Repository: facebook/react
Pull request: Replaced network.onRequestFinished() caching with network.getHAR()

Replaced `network.onRequestFinished()` caching with `network.getHAR()` so that we can avoid redundantly (pre) caching JavaScript content. In the event that the HAR log doesn't contain a match, we'll fall back to fetching from the Network (and hoping for a cache hit from that layer).

I've tested both internally (internal Facebook DEV server) and externally (Code Sandbox) and it seems like this approach results in cache hits, so long as DevTools is opened when the page loads. (Otherwise it falls back to `fetch()`.)

---

Posting as a possible alternative approach to #22282.

Changed files:
filename: packages/react-devtools-extensions/src/main.js
patch: @@ -19,33 +19,13 @@ import {
   localStorageSetItem,
 } from 'react-devtools-shared/src/storage';
 import DevTools from 'react-devtools-shared/src/devtools/views/DevTools';
+import {__DEBUG__} from 'react-devtools-shared/src/constants';
 
 const LOCAL_STORAGE_SUPPORTS_PROFILING_KEY =
   'React::DevTools::supportsProfiling';
 
 const isChrome = getBrowserName() === 'Chrome';
 
-const cachedNetworkEvents = new Map();
-
-// Cache JavaScript resources as the page loads them.
-// This helps avoid unnecessary duplicate requests when hook names are parsed.
-// Responses with a Vary: 'Origin' might not match future requests.
-// This lets us avoid a possible (expensive) cache miss.
-// For more info see: github.com/facebook/react/pull/22198
-chrome.devtools.network.onRequestFinished.addListener(
-  function onRequestFinished(event) {
-    if (event.request.method === 'GET') {
-      switch (event.response.content.mimeType) {
-        case 'application/javascript':
-        case 'application/x-javascript':
-        case 'text/javascript':
-          cachedNetworkEvents.set(event.request.url, event);
-          break;
-      }
-    }
-  },
-);
-
 let panelCreated = false;
 
 // The renderer interface can't read saved component filters directly,
@@ -233,56 +213,113 @@ function createPanelIfReactLoaded() {
           }
         };
 
+        let debugIDCounter = 0;
+
         // For some reason in Firefox, chrome.runtime.sendMessage() from a content script
         // never reaches the chrome.runtime.onMessage event listener.
         let fetchFileWithCaching = null;
         if (isChrome) {
-          // Fetching files from the extension won't make use of the network cache
-          // for resources that have already been loaded by the page.
-          // This helper function allows the extension to request files to be fetched
-          // by the content script (running in the page) to increase the likelihood of a cache hit.
-          fetchFileWithCaching = url => {
-            const event = cachedNetworkEvents.get(url);
-            if (event != null) {
-              // If this resource has already been cached locally,
-              // skip the network queue (which might not be a cache hit anyway)
-              // and just use the cached response.
-              return new Promise(resolve => {
-                event.getContent(content => resolve(content));
-              });
+          const fetchFromNetworkCache = (url, resolve, reject) => {
+            // Debug ID allows us to avoid re-logging (potentially long) URL strings below,
+            // while also still associating (potentially) interleaved logs with the original request.
+            let debugID = null;
+
+            if (__DEBUG__) {
+              debugID = debugIDCounter++;
+              console.log(`[main] fetchFromNetworkCache(${debugID})`, url);
             }
 
-            // If DevTools was opened after the page started loading,
-            // we may have missed some requests.
-            // So fall back to a fetch() and hope we get a cached response.
-
-            return new Promise((resolve, reject) => {
-              function onPortMessage({payload, source}) {
-                if (source === 'react-devtools-content-script') {
-                  switch (payload?.type) {
-                    case 'fetch-file-with-cache-complete':
-                      chrome.runtime.onMessage.removeListener(onPortMessage);
-                      resolve(payload.value);
-                      break;
-                    case 'fetch-file-with-cache-error':
-                      chrome.runtime.onMessage.removeListener(onPortMessage);
-                      reject(payload.value);
-                      break;
+            chrome.devtools.network.getHAR(harLog => {
+              for (let i = 0; i < harLog.entries.length; i++) {
+                const entry = harLog.entries[i];
+                if (url === entry.request.url) {
+                  if (__DEBUG__) {
+                    console.log(
+                      `[main] fetchFromNetworkCache(${debugID}) Found matching URL in HAR`,
+                      url,
+                    );
                   }
+
+                  entry.getContent(content => {
+                    if (content) {
+                      if (__DEBUG__) {
+                        console.log(
+                          `[main] fetchFromNetworkCache(${debugID}) Content retrieved`,
+                        );
+                      }
+
+                      resolve(content);
+                    } else {
+                      if (__DEBUG__) {
+                        console.log(
+                          `[main] fetchFromNetworkCache(${debugID}) Invalid content returned by getContent()`,
+                          content,
+                        );
+                      }
+
+                      // Edge case where getContent() returned null; fall back to fetch.
+                      fetchFromPage(url, resolve);
+                    }
+                  });
+
+                  return;
                 }
               }
 
-              chrome.runtime.onMessage.addListener(onPortMessage);
+              if (__DEBUG__) {
+                console.log(
+                  `[main] fetchFromNetworkCache(${debugID}) No cached request found in getHAR()`,
+                );
+              }
 
-              chrome.devtools.inspectedWindow.eval(`
-                window.postMessage({
-                  source: 'react-devtools-extension',
-                  payload: {
-                    type: 'fetch-file-with-cache',
-                    url: "${url}",
-                  },
-                });
-              `);
+              // No matching URL found; fall back to fetch.
+              fetchFromPage(url, resolve);
+            });
+          };
+
+          const fetchFromPage = (url, resolve, reject) => {
+            if (__DEBUG__) {
+              console.log('[main] fetchFromPage()', url);
+            }
+
+            function onPortMessage({payload, source}) {
+              if (source === 'react-devtools-content-script') {
+                switch (payload?.type) {
+                  case 'fetch-file-with-cache-complete':
+                    chrome.runtime.onMessage.removeListener(onPortMessage);
+                    resolve(payload.value);
+                    break;
+                  case 'fetch-file-with-cache-error':
+                    chrome.runtime.onMessage.removeListener(onPortMessage);
+                    reject(payload.value);
+                    break;
+                }
+              }
+            }
+
+            chrome.runtime.onMessage.addListener(onPortMessage);
+
+            chrome.devtools.inspectedWindow.eval(`
+              window.postMessage({
+                source: 'react-devtools-extension',
+                payload: {
+                  type: 'fetch-file-with-cache',
+                  url: "${url}",
+                },
+              });
+            `);
+          };
+
+          // Fetching files from the extension won't make use of the network cache
+          // for resources that have already been loaded by the page.
+          // This helper function allows the extension to request files to be fetched
+          // by the content script (running in the page) to increase the likelihood of a cache hit.
+          fetchFileWithCaching = url => {
+            return new Promise((resolve, reject) => {
+              // Try fetching from the Network cache first.
+              // If DevTools was opened after the page started loading, we may have missed some requests.
+              // So fall back to a fetch() from the page and hope we get a cached response that way.
+              fetchFromNetworkCache(url, resolve, reject);
             });
           };
         }
@@ -441,9 +478,6 @@ function createPanelIfReactLoaded() {
 
       // Re-initialize DevTools panel when a new page is loaded.
       chrome.devtools.network.onNavigated.addListener(function onNavigated() {
-        // Clear cached requests when a new page is opened.
-        cachedNetworkEvents.clear();
-
         // Re-initialize saved filters on navigation,
         // since global values stored on window get reset in this case.
         syncSavedPreferences();
@@ -460,9 +494,6 @@ function createPanelIfReactLoaded() {
 
 // Load (or reload) the DevTools extension when the user navigates to a new page.
 function checkPageForReact() {
-  // Clear cached requests when a new page is opened.
-  cachedNetworkEvents.clear();
-
   syncSavedPreferences();
   createPanelIfReactLoaded();
 }