Make RCTEventEmitter a RCTJSInvokerModule [2/n]

Summary:
Diff 1/N explained how calls to `[_bridge enqueueJSCall:]` can be replaced with `_invokeJS` in a bridgeless world.

This diff replaces RCTEventEmitter's usage of `enqueueJSCall` if the bridge isn't available.

Changelog: [Internal][iOS] Make RCTEventEmitter a RCTJSInvokerModule

Reviewed By: RSNara

Differential Revision: D18941955

fbshipit-source-id: 1b81e46585432e005cff5aa0ab4d151f50ea051b

React/Modules/RCTEventEmitter.h

@@ -6,12 +6,13 @@
  */
 
 #import <React/RCTBridge.h>
+#import <React/RCTJSInvokerModule.h>
 
 /**
  * RCTEventEmitter is an abstract base class to be used for modules that emit
  * events to be observed by JS.
  */
-@interface RCTEventEmitter : NSObject <RCTBridgeModule>
+@interface RCTEventEmitter : NSObject <RCTBridgeModule, RCTJSInvokerModule>
 
 @property (nonatomic, weak) RCTBridge *bridge;
 

React/Modules/RCTEventEmitter.m

@@ -15,6 +15,8 @@
   NSInteger _listenerCount;
 }
 
+@synthesize invokeJS = _invokeJS;
+
 + (NSString *)moduleName
 {
   return @"";
@@ -35,7 +37,7 @@
 
 - (void)sendEventWithName:(NSString *)eventName body:(id)body
 {
-  RCTAssert(_bridge != nil, @"Error when sending event: %@ with body: %@. "
+  RCTAssert(_bridge != nil || _invokeJS != nil, @"Error when sending event: %@ with body: %@. "
             "Bridge is not set. This is probably because you've "
             "explicitly synthesized the bridge in %@, even though it's inherited "
             "from RCTEventEmitter.", eventName, body, [self class]);
@@ -44,11 +46,13 @@
     RCTLogError(@"`%@` is not a supported event type for %@. Supported events are: `%@`",
                 eventName, [self class], [[self supportedEvents] componentsJoinedByString:@"`, `"]);
   }
-  if (_listenerCount > 0) {
+  if (_listenerCount > 0 && _bridge) {
     [_bridge enqueueJSCall:@"RCTDeviceEventEmitter"
                     method:@"emit"
                       args:body ? @[eventName, body] : @[eventName]
                 completion:NULL];
+  } else if (_listenerCount > 0 && _invokeJS) {
+    _invokeJS(@"RCTDeviceEventEmitter", @"emit", body ? @[eventName, body] : @[eventName]);
   } else {
     RCTLogWarn(@"Sending `%@` with no listeners registered.", eventName);
   }