From 1204206ce968f2c450d6d00d3828a7c4f850675c Mon Sep 17 00:00:00 2001
From: Joe Haddad <joe.haddad@zeit.co>
Date: Fri, 16 Oct 2020 03:32:04 -0400
Subject: [PATCH] adjust code

---
 packages/next/client/index.tsx              |  4 +--
 packages/next/client/performance-relayer.ts | 31 +++++++++++++++++----
 2 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/packages/next/client/index.tsx b/packages/next/client/index.tsx
index 73c8136efde6..43fae5b351d4 100644
--- a/packages/next/client/index.tsx
+++ b/packages/next/client/index.tsx
@@ -743,9 +743,7 @@ function Root({
   // We should ask to measure the Web Vitals after rendering completes so we
   // don't cause any hydration delay:
   React.useEffect(() => {
-    if (onPerfEntry) {
-      measureWebVitals(onPerfEntry)
-    }
+    measureWebVitals(onPerfEntry)
   }, [])
   return children as React.ReactElement
 }
diff --git a/packages/next/client/performance-relayer.ts b/packages/next/client/performance-relayer.ts
index f1b43fa7e10e..1a2dfe1cb899 100644
--- a/packages/next/client/performance-relayer.ts
+++ b/packages/next/client/performance-relayer.ts
@@ -4,13 +4,32 @@ import {
   getFID,
   getLCP,
   getTTFB,
+  Metric,
   ReportHandler,
 } from 'web-vitals'
 
-export default (onPerfEntry: ReportHandler) => {
-  getCLS(onPerfEntry)
-  getFID(onPerfEntry)
-  getFCP(onPerfEntry)
-  getLCP(onPerfEntry)
-  getTTFB(onPerfEntry)
+let isRegistered = false
+let userReportHandler: ReportHandler | undefined
+
+function onReport(metric: Metric) {
+  if (userReportHandler) {
+    userReportHandler(metric)
+  }
+}
+
+export default (onPerfEntry?: ReportHandler) => {
+  // Update function if it changes:
+  userReportHandler = onPerfEntry
+
+  // Only register listeners once:
+  if (isRegistered) {
+    return
+  }
+  isRegistered = true
+
+  getCLS(onReport)
+  getFID(onReport)
+  getFCP(onReport)
+  getLCP(onReport)
+  getTTFB(onReport)
 }