Swup Parallel Plugin

A swup plugin for animating the previous and next page in parallel.

• Combines swup's leave/enter animations into a single animation
• Keeps the previous page visible while the next page is entering
• Allows synchronous animations like overlays, crossfades, or slideshows

Demos

To see parallel animations in action, check out the official demos:

• slideshow animation
• reveal animation

https://github.com/swup/parallel-plugin/assets/22225348/aff0a235-d9aa-472b-9967-1e9fa0e67313

Feel free to explore more examples on the Swup Demos page of the docs.

Installation

Install the plugin from npm and import it into your bundle.

npm install @swup/parallel-plugin

import SwupParallelPlugin from '@swup/parallel-plugin';

Or include the minified production file from a CDN:

<script src="https://unpkg.com/@swup/parallel-plugin@0"></script>

Usage

To run this plugin, include an instance in the swup options.

const swup = new Swup({
  plugins: [new SwupParallelPlugin()]
});

Scenario

When transitioning pages, swup will hide the previous page before replacing the content and showing the next page. This is fine in most cases, and great for performance and accessibility. However, some layouts require the previous page to be visible for the duration of the page transition: think crossfades, overlays, slideshows, 3D effects, etc. For these to be convincing, the old and new containers must be animated at the same time.

Lifecycle

Swup default animations

• Animate out: hide the previous content
• Replace the content entirely
• Animate in: show the next content
• Previous and next content are never in the DOM at the same time

Parallel Plugin animations

• Skip the out-phase of the animation
• Add the next content to the DOM
• Animate in and out: show the next content while hiding the previous content
• Previous and next content are DOM siblings during the animation

Markup

In this example, we want to slide in the new main element while sliding out the previous main element. The markup for parallel animations isn't any different from normal animations: a simple section with a content area.

<section>
  <header>
    My Website
  </header>
  <main id="swup" class="transition-slide">
    <h1>Welcome</h1>
  </main>
</section>

State during the animation

During the animation, both containers will be in the DOM at the same time. Swup has inserted the next container, will wait for any animations to finish, and then remove the previous container.

Note: for accessibility reasons, the next container is always inserted before the previous one and the previous one is marked as hidden from screen readers by setting aria-hidden="true".

<section>
  <header>
    My Website
  </header>
  <!-- Next container -->
  <main id="swup" class="transition-slide is-next-container">
    <h1>Next page</h1>
  </main>
    <!-- Previous container -->
  <main id="swup" class="transition-slide is-previous-container" aria-hidden="true">
    <h1>Previous page</h1>
  </main>
</section>

Styling

Basic styling for parallel animations

Showing both the previous and current container during a page transition requires slightly more complex styling than usual. The containers need to be layered on top of each other while they're both in the DOM.

The details depend on the specific layout, however the easiest way in most cases is a CSS grid with both containers claiming the same row and column. This type of layout avoids messy absolute positioning and scroll offsets.

/* Layout */

section {
  display: grid;
  overflow: hidden;
  grid-template-areas: "header"
                        "main";
}

section > header {
  grid-area: header;
}

section > main {
  grid-area: main;
}

Defining the animations

Instead of using swup's default classes for timing the animation, parallel animations can be controlled using two custom classes applied by this plugin on the containers themselves:

The class is-previous-container defines the final state of the previous container, e.g. already out of view or transparent. It is added directly after the element is added to the DOM to begin the animation.

The class is-next-container defines the initial state of the next container about to enter the page, e.g. still out of view or transparent. It is added before the element is inserted into the DOM and removed directly after insertion to begin the animation. As it's only applied for a split second, it won't be visible when inspecting the element in the browser dev tools.

/* Parallel animation timing */
.is-changing .transition-slide {
  transition: transform 0.3s ease-in-out, opacity 0.3s ease-in-out;
}

/* Style of previous container after leaving */
.transition-slide.is-previous-container {
  transform: translateX(-100%);
  opacity: 0;
}

/* Style of next container before entering */
.transition-slide.is-next-container {
  transform: translateX(100%);
  opacity: 0;
}

Timing

Technically, the plugin will skip the out-animation, add the next container, wait for animations to finish, then remove the previous container. All animations now happen in the in-phase of the lifecycle, after the content:replace hook that normally marks the middle of the animation process. Any containers that are not animated or not animated in parallel (e.g. a static header) will be replaced at the start of the parallel animation.

This also applies when using the JS plugin: the out-animation is skipped entirely and only the in-animation is executed. You'll need to perform both animations in the in handler.

Keeping the previous container

The previous container is removed from the DOM after the animation finishes. If you need to keep one or more copies of it around after the animation, you can set the keep option and adjust your styling.

Warning Keep in mind the accessibility concerns of having duplicate content on the page. While this plugin marks containers as aria-hidden="true", the duplicate content can still be clicked and focussed. Marking previous containers as inert is a possible solution but not applicable to all use cases. Turning a main container into a simple div would be helpful as well, however it's impossible to do without a complete re-render of the element.

new SwupParallelPlugin({ keep: 1 })

Previous containers that will be kept are marked with a class name of .is-kept-container. Those about to be removed are marked with .is-removing-container.

<section>
  <main id="swup" class="transition-slide is-next-container"></main>
  <main id="swup" class="transition-slide is-previous-container is-kept-container" aria-hidden="true"></main>
  <main id="swup" class="transition-slide is-previous-container is-kept-container is-removing-container" aria-hidden="true"></main>
</section>

Use the classes is-removing-container to style the transition of a container to be removed.

.is-changing .transition-slide {
  transition: transform 0.3s ease-in-out, opacity 0.3s ease-in-out;
}

.transition-slide.is-next-container {
  transform: translateX(100%);
  opacity: 0;
}

.transition-slide.is-kept-container {
  transform: translateX(-100%);
}

.transition-slide.is-removing-container {
  transform: translateX(-200%);
  opacity: 0;
}

Options

containers

By default, all content containers are animated in parallel. If you only want to perform parallel animations for specific containers and replace other containers normally, specify the parallel containers here.

{
  containers: ['main']
}

keep

The number of previous containers to keep around after the animation finishes. Useful for layouts like slideshows, stacks, etc. Default: 0.

{
  keep: 1
}

Pass an object indexed by selector to keep around the previous version of certain containers only:

{
  containers: ['main', 'footer'],
  keep: { 'main': 1 }
}

API

Opting out of parallel animations

The plugin will set a flag on the visit object, indicating the current visit as a parallel animation: visit.animation.parallel. You can unset this flag to fall back to a normal animation with leave and enter in series.

// Disable parallel animations for this visit
swup.hooks.on('visit:start', (visit) => {
  if (someCondition) {
    visit.animation.parallel = false;
  }
});

Hooks

The plugin adds two new hooks to swup. Both hooks receive a ContainerSet instance as their only argument:

type ContainerSet = {
  /** Selector to match this container */
  selector: string;
  /** Incoming container element */
  next: HTMLElement;
  /** Outgoing container element */
  previous: HTMLElement;
  /** Container elements to keep around after the animation */
  keep: HTMLElement[];
  /** Container elements to remove after the animation */
  remove: HTMLElement[];
};

content:insert

Triggered when the new content containers are inserted. Hook before this to manipulate the elements before they are inserted into the DOM.

swup.hooks.before('content:insert', (visit, { containers }) => {
  for (const { next } of containers) {
    console.log('About to insert container', next);
  }
});

content:remove

Triggered when the previous containers are removed, after the animation has finished.

swup.hooks.before('content:remove', (visit, { containers }) => {
  for (const { remove } of containers) {
    console.log('About to remove containers', remove);
  }
});