Introducing `RCTBridgeWillBeInvalidatedNotification`

Summary:
This diff introduces a new broadcasting event for Bridge that communicates that the bridge is being invalidated. This notification can be used by interested parties as a signal to start tearing down all bridge-dependant processes.

The biggest difference between the new event and `RCTBridgeWillInvalidateModulesNotification` is that the latter is being called asynchronously on the JavaScript thread, whereas the new one is being called on whatever thread caused the Bridge invalidation (deallocation) *synchronously*. In most cases that happens on the main thread that allows destroying strictly-main-threaded infra (mostly UI) synchronously and avoids races.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D18050889

fbshipit-source-id: 9364c820ea2b0358edf45f0e2b3e16a13d730a9c

React/Base/RCTBridge.h

@@ -102,13 +102,19 @@ RCT_EXTERN NSString *const RCTBridgeWillInvalidateModulesNotification;
  */
 RCT_EXTERN NSString *const RCTBridgeDidInvalidateModulesNotification;
 
+/**
+ * This notification fires right before the bridge starting invalidation process.
+ * Handle this notification to perform additional invalidation.
+ * The notification can be issued on any thread.
+ */
+RCT_EXTERN NSString *const RCTBridgeWillBeInvalidatedNotification;
+
 /**
  * Key for the RCTSource object in the RCTBridgeDidDownloadScriptNotification
  * userInfo dictionary.
  */
 RCT_EXTERN NSString *const RCTBridgeDidDownloadScriptNotificationSourceKey;
 
-
 /**
  * Key for the reload reason in the RCTBridgeWillReloadNotification userInfo dictionary.
  */

React/Base/RCTBridge.m

@@ -36,6 +36,7 @@ NSString *const RCTBridgeWillDownloadScriptNotification = @"RCTBridgeWillDownloa
 NSString *const RCTBridgeDidDownloadScriptNotification = @"RCTBridgeDidDownloadScriptNotification";
 NSString *const RCTBridgeWillInvalidateModulesNotification = @"RCTBridgeWillInvalidateModulesNotification";
 NSString *const RCTBridgeDidInvalidateModulesNotification = @"RCTBridgeDidInvalidateModulesNotification";
+NSString *const RCTBridgeWillBeInvalidatedNotification = @"RCTBridgeWillBeInvalidatedNotification";
 NSString *const RCTBridgeDidDownloadScriptNotificationSourceKey = @"source";
 NSString *const RCTBridgeDidDownloadScriptNotificationReasonKey = @"reason";
 NSString *const RCTBridgeDidDownloadScriptNotificationBridgeDescriptionKey = @"bridgeDescription";
@@ -386,6 +387,9 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 
 - (void)invalidate
 {
+  [[NSNotificationCenter defaultCenter] postNotificationName:RCTBridgeWillBeInvalidatedNotification
+                                                      object:self];
+
   RCTBridge *batchedBridge = self.batchedBridge;
   self.batchedBridge = nil;