Fix onDismiss in Modal

Summary:
# Disclaimer:
I might be missing something as the solution I implemented here seems like something that was considered by original author. If this solution isn't good, I have a plan B.

# Problem:
`onDismiss` prop isn't being called once the modal is dismissed, this diff fixes it.

Also I've noticed that `onDismiss` is meant to only work on iOS, why is that? By landing this diff, it'll be called on Android as well so we need to change the docs (https://facebook.github.io/react-native/docs/modal.html#ondismiss).

## Video that shows the problem
Following code is in playground.js P70222409 which just increments number everytime onDismiss is called

{F166303269}

Reviewed By: shergin

Differential Revision: D16109536

fbshipit-source-id: 3fba56f5671912387b217f03b613dffd89614c9d

Libraries/Modal/Modal.js

@@ -12,20 +12,13 @@
 
 const AppContainer = require('../ReactNative/AppContainer');
 const I18nManager = require('../ReactNative/I18nManager');
-const NativeEventEmitter = require('../EventEmitter/NativeEventEmitter');
-import NativeModalManager from './NativeModalManager';
-const Platform = require('../Utilities/Platform');
 const React = require('react');
 const PropTypes = require('prop-types');
 const ScrollView = require('../Components/ScrollView/ScrollView');
 const StyleSheet = require('../StyleSheet/StyleSheet');
 const View = require('../Components/View/View');
 
 import RCTModalHostView from './RCTModalHostViewNativeComponent';
-const ModalEventEmitter =
-  Platform.OS === 'ios' && NativeModalManager != null
-    ? new NativeEventEmitter(NativeModalManager)
-    : null;
 
 import type EmitterSubscription from '../vendor/emitter/EmitterSubscription';
 import type {ViewProps} from '../Components/View/ViewPropTypes';
@@ -175,22 +168,9 @@ 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._eventSubscription) {
-      this._eventSubscription.remove();
+    if (this.props.onDismiss != null) {
+      this.props.onDismiss();
     }
   }
 

Libraries/Modal/RCTModalHostViewNativeComponent.js

@@ -78,14 +78,6 @@ 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://facebook.github.io/react-native/docs/modal.html#ondismiss
-   */
-  onDismiss?: ?BubblingEventHandler<null>,
-
   /**
    * Deprecated. Use the `animationType` prop instead.
    */

React/Views/RCTModalHostViewManager.m

@@ -10,7 +10,6 @@
 #import "RCTBridge.h"
 #import "RCTModalHostView.h"
 #import "RCTModalHostViewController.h"
-#import "RCTModalManager.h"
 #import "RCTShadowView.h"
 #import "RCTUtils.h"
 
@@ -81,15 +80,10 @@ - (void)presentModalHostView:(RCTModalHostView *)modalHostView withViewControlle
 
 - (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, completionBlock);
+    _dismissalBlock([modalHostView reactViewController], viewController, animated, nil);
   } else {
-    [viewController.presentingViewController dismissViewControllerAnimated:animated completion:completionBlock];
+    [viewController.presentingViewController dismissViewControllerAnimated:animated completion:nil];
   }
 }