Source: packages/spritesheet/src/SpritesheetLoader.js

import { url } from '@pixi/utils';
import { LoaderResource } from '@pixi/loaders';
import { Spritesheet } from './Spritesheet';
/**
 * {@link PIXI.Loader Loader} middleware for loading texture atlases that have been created with
 * TexturePacker or similar JSON-based spritesheet.
 *
 * This middleware automatically generates Texture resources.
 *
 * @class
 * @memberof PIXI
 * @implements PIXI.ILoaderPlugin
 */
export class SpritesheetLoader
{
    /**
     * Called after a resource is loaded.
     * @see PIXI.Loader.loaderMiddleware
     * @param {PIXI.LoaderResource} resource
     * @param {function} next
     */
    static use(resource, next)
    {
        const imageResourceName = `${resource.name}_image`;
        // skip if no data, its not json, it isn't spritesheet data, or the image resource already exists

        if (!resource.data
            || resource.type !== LoaderResource.TYPE.JSON
            || !resource.data.frames
            || this.resources[imageResourceName])
        {
            next();

            return;
        }
        const loadOptions = {
            crossOrigin: resource.crossOrigin,
            metadata: resource.metadata.imageMetadata,
            parentResource: resource,
        };
        const resourcePath = SpritesheetLoader.getResourcePath(resource, this.baseUrl);
        // load the image for this sheet

        this.add(imageResourceName, resourcePath, loadOptions, function onImageLoad(res)
        {
            if (res.error)
            {
                next(res.error);

                return;
            }
            const { baseTexture } = res.texture;
            // Don't need this Texture anymore

            res.texture.destroy();
            const spritesheet = new Spritesheet(baseTexture, resource.data, resource.url);

            spritesheet.parse(() =>
            {
                resource.spritesheet = spritesheet;
                resource.textures = spritesheet.textures;
                next();
            });
        });
    }
    /**
     * Get the spritesheets root path
     * @param {PIXI.LoaderResource} resource - Resource to check path
     * @param {string} baseUrl - Base root url
     */
    static getResourcePath(resource, baseUrl)
    {
        // Prepend url path unless the resource image is a data url
        if (resource.isDataUrl)
        {
            return resource.data.meta.image;
        }

        return url.resolve(resource.url.replace(baseUrl, ''), resource.data.meta.image);
    }
}