[core] Add listenerCount function to the new EventEmitter (#27876)

packages/expo-modules-core/common/cpp/EventEmitter.cpp

@@ -130,6 +130,13 @@ void emitEvent(jsi::Runtime &runtime, const jsi::Object &emitter, const std::str
   }
 }
 
+size_t getListenerCount(jsi::Runtime &runtime, const jsi::Object &emitter, const std::string &eventName) {
+  if (NativeState::Shared state = NativeState::get(runtime, emitter, false)) {
+    return state->listeners.listenersCount(eventName);
+  }
+  return 0;
+}
+
 jsi::Value createEventSubscription(jsi::Runtime &runtime, const std::string &eventName, const jsi::Object &emitter, const jsi::Function &listener) {
   jsi::Object subscription(runtime);
   jsi::PropNameID removeProp = jsi::PropNameID::forAscii(runtime, "remove", 6);
@@ -213,7 +220,7 @@ void installClass(jsi::Runtime &runtime) {
     std::string eventName = args[0].asString(runtime).utf8(runtime);
 
     // Unwrap `this` object if it's a lazy object (e.g. native module).
-    const jsi::Object &thisObject = thisValue.getObject(runtime);
+    const jsi::Object &thisObject = LazyObject::unwrapObjectIfNecessary(runtime, thisValue.getObject(runtime));
 
     // Make a new pointer that skips the first argument which is the event name.
     const jsi::Value *eventArgs = count > 1 ? &args[1] : nullptr;
@@ -222,15 +229,26 @@ void installClass(jsi::Runtime &runtime) {
     return jsi::Value::undefined();
   };
 
+  jsi::HostFunctionType listenerCountHost = [](jsi::Runtime &runtime, const jsi::Value &thisValue, const jsi::Value *args, size_t count) -> jsi::Value {
+    std::string eventName = args[0].asString(runtime).utf8(runtime);
+
+    // Unwrap `this` object if it's a lazy object (e.g. native module).
+    const jsi::Object &thisObject = LazyObject::unwrapObjectIfNecessary(runtime, thisValue.getObject(runtime));
+
+    return jsi::Value((int)getListenerCount(runtime, thisObject, eventName));
+  };
+
   jsi::PropNameID addListenerProp = jsi::PropNameID::forAscii(runtime, "addListener", 11);
   jsi::PropNameID removeListenerProp = jsi::PropNameID::forAscii(runtime, "removeListener", 14);
   jsi::PropNameID removeAllListenersProp = jsi::PropNameID::forAscii(runtime, "removeAllListeners", 18);
   jsi::PropNameID emitProp = jsi::PropNameID::forAscii(runtime, "emit", 4);
+  jsi::PropNameID listenerCountProp = jsi::PropNameID::forAscii(runtime, "listenerCount", 13);
 
   prototype.setProperty(runtime, addListenerProp, jsi::Function::createFromHostFunction(runtime, addListenerProp, 2, addListenerHost));
   prototype.setProperty(runtime, removeListenerProp, jsi::Function::createFromHostFunction(runtime, removeListenerProp, 2, removeListenerHost));
   prototype.setProperty(runtime, removeAllListenersProp, jsi::Function::createFromHostFunction(runtime, removeAllListenersProp, 1, removeAllListenersHost));
   prototype.setProperty(runtime, emitProp, jsi::Function::createFromHostFunction(runtime, emitProp, 2, emit));
+  prototype.setProperty(runtime, listenerCountProp, jsi::Function::createFromHostFunction(runtime, listenerCountProp, 1, listenerCountHost));
 
   common::getCoreObject(runtime)
     .setProperty(runtime, "EventEmitter", eventEmitterClass);

packages/expo-modules-core/common/cpp/EventEmitter.h

@@ -20,6 +20,7 @@ class Listeners {
   friend void removeListener(jsi::Runtime &runtime, const jsi::Object &emitter, const std::string &eventName, const jsi::Function &listener);
   friend void removeAllListeners(jsi::Runtime &runtime, const jsi::Object &emitter, const std::string &eventName);
   friend void emitEvent(jsi::Runtime &runtime, const jsi::Object &emitter, const std::string &eventName, const jsi::Value *args, size_t count);
+  friend size_t getListenerCount(jsi::Runtime &runtime, const jsi::Object &emitter, const std::string &eventName);
 
   /**
    Type of the list containing listeners for the specific event name.

packages/expo-modules-core/ios/Tests/EventEmitterSpec.swift

@@ -87,6 +87,38 @@ final class EventEmitterSpec: ExpoSpec {
         expect(args[2]) == 24
       }
 
+      it("returns listener count with listeners added") {
+        let listenerCount = try runtime.eval([
+          "emitter = new expo.EventEmitter()",
+          "emitter.addListener('test', () => {})",
+          "emitter.addListener('test', () => {})",
+          "emitter.listenerCount('test')"
+        ])
+        expect(listenerCount.kind) == .number
+        expect(try listenerCount.asInt()) == 2
+      }
+
+      it("returns listener count without any listeners") {
+        let listenerCount = try runtime.eval([
+          "emitter = new expo.EventEmitter()",
+          "emitter.listenerCount('test')"
+        ])
+        expect(listenerCount.kind) == .number
+        expect(try listenerCount.asInt()) == 0
+      }
+
+      it("returns listener count for the proper event") {
+        let listenerCount = try runtime.eval([
+          "emitter = new expo.EventEmitter()",
+          "emitter.addListener('test1', () => {})",
+          "emitter.addListener('test1', () => {})",
+          "emitter.addListener('test2', () => {})",
+          "emitter.listenerCount('test2')"
+        ])
+        expect(listenerCount.kind) == .number
+        expect(try listenerCount.asInt()) == 1
+      }
+
       it("calls startObserving on addListener") {
         var calls: Int = 0
         let eventName = "testEvent"