[pull] main from facebook:main #70
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Wait for me to merge, but this has landed everywhere and is ready to remove.
The public API has been deleted a long time ago so this should be unused unless it's used by hacks. It should be replaced with an effect/lifecycle that manually tracks this if you need it. The problem with this API is how the timing implemented because it requires Placement/Hydration flags to be cleared too early. In fact, that's why we also have a separate PlacementDEV flag that works differently. https://github.com/facebook/react/blob/main/packages/react-reconciler/src/ReactFiberCommitWork.js#L2157-L2165 We should be able to remove this code now.
This will provide the opt-in for using [View Transitions](https://developer.mozilla.org/en-US/docs/Web/API/View_Transition_API) in React. View Transitions only trigger for async updates like `startTransition`, `useDeferredValue`, Actions or `<Suspense>` revealing from fallback to content. Synchronous updates provide an opt-out but also guarantee that they commit immediately which View Transitions can't. There's no need to opt-in to View Transitions at the "cause" side like event handlers or actions. They don't know what UI will change and whether that has an animated transition described. Conceptually the `<ViewTransition>` component is like a DOM fragment that transitions its children in its own isolate/snapshot. The API works by wrapping a DOM node or inner component: ```js import {ViewTransition} from 'react'; <ViewTransition><Component /></ViewTransition> ``` The default is `name="auto"` which will automatically assign a `view-transition-name` to the inner DOM node. That way you can add a View Transition to a Component without controlling its DOM nodes styling otherwise. A difference between this and the browser's built-in `view-transition-name: auto` is that switching the DOM nodes within the `<ViewTransition>` component preserves the same name so this example cross-fades between the DOM nodes instead of causing an exit and enter: ```js <ViewTransition>{condition ? <ComponentA /> : <ComponentB />}</ViewTransition> ``` This becomes especially useful with `<Suspense>` as this example cross-fades between Skeleton and Content: ```js <ViewTransition> <Suspense fallback={<Skeleton />}> <Content /> </Suspense> </ViewTransition> ``` Where as this example triggers an exit of the Skeleton and an enter of the Content: ```js <Suspense fallback={<ViewTransition><Skeleton /></ViewTransition>}> <ViewTransition><Content /></ViewTransition> </Suspense> ``` Managing instances and keys becomes extra important. You can also specify an explicit `name` property for example for animating the same conceptual item from one page onto another. However, best practices is to property namespace these since they can easily collide. It's also useful to add an `id` to it if available. ```js <ViewTransition name="my-shared-view"> ``` The model in general is the same as plain `view-transition-name` except React manages a set of heuristics for when to apply it. A problem with the naive View Transitions model is that it overly opts in every boundary that *might* transition into transitioning. This is leads to unfortunate effects like things floating around when unrelated updates happen. This leads the whole document to animate which means that nothing is clickable in the meantime. It makes it not useful for smaller and more local transitions. Best practice is to add `view-transition-name` only right before you're about to need to animate the thing. This is tricky to manage globally on complex apps and is not compositional. Instead we let React manage when a `<ViewTransition>` "activates" and add/remove the `view-transition-name`. This is also when React calls `startViewTransition` behind the scenes while it mutates the DOM. I've come up with a number of heuristics that I think will make a lot easier to coordinate this. The principle is that only if something that updates that particular boundary do we activate it. I hope that one day maybe browsers will have something like these built-in and we can remove our implementation. A `<ViewTransition>` only activates if: - If a mounted Component renders a `<ViewTransition>` within it outside the first DOM node, and it is within the viewport, then that ViewTransition activates as an "enter" animation. This avoids inner "enter" animations trigger when the parent mounts. - If an unmounted Component had a `<ViewTransition>` within it outside the first DOM node, and it was within the viewport, then that ViewTransition activates as an "exit" animation. This avoids inner "exit" animations triggering when the parent unmounts. - If an explicitly named `<ViewTransition name="...">` is deep within an unmounted tree and one with the same name appears in a mounted tree at the same time, then both are activated as a pair, but only if they're both in the viewport. This avoids these triggering "enter" or "exit" animations when going between parents that don't have a pair. - If an already mounted `<ViewTransition>` is visible and a DOM mutation, that might affect how it's painted, happens within its children but outside any nested `<ViewTransition>`. This allows it to "cross-fade" between its updates. - If an already mounted `<ViewTransition>` resizes or moves as the result of direct DOM nodes siblings changing or moving around. This allows insertion, deletion and reorders into a list to animate all children. It is only within one DOM node though, to avoid unrelated changes in the parent to trigger this. If an item is outside the viewport before and after, then it's skipped to avoid things flying across the screen. - If a `<ViewTransition>` boundary changes size, due to a DOM mutation within it, then the parent activates (or the root document if there are no more parents). This ensures that the container can cross-fade to avoid abrupt relayout. This can be avoided by using absolutely positioned children. When this can avoid bubbling to the root document, whatever is not animating is still responsive to clicks during the transition. Conceptually each DOM node has its own default that activates the parent `<ViewTransition>` or no transition if the parent is the root. That means that if you add a DOM node like `<div><ViewTransition><Component /></ViewTransition></div>` this won't trigger an "enter" animation since it was the div that was added, not the ViewTransition. Instead, it might cause a cross-fade of the parent ViewTransition or no transition if it had no parent. This ensures that only explicit boundaries perform coarse animations instead of every single node which is really the benefit of the View Transitions model. This ends up working out well for simple cases like switching between two pages immediately while transitioning one floating item that appears on both pages. Because only the floating item transitions by default. Note that it's possible to add manual `view-transition-name` with CSS or `style={{ viewTransitionName: 'auto' }}` that always transitions as long as something else has a `<ViewTransition>` that activates. For example a `<ViewTransition>` can wrap a whole page for a cross-fade but inside of it an explicit name can be added to something to ensure it animates as a move when something relates else changes its layout. Instead of just cross-fading it along with the Page which would be the default. There's more PRs coming with some optimizations, fixes and expanded APIs. This first PR explores the above core heuristic. --------- Co-authored-by: Sebastian "Sebbie" Silbermann <[email protected]>
…lass (#31999) Stacked on #31975. This is the primary way we recommend styling your View Transitions since it allows for reusable styling such as a CSS library specializing in View Transitions in a way that's composable and without naming conflicts. E.g. ```js <ViewTransition className="enter-slide-in exit-fade-out update-cross-fade"> ``` This doesn't change the HTML `class` attribute. It's not a CSS class. Instead it assign the `view-transition-class` style prop of the underlying DOM node while it's transitioning. You can also just use `<div style={{viewTransitionClass: ...}}>` on the DOM node but it's convenient to control the Transition completely from the outside and conceptually we're transitioning the whole fragment. You can even make Transition components that just wraps existing components. `<RevealTransition><Component /></RevealTransition>` this way. Since you can also have multiple wrappers for different circumstances it allows React's heuristics to use different classes for different scenarios. We'll likely add more options like configuring different classes for different `types` or scenarios that can't be described by CSS alone. ## CSS Modules ```js import transitions from './transitions.module.css'; <ViewTransition className={transitions.bounceIn}>...</ViewTransition> ``` CSS Modules works well with this strategy because you can have globally unique namespaces and define your transitions in the CSS modules as a library that you can import. [As seen in the fixture here.](8b91b37#diff-b4d9854171ffdac4d2c01be92a5eff4f8e9e761e6af953094f99ca243b054a85R11) I did notice an unfortunate bug in how CSS Modules (at least in Webpack) generates class names. Sometimes the `+` character is used in the hash of the class name which is not valid for `view-transition-class` and so it breaks. I had to rename my class names until the hash yielded something different to work around it. Ideally that bug gets fixed soon. ## className, rly? `className` isn't exactly the most loved property name, however, I'm using `className` here too for consistency. Even though in this case there's no direct equivalent DOM property name. The CSS property is named `viewTransitionClass`, but the "viewTransition" prefix is implied by the Component it is on in this case. For most people the fact that this is actually a different namespace than other CSS classes doesn't matter. You'll most just use a CSS library anyway and conceptually you're just assigning classes the same way as `className` on a DOM node. But if we ever rename the `class` prop then we can do that for this one as well.
Stacked on #31975. We're going to recommend that the primary way you style a View Transition is using a View Transition Class (and/or Type). These are only available in the View Transitions v2 spec. When they're not available it's better to fallback to just not animating instead of animating with the wrong styling rules applied. This is already widely supported in Chrome and Safari 18.2. Safari 18.2 usage is still somewhat low but it's rolling out quickly as we speak. A way to detect this is by just passing the object form to `startViewTransition` which throws if it's an earlier version. The object form is required for `types` but luckily classes rolled out at the same time. Therefore we're only indirectly detecting class support. This means that in practice Safari 18.0 and 18.1 won't animate. We could try to only apply the feature detection if you're actually using classes or types, but that would create an unfortunate ecosystem burden to try to support names. It also leads to flaky effects when only some animations work. Better to just disable them all. Firefox has yet to ship anything. We'll have to look out for how the feature detection happens there and if they roll things out in different order but if you ship late, you deal with web compat as the ball lies.
…tion to finish (#32002) Stacked on #31975. View Transitions cannot handle interruptions in that if you start a new one before the previous one has finished, it just stops and then restarts. It doesn't seamlessly transition into the new transition. This is generally considered a bad thing but I actually think it's quite good for fire-and-forget animations (gestures is another story). There are too many examples of bad animations in fast interactions because the scenario wasn't predicted. Like overlapping toasts or stacked layers that look bad. The only case interrupts tend to work well is when you do a strict reversal of an animation like returning to the page you just left or exiting a modal just being opened. However, we're limited by the platform even in that regard. I think one reason interruptions have traditionally been seen as good is because it's hard if you have a synchronous framework to not interrupt since your application state has already moved on. We don't have that limitation since we can suspend commits. We can do all the work to prepare for the next commit by rendering while the animation is going but then delay the commit until the previous one finishes. Another technical limitation earlier animation libraries suffered from is only have the option to either interrupt or sequence animations since it's modeling just one change set. Like showing one toast at a time. That's bad. We don't have that limitation because we can interrupt a previously suspended commit and start working on a new one instead. That's what we do for suspended transitions in general. The net effect is that we batch the commits. Therefore if you get multiple toasts flying in fast, they can animate as a batch in together all at once instead of overlapping slightly or being staggered. Interruptions (often) bad. Staggered animations bad. Batched animations good. This PR stashes the currently active View Transition with an expando on the container that's animating (currently always document). This is similar to what we do with event handlers etc. We reason we do this with an expando is that if you have multiple Reacts on the same page they need to wait for each other. However, one of those might also be the SSR runtime. So this lets us wait for the SSR runtime's animations to finish before starting client ones. This could really be a more generic name since this should ideally be shared across frameworks. It's kind of strange that this property doesn't already exist in the DOM given that there can only be one. It would be useful to be able to coordinate this across libraries.
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