Synchronize access to ViewManagerRegistry

Summary:
Found that we may do multiple allocations of the same ViewManager instance since ViewManagers are both accessed from the UI thread (mounting views) and from the Fabric background thread (for measuring Text), which could lead to multiple instances of the same ViewManager to be created.

As far as I can tell, this issue was harmless since our ViewManager constructors don't have side-effects, but not ideal.

Changelog: [Internall]

Reviewed By: rshest

Differential Revision: D43661306

fbshipit-source-id: 37ef82d41d43c334fdc6cfbeffb225bba87c668e

ReactAndroid/src/main/java/com/facebook/react/uimanager/ViewManagerRegistry.java

@@ -12,6 +12,7 @@
 import androidx.annotation.Nullable;
 import com.facebook.react.bridge.UiThreadUtil;
 import com.facebook.react.common.MapBuilder;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
@@ -51,7 +52,7 @@ public ViewManagerRegistry(Map<String, ViewManager> viewManagerMap) {
    *     view manager registered for the className received as a parameter.
    * @return the {@link ViewManager} registered to the className received as a parameter
    */
-  public ViewManager get(String className) {
+  public synchronized ViewManager get(String className) {
     ViewManager viewManager = mViewManagers.get(className);
     if (viewManager != null) {
       return viewManager;
@@ -84,7 +85,7 @@ public ViewManager get(String className) {
    *     there is no ViewManager associated to the className received as a parameter.
    */
   @Nullable
-  ViewManager getViewManagerIfExists(String className) {
+  /* package */ synchronized ViewManager getViewManagerIfExists(String className) {
     ViewManager viewManager = mViewManagers.get(className);
     if (viewManager != null) {
       return viewManager;
@@ -97,12 +98,16 @@ ViewManager getViewManagerIfExists(String className) {
 
   /** Send lifecycle signal to all ViewManagers that StopSurface has been called. */
   public void onSurfaceStopped(final int surfaceId) {
+    List<ViewManager> viewManagers;
+    synchronized (this) {
+      viewManagers = new ArrayList<>(mViewManagers.values());
+    }
     Runnable runnable =
         new Runnable() {
           @Override
           public void run() {
-            for (Map.Entry<String, ViewManager> entry : mViewManagers.entrySet()) {
-              entry.getValue().onSurfaceStopped(surfaceId);
+            for (ViewManager viewManager : viewManagers) {
+              viewManager.onSurfaceStopped(surfaceId);
             }
           }
         };
@@ -116,12 +121,16 @@ public void run() {
   /** ComponentCallbacks2 method. */
   @Override
   public void onTrimMemory(int level) {
+    List<ViewManager> viewManagers;
+    synchronized (this) {
+      viewManagers = new ArrayList<>(mViewManagers.values());
+    }
     Runnable runnable =
         new Runnable() {
           @Override
           public void run() {
-            for (Map.Entry<String, ViewManager> entry : mViewManagers.entrySet()) {
-              entry.getValue().trimMemory();
+            for (ViewManager viewManager : viewManagers) {
+              viewManager.trimMemory();
             }
           }
         };