Repository: facebook/react
Pull request: [DevTools] Remove use of .alternate in root and recordProfilingDurations

Ideally we shouldn't use the `.alternate` to access previous state because ideally Fibers shouldn't have alternates.

The only case it's ok to use it is when it is used to identity the stateful part of a component's identity. In a non-alternate Fiber model there would instead be another object that represents instance but in the current model it's modeled by the pair. 

It's not ok is to get the previous state of the tree since that would not live on the stateful part.

We don't generally need this though because we have the previous state on instance.data before updating it, or passed from above.

Changed files:
filename: packages/react-devtools-shared/src/backend/fiber/renderer.js
patch: @@ -2371,7 +2371,7 @@ export function attach(
     }
 
     if (isProfilingSupported) {
-      recordProfilingDurations(fiberInstance);
+      recordProfilingDurations(fiberInstance, null);
     }
     return fiberInstance;
   }
@@ -2937,21 +2937,22 @@ export function attach(
     removeChild(instance, null);
   }
 
-  function recordProfilingDurations(fiberInstance: FiberInstance) {
+  function recordProfilingDurations(
+    fiberInstance: FiberInstance,
+    prevFiber: null | Fiber,
+  ) {
     const id = fiberInstance.id;
     const fiber = fiberInstance.data;
     const {actualDuration, treeBaseDuration} = fiber;
 
     fiberInstance.treeBaseDuration = treeBaseDuration || 0;
 
     if (isProfiling) {
-      const {alternate} = fiber;
-
       // It's important to update treeBaseDuration even if the current Fiber did not render,
       // because it's possible that one of its descendants did.
       if (
-        alternate == null ||
-        treeBaseDuration !== alternate.treeBaseDuration
+        prevFiber == null ||
+        treeBaseDuration !== prevFiber.treeBaseDuration
       ) {
         // Tree base duration updates are included in the operations typed array.
         // So we have to convert them from milliseconds to microseconds so we can send them as ints.
@@ -2963,7 +2964,7 @@ export function attach(
         pushOperation(convertedTreeBaseDuration);
       }
 
-      if (alternate == null || didFiberRender(alternate, fiber)) {
+      if (prevFiber == null || didFiberRender(prevFiber, fiber)) {
         if (actualDuration != null) {
           // The actual duration reported by React includes time spent working on children.
           // This is useful information, but it's also useful to be able to exclude child durations.
@@ -2991,7 +2992,7 @@ export function attach(
           );
 
           if (recordChangeDescriptions) {
-            const changeDescription = getChangeDescription(alternate, fiber);
+            const changeDescription = getChangeDescription(prevFiber, fiber);
             if (changeDescription !== null) {
               if (metadata.changeDescriptions !== null) {
                 metadata.changeDescriptions.set(id, changeDescription);
@@ -3314,8 +3315,6 @@ export function attach(
           // Register the new alternate in case it's not already in.
           fiberToFiberInstanceMap.set(nextChild, fiberInstance);
 
-          // Update the Fiber so we that we always keep the current Fiber on the data.
-          fiberInstance.data = nextChild;
           moveChild(fiberInstance, previousSiblingOfExistingInstance);
 
           if (
@@ -3455,6 +3454,8 @@ export function attach(
     const stashedPrevious = previouslyReconciledSibling;
     const stashedRemaining = remainingReconcilingChildren;
     if (fiberInstance !== null) {
+      // Update the Fiber so we that we always keep the current Fiber on the data.
+      fiberInstance.data = nextFiber;
       if (
         mostRecentlyInspectedElement !== null &&
         mostRecentlyInspectedElement.id === fiberInstance.id &&
@@ -3610,7 +3611,7 @@ export function attach(
         const isProfilingSupported =
           nextFiber.hasOwnProperty('treeBaseDuration');
         if (isProfilingSupported) {
-          recordProfilingDurations(fiberInstance);
+          recordProfilingDurations(fiberInstance, prevFiber);
         }
       }
       if (shouldResetChildren) {
@@ -3681,11 +3682,11 @@ export function attach(
       // If we have not been profiling, then we can just walk the tree and build up its current state as-is.
       hook.getFiberRoots(rendererID).forEach(root => {
         const current = root.current;
-        const alternate = current.alternate;
         const newRoot = createFiberInstance(current);
         rootToFiberInstanceMap.set(root, newRoot);
         idToDevToolsInstanceMap.set(newRoot.id, newRoot);
         fiberToFiberInstanceMap.set(current, newRoot);
+        const alternate = current.alternate;
         if (alternate) {
           fiberToFiberInstanceMap.set(alternate, newRoot);
         }
@@ -3755,20 +3756,22 @@ export function attach(
     priorityLevel: void | number,
   ) {
     const current = root.current;
-    const alternate = current.alternate;
 
+    let prevFiber: null | Fiber = null;
     let rootInstance = rootToFiberInstanceMap.get(root);
     if (!rootInstance) {
       rootInstance = createFiberInstance(current);
       rootToFiberInstanceMap.set(root, rootInstance);
       idToDevToolsInstanceMap.set(rootInstance.id, rootInstance);
       fiberToFiberInstanceMap.set(current, rootInstance);
+      const alternate = current.alternate;
       if (alternate) {
         fiberToFiberInstanceMap.set(alternate, rootInstance);
       }
       currentRootID = rootInstance.id;
     } else {
       currentRootID = rootInstance.id;
+      prevFiber = rootInstance.data;
     }
 
     // Before the traversals, remember to start tracking
@@ -3804,13 +3807,13 @@ export function attach(
       };
     }
 
-    if (alternate) {
+    if (prevFiber !== null) {
       // TODO: relying on this seems a bit fishy.
       const wasMounted =
-        alternate.memoizedState != null &&
-        alternate.memoizedState.element != null &&
+        prevFiber.memoizedState != null &&
+        prevFiber.memoizedState.element != null &&
         // A dehydrated root is not considered mounted
-        alternate.memoizedState.isDehydrated !== true;
+        prevFiber.memoizedState.isDehydrated !== true;
       const isMounted =
         current.memoizedState != null &&
         current.memoizedState.element != null &&
@@ -3822,7 +3825,7 @@ export function attach(
         mountFiberRecursively(current, false);
       } else if (wasMounted && isMounted) {
         // Update an existing root.
-        updateFiberRecursively(rootInstance, current, alternate, false);
+        updateFiberRecursively(rootInstance, current, prevFiber, false);
       } else if (wasMounted && !isMounted) {
         // Unmount an existing root.
         unmountInstanceRecursively(rootInstance);