Performance tool: Warn when interrupting an in-progress tree

packages/react-reconciler/src/ReactDebugFiberPerf.js

@@ -318,8 +318,9 @@ export function stopPhaseTimer(): void {
   }
 }
 
-export function startWorkLoopTimer(): void {
+export function startWorkLoopTimer(nextUnitOfWork: Fiber | null): void {
   if (enableUserTimingAPI) {
+    currentFiber = nextUnitOfWork;
     if (!supportsUserTiming) {
       return;
     }
@@ -332,14 +333,22 @@ export function startWorkLoopTimer(): void {
   }
 }
 
-export function stopWorkLoopTimer(): void {
+export function stopWorkLoopTimer(interruptedBy: Fiber | null): void {
   if (enableUserTimingAPI) {
     if (!supportsUserTiming) {
       return;
     }
-    const warning = commitCountInCurrentWorkLoop > 1
-      ? 'There were cascading updates'
-      : null;
+    let warning = null;
+    if (interruptedBy !== null) {
+      if (interruptedBy.tag === HostRoot) {
+        warning = 'A top-level update interrupted the previous render';
+      } else {
+        const componentName = getComponentName(interruptedBy) || 'Unknown';
+        warning = `An update to ${componentName} interrupted the previous render`;
+      }
+    } else if (commitCountInCurrentWorkLoop > 1) {
+      warning = 'There were cascading updates';
+    }
     commitCountInCurrentWorkLoop = 0;
     // Pause any measurements until the next loop.
     pauseTimers();

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -216,6 +216,9 @@ export default function<T, P, I, TI, PI, C, CC, CX, PL>(
   let isCommitting: boolean = false;
   let isUnmounting: boolean = false;
 
+  // Used for performance tracking.
+  let interruptedBy: Fiber | null = null;
+
   function resetContextStack() {
     // Reset the stack
     reset();
@@ -752,8 +755,6 @@ export default function<T, P, I, TI, PI, C, CC, CX, PL>(
     root: FiberRoot,
     expirationTime: ExpirationTime,
   ): Fiber | null {
-    startWorkLoopTimer();
-
     invariant(
       !isWorking,
       'renderRoot was called recursively. This error is likely caused ' +
@@ -772,7 +773,7 @@ export default function<T, P, I, TI, PI, C, CC, CX, PL>(
       expirationTime !== nextRenderExpirationTime ||
       nextUnitOfWork === null
     ) {
-      // This is a restart. Reset the stack.
+      // Reset the stack and start working from the root.
       resetContextStack();
       nextRoot = root;
       nextRenderExpirationTime = expirationTime;
@@ -783,6 +784,8 @@ export default function<T, P, I, TI, PI, C, CC, CX, PL>(
       );
     }
 
+    startWorkLoopTimer(nextUnitOfWork);
+
     let didError = false;
     let error = null;
     if (__DEV__) {
@@ -865,12 +868,12 @@ export default function<T, P, I, TI, PI, C, CC, CX, PL>(
     const uncaughtError = firstUncaughtError;
 
     // We're done performing work. Time to clean up.
+    stopWorkLoopTimer(interruptedBy);
+    interruptedBy = null;
     isWorking = false;
     didFatal = false;
     firstUncaughtError = null;
 
-    stopWorkLoopTimer();
-
     if (uncaughtError !== null) {
       onUncaughtError(uncaughtError);
     }
@@ -1198,7 +1201,11 @@ export default function<T, P, I, TI, PI, C, CC, CX, PL>(
             root === nextRoot &&
             expirationTime <= nextRenderExpirationTime
           ) {
-            // This is an interruption. Restart the root from the top.
+            // Restart the root from the top.
+            if (nextUnitOfWork !== null) {
+              // This is an interruption. (Used for performance tracking.)
+              interruptedBy = fiber;
+            }
             nextRoot = null;
             nextUnitOfWork = null;
             nextRenderExpirationTime = NoWork;

packages/react-reconciler/src/__tests__/ReactIncrementalPerf-test.js

@@ -498,4 +498,18 @@ describe('ReactDebugFiberPerf', () => {
     });
     expect(getFlameChart()).toMatchSnapshot();
   });
+
+  it('warns if an in-progress update is interrupted', () => {
+    function Foo() {
+      return <span />;
+    }
+
+    ReactNoop.render(<Foo />);
+    ReactNoop.flushUnitsOfWork(2);
+    ReactNoop.flushSync(() => {
+      ReactNoop.render(<Foo />);
+    });
+    ReactNoop.flush();
+    expect(getFlameChart()).toMatchSnapshot();
+  });
 });

packages/react-reconciler/src/__tests__/__snapshots__/ReactIncrementalPerf-test.js.snap

@@ -243,6 +243,17 @@ exports[`ReactDebugFiberPerf skips parents during setState 1`] = `
 "
 `;
 
+exports[`ReactDebugFiberPerf supports portals 1`] = `
+"⚛ (React Tree Reconciliation)
+  ⚛ Parent [mount]
+    ⚛ Child [mount]
+
+⚛ (Committing Changes)
+  ⚛ (Committing Host Effects: 2 Total)
+  ⚛ (Calling Lifecycle Methods: 0 Total)
+"
+`;
+
 exports[`ReactDebugFiberPerf supports returns 1`] = `
 "⚛ (React Tree Reconciliation)
   ⚛ App [mount]
@@ -260,14 +271,22 @@ exports[`ReactDebugFiberPerf supports returns 1`] = `
 "
 `;
 
-exports[`ReactDebugFiberPerf supports portals 1`] = `
+exports[`ReactDebugFiberPerf warns if an in-progress update is interrupted 1`] = `
 "⚛ (React Tree Reconciliation)
-  ⚛ Parent [mount]
-    ⚛ Child [mount]
+  ⚛ Foo [mount]
+
+⛔ (React Tree Reconciliation) Warning: A top-level update interrupted the previous render
+  ⚛ Foo [mount]
 
 ⚛ (Committing Changes)
-  ⚛ (Committing Host Effects: 2 Total)
+  ⚛ (Committing Host Effects: 1 Total)
   ⚛ (Calling Lifecycle Methods: 0 Total)
+
+⚛ (React Tree Reconciliation)
+
+⚛ (Committing Changes)
+  ⚛ (Committing Host Effects: 1 Total)
+  ⚛ (Calling Lifecycle Methods: 1 Total)
 "
 `;