Refactor performance memory API implementation

Summary:
This diff refactors performance memory API and the RN Tester example.

- The returned value from C++ is number, so no need to cast
- Reuse `MemoryInfo` in RNTester example

Changelog:
[General][Internal] - Code refactor for performance memory API implementation

Reviewed By: rubennorte

Differential Revision: D43523878

fbshipit-source-id: 37d1f6a829a8eac45f7e3791ad36be0c199c6041

Libraries/WebPerformance/MemoryInfo.js

@@ -31,14 +31,23 @@ export default class MemoryInfo {
     }
   }
 
+  /**
+   * The maximum size of the heap, in bytes, that is available to the context
+   */
   get jsHeapSizeLimit(): ?number {
     return this._jsHeapSizeLimit;
   }
 
+  /**
+   * The total allocated heap size, in bytes
+   */
   get totalJSHeapSize(): ?number {
     return this._totalJSHeapSize;
   }
 
+  /**
+   * The currently active segment of JS heap, in bytes.
+   */
   get usedJSHeapSize(): ?number {
     return this._usedJSHeapSize;
   }

Libraries/WebPerformance/Performance.js

@@ -110,14 +110,15 @@ export default class Performance {
       const memoryInfo = NativePerformance.getSimpleMemoryInfo();
       if (memoryInfo.hasOwnProperty('hermes_heapSize')) {
         // We got memory information from Hermes
-        const {hermes_heapSize, hermes_allocatedBytes} = memoryInfo;
-        const totalJSHeapSize = Number(hermes_heapSize);
-        const usedJSHeapSize = Number(hermes_allocatedBytes);
+        const {
+          hermes_heapSize: totalJSHeapSize,
+          hermes_allocatedBytes: usedJSHeapSize,
+        } = memoryInfo;
 
         return new MemoryInfo({
           jsHeapSizeLimit: null, // We don't know the heap size limit from Hermes.
-          totalJSHeapSize: isNaN(totalJSHeapSize) ? null : totalJSHeapSize,
-          usedJSHeapSize: isNaN(usedJSHeapSize) ? null : usedJSHeapSize,
+          totalJSHeapSize,
+          usedJSHeapSize,
         });
       } else {
         // JSC and V8 has no native implementations for memory information in JSI::Instrumentation

packages/rn-tester/js/examples/Performance/PerformanceApiExample.js

@@ -10,6 +10,7 @@
  */
 
 'use strict';
+import type MemoryInfo from '../../../../../Libraries/WebPerformance/MemoryInfo';
 import type ReactNativeStartupTiming from '../../../../../Libraries/WebPerformance/ReactNativeStartupTiming';
 
 import * as React from 'react';
@@ -22,11 +23,7 @@ const performance = new Performance();
 
 function MemoryExample(): React.Node {
   // Memory API testing
-  const [memoryInfo, setMemoryInfo] = useState<{
-    jsHeapSizeLimit?: ?number,
-    totalJSHeapSize?: ?number,
-    usedJSHeapSize?: ?number,
-  }>({});
+  const [memoryInfo, setMemoryInfo] = useState<?MemoryInfo>(null);
   const onGetMemoryInfo = useCallback(() => {
     // performance.memory is not included in bom.js yet.
     // Once we release the change in flow this can be removed.
@@ -40,25 +37,13 @@ function MemoryExample(): React.Node {
         <Button onPress={onGetMemoryInfo} title="Click to update memory info" />
         <View>
           <Text>
-            {`jsHeapSizeLimit: ${
-              memoryInfo.jsHeapSizeLimit == null
-                ? 'N/A'
-                : memoryInfo.jsHeapSizeLimit
-            } bytes`}
+            {`jsHeapSizeLimit: ${String(memoryInfo?.jsHeapSizeLimit)} bytes`}
           </Text>
           <Text>
-            {`totalJSHeapSize: ${
-              memoryInfo.totalJSHeapSize == null
-                ? 'N/A'
-                : memoryInfo.totalJSHeapSize
-            } bytes`}
+            {`totalJSHeapSize: ${String(memoryInfo?.totalJSHeapSize)} bytes`}
           </Text>
           <Text>
-            {`usedJSHeapSize: ${
-              memoryInfo.usedJSHeapSize == null
-                ? 'N/A'
-                : memoryInfo.usedJSHeapSize
-            } bytes`}
+            {`usedJSHeapSize: ${String(memoryInfo?.usedJSHeapSize)} bytes`}
           </Text>
         </View>
       </View>