rs-react-native-image-gallery/src/_helpers.ts

React Native Image Gallery with Thumbnails

import { Image, type NativeTouchEvent } from 'react-native';
import { SCALE_MULTIPLIER } from './constants';

/**
 * Calculate squared distance between two points
 * @param a First coordinate
 * @param b Second coordinate
 */
export const pow2abs = (a: number, b: number): number => (a - b) ** 2;

/**
 * Get distance between two touch points
 * @param touches Array of touch events
 */
export const getDistance = (touches: Array<NativeTouchEvent>): number => {
	// Early validation
	if (!touches || touches.length < 2) return 0;

	const [a, b] = touches;

	// Null check with optional chaining
	if (!a?.pageX || !b?.pageX || !a?.pageY || !b?.pageY) return 0;

	return Math.sqrt(pow2abs(a.pageX, b.pageX) + pow2abs(a.pageY, b.pageY));
};

// Cache for memoized distance calculations - improves performance on pinch gestures
const distanceCache = new Map<string, number>();

/**
 * Memoized version of getDistance - caches results for better performance
 * Only use this when you expect the same touch points to be calculated repeatedly
 * @param touches Array of touch events
 */
export const getMemoizedDistance = (touches: Array<NativeTouchEvent>): number => {
	// Early validation
	if (!touches || touches.length < 2) return 0;

	const [a, b] = touches;

	// Null check with optional chaining
	if (!a?.pageX || !b?.pageX || !a?.pageY || !b?.pageY) return 0;

	// Create cache key from touch coordinates
	const key = `${a.pageX},${a.pageY}-${b.pageX},${b.pageY}`;

	if (distanceCache.has(key)) {
		return distanceCache.get(key)!;
	}

	const distance = Math.sqrt(pow2abs(a.pageX, b.pageX) + pow2abs(a.pageY, b.pageY));

	// Cache result - limit cache size to avoid memory leaks
	if (distanceCache.size > 100) {
		// Clear cache when it gets too big
		distanceCache.clear();
	}

	distanceCache.set(key, distance);
	return distance;
};

/**
 * Calculate scale based on distance ratio with a multiplier
 * @param currentDistance Current distance between touch points
 * @param initialDistance Initial distance between touch points
 */
export const getScale = (currentDistance: number, initialDistance: number): number => {
	// Prevent division by zero
	if (!initialDistance) return 1;

	return (currentDistance / initialDistance) * SCALE_MULTIPLIER;
};

/**
 * Pre-loads images to avoid spinner issues
 * @param urls Array of image URLs to preload (only URLs are supported for prefetch)
 */
export const preLoadImages = (urls: string[]): void => {
	urls.forEach((url) => {
		if (!url || typeof url !== 'string') return;

		// Use Image.prefetch for native platforms (only works with URL strings)
		Image.prefetch(url).catch(() => {
			// Silent catch - we don't need to handle prefetch errors
		});
	});
};