Reconcile element types of lazy component yielding the same type (#20357)

sebmarkbage · web-flow · commit 5711811da17c · 2020-12-01T07:14:04.000-08:00
* Reconcile element types of lazy component yielding the same type

* Add some legacy mode and suspense boundary flushing tests

* Fix infinite loop in legacy mode

In legacy mode we typically commit the suspending fiber and then rerender
the nearest boundary to render the fallback in a separate commit.

We can't do that when the boundary itself suspends because when we try to
do the second pass, it'll suspend again and infinite loop.

Interestingly the legacy semantics are not needed in this case because
they exist to let an existing partial render fully commit its partial state.

In this case there's no partial state, so we can just render the fallback
immediately instead.

* Check fast refresh compatibility first

resolveLazy can suspend and if it does, it can resuspend. Fast refresh
assumes that we don't resuspend. Instead it relies on updating the inner
component later.

* Use timers instead of act to force fallbacks to show
diff --git a/packages/react-reconciler/src/ReactChildFiber.new.js b/packages/react-reconciler/src/ReactChildFiber.new.js
@@ -246,6 +246,12 @@ function warnOnFunctionType(returnFiber: Fiber) {
   }
 }
 
+function resolveLazy(lazyType) {
+  const payload = lazyType._payload;
+  const init = lazyType._init;
+  return init(payload);
+}
+
 // This wrapper function exists because I expect to clone the code in each path
 // to be able to optimize each path individually by branching early. This needs
 // a compiler or we can do it manually. Helpers that don't need this branching
@@ -383,11 +389,32 @@ function ChildReconciler(shouldTrackSideEffects) {
     element: ReactElement,
     lanes: Lanes,
   ): Fiber {
+    const elementType = element.type;
+    if (elementType === REACT_FRAGMENT_TYPE) {
+      return updateFragment(
+        returnFiber,
+        current,
+        element.props.children,
+        lanes,
+        element.key,
+      );
+    }
     if (current !== null) {
       if (
-        current.elementType === element.type ||
+        current.elementType === elementType ||
         // Keep this check inline so it only runs on the false path:
-        (__DEV__ ? isCompatibleFamilyForHotReloading(current, element) : false)
+        (__DEV__
+          ? isCompatibleFamilyForHotReloading(current, element)
+          : false) ||
+        // Lazy types should reconcile their resolved type.
+        // We need to do this after the Hot Reloading check above,
+        // because hot reloading has different semantics than prod because
+        // it doesn't resuspend. So we can't let the call below suspend.
+        (enableLazyElements &&
+          typeof elementType === 'object' &&
+          elementType !== null &&
+          elementType.$$typeof === REACT_LAZY_TYPE &&
+          resolveLazy(elementType) === current.type)
       ) {
         // Move based on index
         const existing = useFiber(current, element.props);
@@ -551,15 +578,6 @@ function ChildReconciler(shouldTrackSideEffects) {
       switch (newChild.$$typeof) {
         case REACT_ELEMENT_TYPE: {
           if (newChild.key === key) {
-            if (newChild.type === REACT_FRAGMENT_TYPE) {
-              return updateFragment(
-                returnFiber,
-                oldFiber,
-                newChild.props.children,
-                lanes,
-                key,
-              );
-            }
             return updateElement(returnFiber, oldFiber, newChild, lanes);
           } else {
             return null;
@@ -622,15 +640,6 @@ function ChildReconciler(shouldTrackSideEffects) {
             existingChildren.get(
               newChild.key === null ? newIdx : newChild.key,
             ) || null;
-          if (newChild.type === REACT_FRAGMENT_TYPE) {
-            return updateFragment(
-              returnFiber,
-              matchedFiber,
-              newChild.props.children,
-              lanes,
-              newChild.key,
-            );
-          }
           return updateElement(returnFiber, matchedFiber, newChild, lanes);
         }
         case REACT_PORTAL_TYPE: {
@@ -1101,39 +1110,44 @@ function ChildReconciler(shouldTrackSideEffects) {
       // TODO: If key === null and child.key === null, then this only applies to
       // the first item in the list.
       if (child.key === key) {
-        switch (child.tag) {
-          case Fragment: {
-            if (element.type === REACT_FRAGMENT_TYPE) {
-              deleteRemainingChildren(returnFiber, child.sibling);
-              const existing = useFiber(child, element.props.children);
-              existing.return = returnFiber;
-              if (__DEV__) {
-                existing._debugSource = element._source;
-                existing._debugOwner = element._owner;
-              }
-              return existing;
+        const elementType = element.type;
+        if (elementType === REACT_FRAGMENT_TYPE) {
+          if (child.tag === Fragment) {
+            deleteRemainingChildren(returnFiber, child.sibling);
+            const existing = useFiber(child, element.props.children);
+            existing.return = returnFiber;
+            if (__DEV__) {
+              existing._debugSource = element._source;
+              existing._debugOwner = element._owner;
             }
-            break;
+            return existing;
           }
-          default: {
-            if (
-              child.elementType === element.type ||
-              // Keep this check inline so it only runs on the false path:
-              (__DEV__
-                ? isCompatibleFamilyForHotReloading(child, element)
-                : false)
-            ) {
-              deleteRemainingChildren(returnFiber, child.sibling);
-              const existing = useFiber(child, element.props);
-              existing.ref = coerceRef(returnFiber, child, element);
-              existing.return = returnFiber;
-              if (__DEV__) {
-                existing._debugSource = element._source;
-                existing._debugOwner = element._owner;
-              }
-              return existing;
+        } else {
+          if (
+            child.elementType === elementType ||
+            // Keep this check inline so it only runs on the false path:
+            (__DEV__
+              ? isCompatibleFamilyForHotReloading(child, element)
+              : false) ||
+            // Lazy types should reconcile their resolved type.
+            // We need to do this after the Hot Reloading check above,
+            // because hot reloading has different semantics than prod because
+            // it doesn't resuspend. So we can't let the call below suspend.
+            (enableLazyElements &&
+              typeof elementType === 'object' &&
+              elementType !== null &&
+              elementType.$$typeof === REACT_LAZY_TYPE &&
+              resolveLazy(elementType) === child.type)
+          ) {
+            deleteRemainingChildren(returnFiber, child.sibling);
+            const existing = useFiber(child, element.props);
+            existing.ref = coerceRef(returnFiber, child, element);
+            existing.return = returnFiber;
+            if (__DEV__) {
+              existing._debugSource = element._source;
+              existing._debugOwner = element._owner;
             }
-            break;
+            return existing;
           }
         }
         // Didn't match.
diff --git a/packages/react-reconciler/src/ReactChildFiber.old.js b/packages/react-reconciler/src/ReactChildFiber.old.js
@@ -246,6 +246,12 @@ function warnOnFunctionType(returnFiber: Fiber) {
   }
 }
 
+function resolveLazy(lazyType) {
+  const payload = lazyType._payload;
+  const init = lazyType._init;
+  return init(payload);
+}
+
 // This wrapper function exists because I expect to clone the code in each path
 // to be able to optimize each path individually by branching early. This needs
 // a compiler or we can do it manually. Helpers that don't need this branching
@@ -383,11 +389,32 @@ function ChildReconciler(shouldTrackSideEffects) {
     element: ReactElement,
     lanes: Lanes,
   ): Fiber {
+    const elementType = element.type;
+    if (elementType === REACT_FRAGMENT_TYPE) {
+      return updateFragment(
+        returnFiber,
+        current,
+        element.props.children,
+        lanes,
+        element.key,
+      );
+    }
     if (current !== null) {
       if (
-        current.elementType === element.type ||
+        current.elementType === elementType ||
         // Keep this check inline so it only runs on the false path:
-        (__DEV__ ? isCompatibleFamilyForHotReloading(current, element) : false)
+        (__DEV__
+          ? isCompatibleFamilyForHotReloading(current, element)
+          : false) ||
+        // Lazy types should reconcile their resolved type.
+        // We need to do this after the Hot Reloading check above,
+        // because hot reloading has different semantics than prod because
+        // it doesn't resuspend. So we can't let the call below suspend.
+        (enableLazyElements &&
+          typeof elementType === 'object' &&
+          elementType !== null &&
+          elementType.$$typeof === REACT_LAZY_TYPE &&
+          resolveLazy(elementType) === current.type)
       ) {
         // Move based on index
         const existing = useFiber(current, element.props);
@@ -551,15 +578,6 @@ function ChildReconciler(shouldTrackSideEffects) {
       switch (newChild.$$typeof) {
         case REACT_ELEMENT_TYPE: {
           if (newChild.key === key) {
-            if (newChild.type === REACT_FRAGMENT_TYPE) {
-              return updateFragment(
-                returnFiber,
-                oldFiber,
-                newChild.props.children,
-                lanes,
-                key,
-              );
-            }
             return updateElement(returnFiber, oldFiber, newChild, lanes);
           } else {
             return null;
@@ -622,15 +640,6 @@ function ChildReconciler(shouldTrackSideEffects) {
             existingChildren.get(
               newChild.key === null ? newIdx : newChild.key,
             ) || null;
-          if (newChild.type === REACT_FRAGMENT_TYPE) {
-            return updateFragment(
-              returnFiber,
-              matchedFiber,
-              newChild.props.children,
-              lanes,
-              newChild.key,
-            );
-          }
           return updateElement(returnFiber, matchedFiber, newChild, lanes);
         }
         case REACT_PORTAL_TYPE: {
@@ -1101,39 +1110,43 @@ function ChildReconciler(shouldTrackSideEffects) {
       // TODO: If key === null and child.key === null, then this only applies to
       // the first item in the list.
       if (child.key === key) {
-        switch (child.tag) {
-          case Fragment: {
-            if (element.type === REACT_FRAGMENT_TYPE) {
-              deleteRemainingChildren(returnFiber, child.sibling);
-              const existing = useFiber(child, element.props.children);
-              existing.return = returnFiber;
-              if (__DEV__) {
-                existing._debugSource = element._source;
-                existing._debugOwner = element._owner;
-              }
-              return existing;
+        const elementType = element.type;
+        if (elementType === REACT_FRAGMENT_TYPE) {
+          if (child.tag === Fragment) {
+            deleteRemainingChildren(returnFiber, child.sibling);
+            const existing = useFiber(child, element.props.children);
+            existing.return = returnFiber;
+            if (__DEV__) {
+              existing._debugSource = element._source;
+              existing._debugOwner = element._owner;
             }
-            break;
+            return existing;
           }
-          default: {
-            if (
-              child.elementType === element.type ||
-              // Keep this check inline so it only runs on the false path:
-              (__DEV__
-                ? isCompatibleFamilyForHotReloading(child, element)
-                : false)
-            ) {
-              deleteRemainingChildren(returnFiber, child.sibling);
-              const existing = useFiber(child, element.props);
-              existing.ref = coerceRef(returnFiber, child, element);
-              existing.return = returnFiber;
-              if (__DEV__) {
-                existing._debugSource = element._source;
-                existing._debugOwner = element._owner;
-              }
-              return existing;
+        } else {
+          if (
+            child.elementType === elementType ||
+            (__DEV__
+              ? isCompatibleFamilyForHotReloading(child, element)
+              : false) ||
+            // Lazy types should reconcile their resolved type.
+            // We need to do this after the Hot Reloading check above,
+            // because hot reloading has different semantics than prod because
+            // it doesn't resuspend. So we can't let the call below suspend.
+            (enableLazyElements &&
+              typeof elementType === 'object' &&
+              elementType !== null &&
+              elementType.$$typeof === REACT_LAZY_TYPE &&
+              resolveLazy(elementType) === child.type)
+          ) {
+            deleteRemainingChildren(returnFiber, child.sibling);
+            const existing = useFiber(child, element.props);
+            existing.ref = coerceRef(returnFiber, child, element);
+            existing.return = returnFiber;
+            if (__DEV__) {
+              existing._debugSource = element._source;
+              existing._debugOwner = element._owner;
             }
-            break;
+            return existing;
           }
         }
         // Didn't match.
diff --git a/packages/react-reconciler/src/ReactFiberThrow.new.js b/packages/react-reconciler/src/ReactFiberThrow.new.js
@@ -256,7 +256,15 @@ function throwException(
         // Note: It doesn't matter whether the component that suspended was
         // inside a blocking mode tree. If the Suspense is outside of it, we
         // should *not* suspend the commit.
-        if ((workInProgress.mode & BlockingMode) === NoMode) {
+        //
+        // If the suspense boundary suspended itself suspended, we don't have to
+        // do this trick because nothing was partially started. We can just
+        // directly do a second pass over the fallback in this render and
+        // pretend we meant to render that directly.
+        if (
+          (workInProgress.mode & BlockingMode) === NoMode &&
+          workInProgress !== returnFiber
+        ) {
           workInProgress.flags |= DidCapture;
           sourceFiber.flags |= ForceUpdateForLegacySuspense;
 
diff --git a/packages/react-reconciler/src/ReactFiberThrow.old.js b/packages/react-reconciler/src/ReactFiberThrow.old.js
@@ -256,7 +256,15 @@ function throwException(
         // Note: It doesn't matter whether the component that suspended was
         // inside a blocking mode tree. If the Suspense is outside of it, we
         // should *not* suspend the commit.
-        if ((workInProgress.mode & BlockingMode) === NoMode) {
+        //
+        // If the suspense boundary suspended itself suspended, we don't have to
+        // do this trick because nothing was partially started. We can just
+        // directly do a second pass over the fallback in this render and
+        // pretend we meant to render that directly.
+        if (
+          (workInProgress.mode & BlockingMode) === NoMode &&
+          workInProgress !== returnFiber
+        ) {
           workInProgress.flags |= DidCapture;
           sourceFiber.flags |= ForceUpdateForLegacySuspense;
 
diff --git a/packages/react-reconciler/src/__tests__/ReactLazy-test.internal.js b/packages/react-reconciler/src/__tests__/ReactLazy-test.internal.js
