Cross Fade Page Transition

// -----------------------------------------
// OSMO PAGE TRANSITION BOILERPLATE
// -----------------------------------------

gsap.registerPlugin(ScrollTrigger, CustomEase);
history.scrollRestoration = "manual";

let lenis = null;
let nextPage = document;
let onceFunctionsInitialized = false;

const rmMQ = window.matchMedia("(prefers-reduced-motion: reduce)");
let reducedMotion = rmMQ.matches;
rmMQ.addEventListener?.("change", e => (reducedMotion = e.matches));
rmMQ.addListener?.(e => (reducedMotion = e.matches)); 

const has = (s) => !!nextPage.querySelector(s);

let staggerDefault = 0.05;
let durationDefault = 0.6;

CustomEase.create("osmo", "0.625, 0.05, 0, 1");
gsap.defaults({ ease: "osmo", duration: durationDefault });


// -----------------------------------------
// FUNCTION REGISTRY
// -----------------------------------------

function initOnceFunctions() {
  initLenis();
  if (onceFunctionsInitialized) return;
  onceFunctionsInitialized = true;
  
  // Runs once on first load
  // if (has('[data-something]')) initSomething();
}

function initBeforeEnterFunctions(next) {
  nextPage = next || document;
  
  // Runs before the enter animation
  // if (has('[data-something]')) initSomething();
}

function initAfterEnterFunctions(next) {
  nextPage = next || document;
  
  // Runs after enter animation completes
  // if (has('[data-something]')) initSomething();
  
  lenis.resize();
  ScrollTrigger.refresh();
}


// -----------------------------------------
// PAGE TRANSITIONS
// -----------------------------------------

function runPageOnceAnimation(next) {
  const tl = gsap.timeline();

  tl.call(() => {
    window.scrollTo(0, 0);
  }, null, 0);

  return tl;
}

function runPageLeaveAnimation(current, next) {
  const tl = gsap.timeline({
    onComplete: () => { current.remove() }
  });
  
  if (reducedMotion) {
    // Immediate swap behavior if user prefers reduced motion
    return tl.set(current, { autoAlpha: 0 });
  }

  tl.to(current, { autoAlpha: 0, duration: 0.4 });

  return tl;
}

function runPageEnterAnimation(next){
  const tl = gsap.timeline();
  
  if (reducedMotion) {
    // Immediate swap behavior if user prefers reduced motion
    tl.set(next, { autoAlpha: 1 });
    tl.add("pageReady")
    tl.call(resetPage, [next], "pageReady");
    return new Promise(resolve => tl.call(resolve, null, "pageReady"));
  }
  
  tl.add("startEnter", 0.6);
  
  tl.fromTo(next, {
    autoAlpha: 0,
  },{
    autoAlpha: 1,
  }, "startEnter");

  tl.add("pageReady");
  tl.call(resetPage, [next], "pageReady");

  return new Promise(resolve => {
    tl.call(resolve, null, "pageReady");
  });
}


// -----------------------------------------
// BARBA HOOKS + INIT
// -----------------------------------------

barba.hooks.beforeEnter(data => {
  // Position new container on top
  gsap.set(data.next.container, {
    position: "fixed",
    top: 0,
    left: 0,
    right: 0,
  });
  
  if(lenis && typeof lenis.stop === "function"){
    lenis.stop();
  }
  
  initBeforeEnterFunctions(data.next.container);
  applyThemeFrom(data.next.container);
});

barba.hooks.afterLeave(() => {
  ScrollTrigger.getAll().forEach(trigger => trigger.kill());
});

barba.hooks.enter(data => {
  initBarbaNavUpdate(data);
})

barba.hooks.afterEnter(data => {
  // Run page functions
  initAfterEnterFunctions(data.next.container);
  
  // Settle
  lenis.resize();
  lenis.start();
  ScrollTrigger.refresh();
});

barba.init({
  debug: true, // Set to 'false' in production
  timeout: 7000,
  preventRunning: true,
  transitions: [
    {
      name: "default",
      sync: true,
      
      // First load
      async once(data) {
        initOnceFunctions();

        return runPageOnceAnimation(data.next.container);
      },

      // Current page leaves
      async leave(data) {
        return runPageLeaveAnimation(data.current.container, data.next.container);
      },

      // New page enters
      async enter(data) {
        return runPageEnterAnimation(data.next.container);
      }
    }
  ],
});


// -----------------------------------------
// GENERIC + HELPERS
// -----------------------------------------

const themeConfig = {
  light: {
    nav: "dark",
    transition: "light"
  },
  dark: {
    nav: "light",
    transition: "dark"
  }
};

function applyThemeFrom(container) {
  const pageTheme = container?.dataset?.pageTheme || "light";
  const config = themeConfig[pageTheme] || themeConfig.light;
  
  document.body.dataset.pageTheme = pageTheme;
  const transitionEl = document.querySelector('[data-theme-transition]');
  if (transitionEl) {
    transitionEl.dataset.themeTransition = config.transition;
  }

  const nav = document.querySelector('[data-theme-nav]');
  if (nav) {
    nav.dataset.themeNav = config.nav;
  }
}

function initLenis() {
  if (lenis) return; // Already exists
  
  lenis = new Lenis({
    lerp: 0.165,
    wheelMultiplier: 1.25,
  });

  lenis.on('scroll', ScrollTrigger.update);
  gsap.ticker.add((time) => {lenis.raf(time * 1000);});
  gsap.ticker.lagSmoothing(0);
}

function resetPage(container){
  window.scrollTo(0, 0);
  gsap.set(container, { clearProps: "position,top,left,right" });
  lenis.resize();
  lenis.start();  
}

function debounceOnWidthChange(fn, ms) {
  let last = innerWidth,
    timer;
  return function (...args) {
    clearTimeout(timer);
    timer = setTimeout(() => {
      if (innerWidth !== last) {
        last = innerWidth;
        fn.apply(this, args);
      }
    }, ms);
  };
}

function initBarbaNavUpdate(data) {
  var tpl = document.createElement('template');
  tpl.innerHTML = data.next.html.trim();
  var nextNodes = tpl.content.querySelectorAll('[data-barba-update]');
  var currentNodes = document.querySelectorAll('nav [data-barba-update]');

  currentNodes.forEach(function (curr, index) {
    var next = nextNodes[index];
    if (!next) return;

    // Aria-current sync
    var newStatus = next.getAttribute('aria-current');
    if (newStatus !== null) {
      curr.setAttribute('aria-current', newStatus);
    } else {
      curr.removeAttribute('aria-current');
    }

    // Class list sync
    var newClassList = next.getAttribute('class') || '';
    curr.setAttribute('class', newClassList);
  });
}


// -----------------------------------------
// YOUR FUNCTIONS GO BELOW HERE
// -----------------------------------------