Fix cannot working Modal's onDismiss

Summary:
Fixes: facebook#29455

Modal's onDismiss is not called on iOS.
This issue occurred the commit facebook@bd2b7d6 and was fixed the commit facebook@27a3248.
However, the master and stable-0.63 branches do not have this modified commit applied to them.

<!-- Help reviewers and the release process by writing your own changelog entry. For an example, see:
https://github.com/facebook/react-native/wiki/Changelog
-->

[iOS] [Fixed] - Modal's onDismiss prop will now be called successfully.

Test Plan:
Tested on iOS with this change:

1. Set function Modal's onDismiss prop.
1. Set Modal's visible props is true. (show Modal)
1. Set Modal's visible props is false. (close Modal)
1. The set function in onDismiss is called.

Libraries/Modal/Modal.js

@@ -13,6 +13,9 @@
 const AppContainer = require('../ReactNative/AppContainer');
 const I18nManager = require('../ReactNative/I18nManager');
 const PropTypes = require('prop-types');
+import NativeEventEmitter from '../EventEmitter/NativeEventEmitter';
+import NativeModalManager from './NativeModalManager';
+const Platform = require('../Utilities/Platform');
 const React = require('react');
 const ScrollView = require('../Components/ScrollView/ScrollView');
 const StyleSheet = require('../StyleSheet/StyleSheet');
@@ -22,6 +25,12 @@ import type {ViewProps} from '../Components/View/ViewPropTypes';
 import type {DirectEventHandler} from '../Types/CodegenTypes';
 import type EmitterSubscription from '../vendor/emitter/EmitterSubscription';
 import RCTModalHostView from './RCTModalHostViewNativeComponent';
+
+const ModalEventEmitter =
+  Platform.OS === 'ios' && NativeModalManager != null
+    ? new NativeEventEmitter(NativeModalManager)
+    : null;
+
 /**
  * The Modal component is a simple way to present content above an enclosing view.
  *
@@ -173,9 +182,22 @@ class Modal extends React.Component<Props> {
     };
   }
 
+  componentDidMount() {
+    if (ModalEventEmitter) {
+      this._eventSubscription = ModalEventEmitter.addListener(
+        'modalDismissed',
+        event => {
+          if (event.modalID === this._identifier && this.props.onDismiss) {
+            this.props.onDismiss();
+          }
+        },
+      );
+    }
+  }
+
   componentWillUnmount() {
-    if (this.props.onDismiss != null) {
-      this.props.onDismiss();
+    if (this._eventSubscription) {
+      this._eventSubscription.remove();
     }
   }
 

Libraries/Modal/RCTModalHostViewNativeComponent.js

@@ -15,6 +15,7 @@ import type {HostComponent} from '../Renderer/shims/ReactNativeTypes';
 import type {
   WithDefault,
   DirectEventHandler,
+  BubblingEventHandler,
   Int32,
 } from '../Types/CodegenTypes';
 
@@ -86,6 +87,14 @@ type NativeProps = $ReadOnly<{|
    */
   onShow?: ?DirectEventHandler<null>,
 
+  /**
+   * The `onDismiss` prop allows passing a function that will be called once
+   * the modal has been dismissed.
+   *
+   * See https://reactnative.dev/docs/modal.html#ondismiss
+   */
+  onDismiss?: ?BubblingEventHandler<null>,
+
   /**
    * Deprecated. Use the `animationType` prop instead.
    */

React/Views/RCTModalHostViewManager.m

@@ -10,6 +10,7 @@
 #import "RCTBridge.h"
 #import "RCTModalHostView.h"
 #import "RCTModalHostViewController.h"
+#import "RCTModalManager.h"
 #import "RCTShadowView.h"
 #import "RCTUtils.h"
 
@@ -89,10 +90,15 @@ - (void)dismissModalHostView:(RCTModalHostView *)modalHostView
           withViewController:(RCTModalHostViewController *)viewController
                     animated:(BOOL)animated
 {
+  dispatch_block_t completionBlock = ^{
+    if (modalHostView.identifier) {
+      [[self.bridge moduleForClass:[RCTModalManager class]] modalDismissed:modalHostView.identifier];
+    }
+  };
   if (_dismissalBlock) {
-    _dismissalBlock([modalHostView reactViewController], viewController, animated, nil);
+    _dismissalBlock([modalHostView reactViewController], viewController, animated, completionBlock);
   } else {
-    [viewController.presentingViewController dismissViewControllerAnimated:animated completion:nil];
+    [viewController.presentingViewController dismissViewControllerAnimated:animated completion:completionBlock];
   }
 }
 

React/Views/RCTModalManager.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import <UIKit/UIKit.h>
+
+#import <React/RCTBridgeModule.h>
+#import <React/RCTEventEmitter.h>
+
+@interface RCTModalManager : RCTEventEmitter <RCTBridgeModule>
+
+- (void)modalDismissed:(NSNumber *)modalID;
+
+@end

React/Views/RCTModalManager.m

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import "RCTModalManager.h"
+
+@interface RCTModalManager ()
+
+@property BOOL shouldEmit;
+
+@end
+
+@implementation RCTModalManager
+
+RCT_EXPORT_MODULE();
+
+- (NSArray<NSString *> *)supportedEvents
+{
+  return @[ @"modalDismissed" ];
+}
+
+- (void)startObserving
+{
+  _shouldEmit = YES;
+}
+
+- (void)stopObserving
+{
+  _shouldEmit = NO;
+}
+
+- (void)modalDismissed:(NSNumber *)modalID
+{
+  if (_shouldEmit) {
+    [self sendEventWithName:@"modalDismissed" body:@{@"modalID" : modalID}];
+  }
+}
+
+@end