Dispatch events even when there are no listeners

Summary:
## Rationale
For every 1 call to RCTNetworking.sendRequest, we execute 6 calls to RCTNetworking.addListener. This is followed by at least one call to RCTNetworking.removeListeners. Aside from incrementing and decrementing the `_listeners` integer, these two methods accomplish nothing else: RCTNetworking doesn't implement the `startObserving` and `stopObserving` methods.

This diff makes RCTEventEmitter dispatch events without looking at the listeners integer. In the future, this will allow us to stop making these ~8 unnecessary NativeModule calls for every Network request we send.

Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D24272560

fbshipit-source-id: 7996eba5abfa4669a89c43a3ffa536c0faa214a8

Libraries/Network/RCTNetworking.mm

@@ -159,7 +159,7 @@ RCT_EXPORT_MODULE()
 
 - (instancetype)initWithHandlersProvider:(NSArray<id<RCTURLRequestHandler>> * (^)(void))getHandlers
 {
-  if (self = [super init]) {
+  if (self = [super initWithDisabledObservation]) {
     _handlersProvider = getHandlers;
   }
   return self;

React/Modules/RCTEventEmitter.h

@@ -16,6 +16,8 @@
 
 @property (nonatomic, weak) RCTBridge *bridge;
 
+- (instancetype)initWithDisabledObservation;
+
 /**
  * Override this method to return an array of supported event names. Attempting
  * to observe or send an event that isn't included in this list will result in

React/Modules/RCTEventEmitter.m

@@ -12,6 +12,7 @@
 
 @implementation RCTEventEmitter {
   NSInteger _listenerCount;
+  BOOL _observationDisabled;
 }
 
 @synthesize invokeJS = _invokeJS;
@@ -32,6 +33,13 @@
   }
 }
 
+- (instancetype)initWithDisabledObservation
+{
+  self = [super init];
+  _observationDisabled = YES;
+  return self;
+}
+
 - (NSArray<NSString *> *)supportedEvents
 {
   return nil;
@@ -56,12 +64,15 @@
         [self class],
         [[self supportedEvents] componentsJoinedByString:@"`, `"]);
   }
-  if (_listenerCount > 0 && _bridge) {
+
+  BOOL shouldEmitEvent = (_observationDisabled || _listenerCount > 0);
+
+  if (shouldEmitEvent && _bridge) {
     [_bridge enqueueJSCall:@"RCTDeviceEventEmitter"
                     method:@"emit"
                       args:body ? @[ eventName, body ] : @[ eventName ]
                 completion:NULL];
-  } else if (_listenerCount > 0 && _invokeJS) {
+  } else if (shouldEmitEvent && _invokeJS) {
     _invokeJS(@"RCTDeviceEventEmitter", @"emit", body ? @[ eventName, body ] : @[ eventName ]);
   } else {
     RCTLogWarn(@"Sending `%@` with no listeners registered.", eventName);
@@ -80,6 +91,10 @@
 
 - (void)invalidate
 {
+  if (_observationDisabled) {
+    return;
+  }
+
   if (_listenerCount > 0) {
     [self stopObserving];
   }
@@ -87,6 +102,10 @@
 
 RCT_EXPORT_METHOD(addListener : (NSString *)eventName)
 {
+  if (_observationDisabled) {
+    return;
+  }
+
   if (RCT_DEBUG && ![[self supportedEvents] containsObject:eventName]) {
     RCTLogError(
         @"`%@` is not a supported event type for %@. Supported events are: `%@`",
@@ -102,6 +121,10 @@ RCT_EXPORT_METHOD(addListener : (NSString *)eventName)
 
 RCT_EXPORT_METHOD(removeListeners : (double)count)
 {
+  if (_observationDisabled) {
+    return;
+  }
+
   int currentCount = (int)count;
   if (RCT_DEBUG && currentCount > _listenerCount) {
     RCTLogError(@"Attempted to remove more %@ listeners than added", [self class]);