
                    total += toAdd;
                    count++;
                }
            }
        }
        return count > 0 ? total / count : TouchHistoryMath.noCentroid;
    },

    currentCentroidXOfTouchesChangedAfter: function (
        touchHistory,
        touchesChangedAfter
    ) {
        return TouchHistoryMath.centroidDimension(
            touchHistory,
            touchesChangedAfter,
            true, // isXAxis
            true // ofCurrent
        );
    },

    currentCentroidYOfTouchesChangedAfter: function (
        touchHistory,
        touchesChangedAfter
    ) {
        return TouchHistoryMath.centroidDimension(
            touchHistory,
            touchesChangedAfter,
            false, // isXAxis
            true // ofCurrent
        );
    },

    previousCentroidXOfTouchesChangedAfter: function (
        touchHistory,
        touchesChangedAfter
    ) {
        return TouchHistoryMath.centroidDimension(
            touchHistory,
            touchesChangedAfter,
            true, // isXAxis
            false // ofCurrent
        );
    },

    previousCentroidYOfTouchesChangedAfter: function (
        touchHistory,
        touchesChangedAfter
    ) {
        return TouchHistoryMath.centroidDimension(
            touchHistory,
            touchesChangedAfter,
            false, // isXAxis
            false // ofCurrent
        );
    },

    currentCentroidX: function (touchHistory) {
        return TouchHistoryMath.centroidDimension(
            touchHistory,
            0, // touchesChangedAfter
            true, // isXAxis
            true // ofCurrent
        );
    },

    currentCentroidY: function (touchHistory) {
        return TouchHistoryMath.centroidDimension(
            touchHistory,
            0, // touchesChangedAfter
            false, // isXAxis
            true // ofCurrent
        );
    },

    noCentroid: -1
};

module.exports = TouchHistoryMath;