web/_includes/mode-toggle.html

<!--
  Switch the mode between dark and light.
-->

<script type="text/javascript">
  class ModeToggle {
    static get MODE_KEY() { return "mode"; }
    static get MODE_ATTR() { return "data-mode"; }
    static get DARK_MODE() { return "dark"; }
    static get LIGHT_MODE() { return "light"; }
    static get ID() { return "mode-toggle"; }

    constructor() {
      if (this.hasMode) {
        if (this.isDarkMode) {
          if (!this.isSysDarkPrefer) {
            this.setDark();
          }
        } else {
          if (this.isSysDarkPrefer) {
            this.setLight();
          }
        }
      }

      let self = this;

      /* always follow the system prefers */
      this.sysDarkPrefers.addEventListener("change", () => {
        if (self.hasMode) {
          if (self.isDarkMode) {
            if (!self.isSysDarkPrefer) {
              self.setDark();
            }

          } else {
            if (self.isSysDarkPrefer) {
              self.setLight();
            }
          }

          self.clearMode();
        }

        self.notify();

      });

    } /* constructor() */

    get sysDarkPrefers() { return window.matchMedia("(prefers-color-scheme: dark)"); }

    get isSysDarkPrefer() { return this.sysDarkPrefers.matches; }

    get isDarkMode() { return this.mode === ModeToggle.DARK_MODE; }

    get isLightMode() { return this.mode === ModeToggle.LIGHT_MODE; }

    get hasMode() { return this.mode != null; }

    get mode() { return sessionStorage.getItem(ModeToggle.MODE_KEY); }

    /* get the current mode on screen */
    get modeStatus() {
      if (this.isDarkMode
        || (!this.hasMode && this.isSysDarkPrefer)) {
        return ModeToggle.DARK_MODE;
      } else {
        return ModeToggle.LIGHT_MODE;
      }
    }

    setDark() {
      $('html').attr(ModeToggle.MODE_ATTR, ModeToggle.DARK_MODE);
      sessionStorage.setItem(ModeToggle.MODE_KEY, ModeToggle.DARK_MODE);
    }

    setLight() {
      $('html').attr(ModeToggle.MODE_ATTR, ModeToggle.LIGHT_MODE);
      sessionStorage.setItem(ModeToggle.MODE_KEY, ModeToggle.LIGHT_MODE);
    }

    clearMode() {
      $('html').removeAttr(ModeToggle.MODE_ATTR);
      sessionStorage.removeItem(ModeToggle.MODE_KEY);
    }

    /* Notify another plugins that the theme mode has changed */
    notify() {
      window.postMessage({
        direction: ModeToggle.ID,
        message: this.modeStatus
      }, "*");
    }

  } /* ModeToggle */

  const toggle = new ModeToggle();

  function flipMode() {
    if (toggle.hasMode) {
      if (toggle.isSysDarkPrefer) {
        if (toggle.isLightMode) {
          toggle.clearMode();
        } else {
          toggle.setLight();
        }

      } else {
        if (toggle.isDarkMode) {
          toggle.clearMode();
        } else {
          toggle.setDark();
        }
      }

    } else {
      if (toggle.isSysDarkPrefer) {
        toggle.setLight();
      } else {
        toggle.setDark();
      }
    }

    toggle.notify();

  } /* flipMode() */

</script>