Repository: facebook/react
Pull request: Support highlights for React Native apps in dev tools

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## Summary

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 Explain the **motivation** for making this change. What existing problem does the pull request solve?
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This pull request emit the trace update events `drawTraceUpdates` with the trace frame information when the trace update drawer runs outside of web environment. This allows React Devtool running in mobile or other platforms have a chance to render such highlights and provide similar feature on web to provide re-render highlights. This is a feature needed for identifying unnecessary re-renders.

## How did you test this change?

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I tested this change with Flipper desktop app running against mobile app, and verified that the event with correct array of frames are passing through properly.

Changed files:
filename: packages/react-devtools-core/src/standalone.js
patch: @@ -262,6 +262,7 @@ function initialize(socket: WebSocket) {
   store = new Store(bridge, {
     checkBridgeProtocolCompatibility: true,
     supportsNativeInspection: true,
+    supportsTraceUpdates: true,
   });
 
   log('Connected');

filename: packages/react-devtools-shared/src/backend/agent.js
patch: @@ -148,6 +148,8 @@ export default class Agent extends EventEmitter<{
   stopInspectingNative: [],
   shutdown: [],
   traceUpdates: [Set<NativeType>],
+  drawTraceUpdates: [Array<NativeType>],
+  disableTraceUpdates: [],
 }> {
   _bridge: BackendBridge;
   _isProfiling: boolean = false;

filename: packages/react-devtools-shared/src/backend/views/TraceUpdates/canvas.js
patch: @@ -10,6 +10,7 @@
 import type {Data} from './index';
 import type {Rect} from '../utils';
 import type {NativeType} from '../../types';
+import type Agent from '../../agent';
 
 const OUTLINE_COLOR = '#f0f0f0';
 
@@ -29,7 +30,17 @@ const COLORS = [
 
 let canvas: HTMLCanvasElement | null = null;
 
-export function draw(nodeToData: Map<NativeType, Data>): void {
+export function draw(nodeToData: Map<NativeType, Data>, agent: Agent): void {
+  if (window.document == null) {
+    const nodesToDraw = [];
+    iterateNodes(nodeToData, (_, color, node) => {
+      nodesToDraw.push({node, color});
+    });
+
+    agent.emit('drawTraceUpdates', nodesToDraw);
+    return;
+  }
+
   if (canvas === null) {
     initialize();
   }
@@ -40,17 +51,24 @@ export function draw(nodeToData: Map<NativeType, Data>): void {
 
   const context = canvasFlow.getContext('2d');
   context.clearRect(0, 0, canvasFlow.width, canvasFlow.height);
-
-  nodeToData.forEach(({count, rect}) => {
+  iterateNodes(nodeToData, (rect, color) => {
     if (rect !== null) {
-      const colorIndex = Math.min(COLORS.length - 1, count - 1);
-      const color = COLORS[colorIndex];
-
       drawBorder(context, rect, color);
     }
   });
 }
 
+function iterateNodes(
+  nodeToData: Map<NativeType, Data>,
+  execute: (rect: Rect | null, color: string, node: NativeType) => void,
+) {
+  nodeToData.forEach(({count, rect}, node) => {
+    const colorIndex = Math.min(COLORS.length - 1, count - 1);
+    const color = COLORS[colorIndex];
+    execute(rect, color, node);
+  });
+}
+
 function drawBorder(
   context: CanvasRenderingContext2D,
   rect: Rect,
@@ -79,7 +97,12 @@ function drawBorder(
   context.setLineDash([0]);
 }
 
-export function destroy(): void {
+export function destroy(agent: Agent): void {
+  if (window.document == null) {
+    agent.emit('disableTraceUpdates');
+    return;
+  }
+
   if (canvas !== null) {
     if (canvas.parentNode != null) {
       canvas.parentNode.removeChild(canvas);

filename: packages/react-devtools-shared/src/backend/views/TraceUpdates/index.js
patch: @@ -66,7 +66,7 @@ export function toggleEnabled(value: boolean): void {
       redrawTimeoutID = null;
     }
 
-    destroyCanvas();
+    destroyCanvas(agent);
   }
 }
 
@@ -126,7 +126,7 @@ function prepareToDraw(): void {
     }
   });
 
-  draw(nodeToData);
+  draw(nodeToData, agent);
 
   if (earliestExpiration !== Number.MAX_VALUE) {
     redrawTimeoutID = setTimeout(prepareToDraw, earliestExpiration - now);