docbrown/app/javascript/admin/controllers/creator_settings_controller.js

import { Controller } from '@hotwired/stimulus';
import { isLowContrast } from '@utilities/color/contrastValidator';
import { brightness } from '@utilities/color/accentCalculator';

const MAX_LOGO_PREVIEW_HEIGHT = 80;

/**
 * Manages interactions on the Creator Settings page.
 */
export class CreatorSettingsController extends Controller {
  static targets = ['previewLogo', 'colorContrastError', 'brandColor'];

  /**
   * Displays a preview of the image selected by the user.
   *
   * @param {Event} event
   */
  previewLogo(event) {
    const {
      target: {
        files: [firstFile],
      },
    } = event;

    if (!firstFile) {
      // Most likely the user cancelled the file selection.
      return;
    }

    const reader = new FileReader();

    reader.onload = () => {
      const imageURL = reader.result;
      const image = document.createElement('img');
      image.src = imageURL;
      image.className = 'site-logo';

      // The logo preview image is purely visual so no need to communicate this to assistive technology.
      image.alt = 'preview of logo selected';

      image.addEventListener(
        'load',
        (event) => {
          let {
            target: { width, height },
          } = event;

          this.previewLogoTarget.replaceChild(
            image,
            this.previewLogoTarget.firstChild,
          );

          const maxLogoPreviewWidth = parseInt(
            getComputedStyle(image).getPropertyValue('--max-width'),
            10,
          );

          if (height > MAX_LOGO_PREVIEW_HEIGHT) {
            width = (MAX_LOGO_PREVIEW_HEIGHT / height) * width;
            height = MAX_LOGO_PREVIEW_HEIGHT;
          }

          if (width > maxLogoPreviewWidth) {
            height = (maxLogoPreviewWidth / width) * height;
            width = maxLogoPreviewWidth;
          }

          image.width = width;
          image.height = height;
        },
        { once: true },
      );
    };

    reader.readAsDataURL(firstFile);
  }

  /**
   * Validates the color contrast for accessibility,
   * if the contrast is okay, it updates the branding,
   * else it displays the error.
   * @param {Event} event
   */
  handleValidationsAndUpdates(event) {
    const { value: color } = event.target;

    if (isLowContrast(color)) {
      this.colorContrastErrorTarget.innerText =
        'The selected color must be darker for accessibility purposes.';
    } else {
      this.updateBranding(color);
      this.colorContrastErrorTarget.innerText = '';
    }
  }

  /**
   * Updates ths branding/colors on the Creator Settings Page.
   * by overriding the accent-color in the :root object
   *
   * @param {String} color
   */
  updateBranding(color) {
    if (!new RegExp(event.target.getAttribute('pattern')).test(color)) {
      return;
    }

    document.documentElement.style.setProperty('--accent-brand', color);

    // We need to recalculate '--accent-brand-darker' in javascript as it's
    // currently being calculated in ruby. It is used for the hover effect
    // over the button.
    // 0.85 represents the brightness value set in Ruby to calculate
    // '--accent-brand-darker'
    document.documentElement.style.setProperty(
      '--accent-brand-darker',
      brightness(color, 0.85),
    );
  }

  /**
   * Prevents a submission of the form if the
   * color contrast is low.
   *
   * @param {Event} event
   */
  formValidations(event) {
    const { value: color } = this.brandColorTarget;
    if (isLowContrast(color)) {
      event.preventDefault();
      this.colorContrastErrorTarget.classList.remove('hidden');
      //  we don't want the form to submit if the contrast is low
    }
  }
}