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Bag of Tricks

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Astro Transition Directives

Astro has several directives that you can apply to html elements in your *.astro components1 to control view transitions. See Astro Docs↗ for the official documentation. Below are some additional details and background information.

  • transition:name introduces a transition group for this element. If there is not also a transition:animate directive for the same element, Astro automatically inserts transition:animate="fade" for you.
  • transition:animate adds styling to the transition group. If there is not also a transition:name directive on the same element, Astro will automatically add one with a generated name like astro-hg6bdnwq-1
  • transition:persist="name" marks the element so that it will be copied into the updated document during the swap phase. Technically, it is possible to omit the name, i.e. to only use transition:persist, but I do not recommend it. This is only OK if you also use transition:name for the same element. Astro will then also use the transition name for persist.

There are also custom data attributes that influence the way view transitions work

  • data-astro-transition-persist and data-astro-transition-persist-props, see transition:persist
  • data-astro-reload on links and forms opt-out of view transitions and forces a full page load instead
  • data-astro-rerun on <script> tags forces the script to be re-run even if it was already present on the previous page.

transition:name

You use transition:name to explicitly set the view-transition-name CSS property of an element. A cool thing about transition:name="xxx" is that you can use any Unicode characters and Astro will automatically do the necessary encoding for you.

Astro uses a mechanism similar to scoped styles to set the view-transition-name. It converts this …

Component.astro
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<p transition:name="myName">...</p>

… into this …

Component.astro
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<p data-astro-transition-scope="astro-fgupyhpc-2">...</p>
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<style>
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  [data-astro-transition-scope="astro-fgupyhpc-2"] { view-transition-name: myName; }
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</style>

… setting a data attribute on the element and adding the view-transition-name using a CSS rule.

The main reason for setting the CSS property with a data attribute is that this attribute can be used as a fallback for the CSS property on browsers without native view transition support. The next section shows how this works.

Normally you should not set the view-transition-name CSS property directly in Astro. This will not work with the fallback simulation of view transitions and you will have to take care of the name encoding yourself.

But there are situations where setting the view-transition-name makes sense: if you are only interested in morph animations — which can not be supported by the simulation anyway — and you need them in places where Astro’s transition directives are not supported, e.g. in *.mdx files or framework components.

Setting the view-transition-name works only for browsers with native support for view transitions. Other browsers ignore the attribute and you cannot use it in CSS rules for styling. If used carelessly, it can also lead to unexpected loss of styling.

transition:animate

You can use transition:animate with predefined animation functions↗ or provide your own version↗.

Initial, fade, slide, none: The predefined functions are fade() and slide(). You can also also use transition:animate="none" on the <html> root tag to disable the default animation for this element. The value initial can be used prohibit the generation of styling for a new fade animation and instead use the browser’s default animation.

Ever asked yourself how Astro’s fade differs from the browser`s default animation? Well,

  • it works for browsers without native view transition support,
  • it does define entry and exit animations but no group / morph animation and
  • it is with 180ms about 30% faster than the browser’s 250ms fade and has a steep, kind of ease-in-out timing-function instead of the default ease.

Each transition:animate directive (with a value other than “none” or “initial”) generates a stylesheet with animations for the HTML element to which it is added. Astro directly supports the definition of exit animations for old images and entry animations for new images. Since Astro’s fallback simulation cannot support morph animations, no CSS is generated for them. As a result, browsers with native support for view transitions show the default morph animation and other browsers only show the exit and entry animations, but no morph animation.

To enable animations during view transitions even for browsers that do not support view transitions natively, Astro uses a special scheme of custom data attributes and CSS rules. This is the general structure that Astro generates if you use transition:animate (explicit or implicit)

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[data-astro-transition-scope="astro-fgupyhpc-2"] { view-transition-name: myName; }
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@layer astro {
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  ::view-transition-old(myName) { ... forward exit animation ... }
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  ::view-transition-new(myName) { ... forward entry animation ... }
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  [data-astro-transition=back]::view-transition-old(myName) { ... backward exit animation ... }
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  [data-astro-transition=back]::view-transition-new(myName) { ... backward entry animation ... }
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}
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[data-astro-transition-fallback="old"] [data-astro-transition-scope="astro-fgupyhpc-2"],
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[data-astro-transition-fallback="old"][data-astro-transition-scope="astro-fgupyhpc-2"] { ... forward exit animation ... }
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[data-astro-transition-fallback="new"] [data-astro-transition-scope="astro-fgupyhpc-2"],
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[data-astro-transition-fallback="new"][data-astro-transition-scope="astro-fgupyhpc-2"] { ... forward entry animation ... }
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[data-astro-transition=back][data-astro-transition-fallback="old"] [data-astro-transition-scope="astro-fgupyhpc-2"],
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[data-astro-transition=back][data-astro-transition-fallback="old"][data-astro-transition-scope="astro-fgupyhpc-2"] { ... backward exit animation ... }
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[data-astro-transition=back][data-astro-transition-fallback="new"] [data-astro-transition-scope="astro-fgupyhpc-2"],
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[data-astro-transition=back][data-astro-transition-fallback="new"][data-astro-transition-scope="astro-fgupyhpc-2"] { ... backward entry animation ... }

In the stylesheet above, the first line is the one introduced in the last section for transition:name. The CSS in the Astro layer defines entry and exit animations that are triggered when the view transition pseudo-elements are inserted into the DOM. During view transitions, Astro also sets the data-astro-transition attribute on the <html> document element. In this way, it is possible to give the browser back navigation a styling that differs from the forward animation. Browsers without native view transition support will ignore these declarations all together as they do not recognize the ::view-transition pseudo elements.

The lines in the lower part of the style sheet are for browsers that do not natively support view transitions. Astro’s client-side router recognizes those browsers and if the <ViewTransitions/> fallback option isn’t switched off, it inserts the data-astro-transition-fallback attribute for them to trigger the animation. As this does not happen in browsers with native support for view transitions, the two parts of the stylesheet are mutually exclusive.

In Astro’s fallback simulation, the exit and entry animations do not run in parallel but one after the other:

data-astro-transition-fallback is first set to old, directly before the astro:before-swap event, which is raised as soon as all animations triggered in this way have ended. Right after the astro:after-swap event data-astro-transition-fallback is then set to new. As soon as all animations triggered in this way have ended, the attribute is removed. The variant without a space in front of the data-astro-transition-scope selector is required to be able to select transitions on the <html> element itself.

transition:persist

This directive↗ has nothing to do with animations. Its purpose is to control how the standard swap() operation replaces the current DOM with the version of the target page.

It is intended for situations where you have an element with a certain state on the page where the navigation starts and you want to transfer this element, including its state, to the next page.

This might come in handy for example to preserve the scroll position of a lengthy navigation bar during transitions or a playing audio/video element.

You mark this element on the original page with transition:persist="a-name" and use the same attribute and name for an element in the target DOM. The element itself does not have to be the same, not even similar. The element in the original DOM is used to replace the equally named element in the target DOM. For Astro Islands↗ you can set the additional transition:persist-props attribute if you want to keep the original properties. Otherwise the island’s properties are reevaluated on copy.

Both directives can be simply replaced by their corresponding custom data attributes: transition:persist="x" translates to data-astro-transition-persist="x" and transition:persist-props translates to data-astro-transition-persist-props. This is can be helpful in places where the transition directives are not recognized e.g. in *.mdx and *.html files or framework components.

Footnotes

  1. Transition directives are not supported in framework components, *.md or *.mdx files. You use these directives to define what should be animated during view transitions and how. ↩