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Timothy Yung
ea3ac58669
## Summary We're seeing certain situations in React Native development where ref callbacks in `<Activity mode="hidden">` are sometimes invoked exactly once with `null` without ever being called with a "current" value. This violates the contract for refs because refs are expected to always attach before detach (and to always eventually detach after attach). This is *particularly* bad for refs that return cleanup functions, because refs that return cleanup functions expect to never be invoked with `null`. This bug causes such refs to be invoked with `null` (because since `safelyAttachRef` was never called, `safelyDetachRef` thinks the ref does not return a cleanup function and invokes it with `null`). This fix makes use of `offscreenSubtreeWasHidden` in `commitDeletionEffectsOnFiber`, similar to how https://github.com/facebook/react/commit/ec52a5698e2dfea7050a0b015f0b79abfb2d81b7 did this for `commitDeletionEffectsOnFiber`. ## How did you test this change? We were able to isolate the repro steps to isolate the React Native experimental changes. However, the repro steps depend on Fast Refresh. ``` function callbackRef(current) { // Called once with `current` as null, upon triggering Fast Refresh. } <Activity mode="hidden"> <View ref={callbackRef} />; </Activity> ``` Ideally, we would have a unit test that verifies this behavior without Fast Refresh. (We have evidence that this bug occurs without Fast Refresh in real product implementations. However, we have not successfully deduced the root cause, yet.) This PR currently includes a unit test that reproduces the Fast Refresh scenario, which is also demonstrated in this CodeSandbox: https://codesandbox.io/p/sandbox/hungry-darkness-33wxy7 Verified unit tests pass: ``` $ yarn $ yarn test # Run with `-r=www-classic` for `enableScopeAPI` tests. $ yarn test -r=www-classic ``` Verified on the internal React Native development branch that the bug no longer repros. --------- Co-authored-by: Rick Hanlon <rickhanlonii@fb.com>
react-refresh
This package implements the wiring necessary to integrate Fast Refresh into bundlers. Fast Refresh is a feature that lets you edit React components in a running application without losing their state. It is similar to an old feature known as "hot reloading", but Fast Refresh is more reliable and officially supported by React.
This package is primarily aimed at developers of bundler plugins. If you’re working on one, here is a rough guide for Fast Refresh integration using this package.