Refactor subclassing of RCTEventEmitter (facebook#35106)

Summary:
Pull Request resolved: facebook#35106

This Diff remove the assert on the initializer in the EventEmitter in place of a more idiomatic check when the method is invoked.

It aims to solve an internal error and promotes best practices for the iOS platform.

## Changelog:
[iOS][Changed] Refactor RCTEventEmitter initialization

Reviewed By: sammy-SC

Differential Revision: D40762253

fbshipit-source-id: 83d26616ce147914948e536e9e4b1010758fb690

Author: cipolleschi

React/Modules/RCTEventEmitter.m

@@ -23,17 +23,6 @@ + (NSString *)moduleName
   return @"";
 }
 
-+ (void)initialize
-{
-  [super initialize];
-  if (self != [RCTEventEmitter class]) {
-    RCTAssert(
-        RCTClassOverridesInstanceMethod(self, @selector(supportedEvents)),
-        @"You must override the `supportedEvents` method of %@",
-        self);
-  }
-}
-
 - (instancetype)initWithDisabledObservation
 {
   self = [super init];
@@ -43,6 +32,9 @@ - (instancetype)initWithDisabledObservation
 
 - (NSArray<NSString *> *)supportedEvents
 {
+  NSString *message =
+      [NSString stringWithFormat:@"%@ must implement the supportedEvents method", NSStringFromClass(self.class)];
+  [self _log:message];
   return nil;
 }
 
@@ -147,4 +139,15 @@ - (void)invalidate
   }
 }
 
+#pragma mark - Test utilities
+
+// For testing purposes only.
+// This is supposed to be overriden by a subclass in the Tests
+// to verified that the error message is actually emitted.
+// This is the less intrusive way found to mock the RCTLogError function in unit tests.
+- (void)_log:(NSString *)message
+{
+  RCTLogError(@"%@", message);
+}
+
 @end

packages/rn-tester/RNTesterPods.xcodeproj/project.pbxproj

@@ -17,6 +17,7 @@
 		5C60EB1C226440DB0018C04F /* AppDelegate.mm in Sources */ = {isa = PBXBuildFile; fileRef = 5C60EB1B226440DB0018C04F /* AppDelegate.mm */; };
 		5CB07C9B226467E60039471C /* RNTesterTurboModuleProvider.mm in Sources */ = {isa = PBXBuildFile; fileRef = 5CB07C99226467E60039471C /* RNTesterTurboModuleProvider.mm */; };
 		8145AE06241172D900A3F8DA /* LaunchScreen.storyboard in Resources */ = {isa = PBXBuildFile; fileRef = 8145AE05241172D900A3F8DA /* LaunchScreen.storyboard */; };
+		CD10C7A5290BD4EB0033E1ED /* RCTEventEmitterTests.m in Sources */ = {isa = PBXBuildFile; fileRef = CD10C7A4290BD4EB0033E1ED /* RCTEventEmitterTests.m */; };
 		D19DB1AC8A1EBBFA0D14DB66 /* libPods-RNTester.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 25B78D39CC03C49968A739B2 /* libPods-RNTester.a */; };
 		DCD006323AC907670B0D60A1 /* libPods-RNTesterUnitTests.a in Frameworks */ = {isa = PBXBuildFile; fileRef = D4E0A9AD185CE086FAC9BD09 /* libPods-RNTesterUnitTests.a */; };
 		E7C1241A22BEC44B00DA25C0 /* RNTesterIntegrationTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E7C1241922BEC44B00DA25C0 /* RNTesterIntegrationTests.m */; };
@@ -101,6 +102,7 @@
 		65B6EA573FF170102920BEF4 /* libPods-RNTesterIntegrationTests.a */ = {isa = PBXFileReference; explicitFileType = archive.ar; includeInIndex = 0; path = "libPods-RNTesterIntegrationTests.a"; sourceTree = BUILT_PRODUCTS_DIR; };
 		8145AE05241172D900A3F8DA /* LaunchScreen.storyboard */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = file.storyboard; name = LaunchScreen.storyboard; path = RNTester/LaunchScreen.storyboard; sourceTree = "<group>"; };
 		8A0B7257DD8B2945456B0F61 /* Pods-RNTesterUnitTests.release.xcconfig */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.xcconfig; name = "Pods-RNTesterUnitTests.release.xcconfig"; path = "Target Support Files/Pods-RNTesterUnitTests/Pods-RNTesterUnitTests.release.xcconfig"; sourceTree = "<group>"; };
+		CD10C7A4290BD4EB0033E1ED /* RCTEventEmitterTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = RCTEventEmitterTests.m; sourceTree = "<group>"; };
 		D4E0A9AD185CE086FAC9BD09 /* libPods-RNTesterUnitTests.a */ = {isa = PBXFileReference; explicitFileType = archive.ar; includeInIndex = 0; path = "libPods-RNTesterUnitTests.a"; sourceTree = BUILT_PRODUCTS_DIR; };
 		E771AEEA22B44E3100EA1189 /* Info.plist */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.xml; name = Info.plist; path = RNTester/Info.plist; sourceTree = "<group>"; };
 		E7C1241922BEC44B00DA25C0 /* RNTesterIntegrationTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = RNTesterIntegrationTests.m; sourceTree = "<group>"; };
@@ -329,6 +331,7 @@
 		E7DB20A022B2BA84005AC45F /* RNTesterUnitTests */ = {
 			isa = PBXGroup;
 			children = (
+				CD10C7A4290BD4EB0033E1ED /* RCTEventEmitterTests.m */,
 				E7DB20A322B2BA84005AC45F /* Info.plist */,
 				E7DB20C622B2BAA5005AC45F /* RCTAllocationTests.m */,
 				E7DB20B222B2BAA4005AC45F /* RCTAnimationUtilsTests.m */,
@@ -774,6 +777,7 @@
 				E7DB20D722B2BAA6005AC45F /* RCTModuleInitTests.m in Sources */,
 				E7DB20E522B2BAA6005AC45F /* RCTDevMenuTests.m in Sources */,
 				E7DB20DE22B2BAA6005AC45F /* RCTUnicodeDecodeTests.m in Sources */,
+				CD10C7A5290BD4EB0033E1ED /* RCTEventEmitterTests.m in Sources */,
 				E7DB20E422B2BAA6005AC45F /* RCTFormatErrorTests.m in Sources */,
 				E7DB20EB22B2BAA6005AC45F /* RCTConvert_YGValueTests.m in Sources */,
 				E7DB20E922B2BAA6005AC45F /* RCTComponentPropsTests.m in Sources */,

packages/rn-tester/RNTesterUnitTests/RCTEventEmitterTests.m

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import <React/RCTEventEmitter.h>
+#import <XCTest/XCTest.h>
+
+#pragma mark - Faulty EventEmitter
+
+@interface RCTFaultyEventEmitter : RCTEventEmitter
+@end
+
+@implementation RCTFaultyEventEmitter {
+  NSString *_capturedMessage;
+}
+
+- (NSString *)capturedMessage
+{
+  return _capturedMessage;
+}
+
+- (void)_log:(NSString *)message
+{
+  _capturedMessage = message;
+}
+@end
+
+#pragma mark - Proper EventEmitter
+
+@interface RCTProperEventEmitter : RCTEventEmitter
+@end
+
+@implementation RCTProperEventEmitter
+
+- (NSArray<NSString *> *)supportedEvents
+{
+  return @[ @"myEvent" ];
+}
+@end
+
+#pragma mark - Tests Code
+
+@interface RCTEventEmitterTests : XCTestCase
+
+@end
+
+@implementation RCTEventEmitterTests
+
+- (void)testEventEmitterSubclass_whenFaultySubclassInvokesSupportedEvents_raiseException
+{
+  RCTEventEmitter *emitter = [[RCTFaultyEventEmitter alloc] init];
+
+  NSArray<NSString *> *events = emitter.supportedEvents;
+  XCTAssertNil(events);
+  XCTAssertEqualObjects(
+      ((RCTFaultyEventEmitter *)emitter).capturedMessage,
+      @"RCTFaultyEventEmitter must implement the supportedEvents method");
+}
+
+- (void)testEventEmitterSubclass_whenProperSubclassInvokesSupportedEvents_itreturnsTheEvents
+{
+  RCTEventEmitter *emitter = [[RCTProperEventEmitter alloc] init];
+
+  NSArray<NSString *> *events = emitter.supportedEvents;
+
+  XCTAssertEqualObjects(events, @[ @"myEvent" ]);
+}
+
+@end