Send onDismiss event when Modal is dismissed

Summary:
Changelog: [internal]

"onDismiss" event wasn't called in Fabric. This diff adds it.
Paper implementation of Modal uses `RCTEventEmitter` instead of callback to deliver the event. To align better with Paper, Fabric will follow this pattern.

Reviewed By: shergin

Differential Revision: D26911312

fbshipit-source-id: b0de619c5a02c3378d1f7ac3ce1b705bb5fb634d

Author: sammy-SC

React/Fabric/Mounting/ComponentViews/Modal/RCTModalHostViewComponentView.h

@@ -27,4 +27,9 @@
  */
 - (void)dismissViewController:(UIViewController *)modalViewController animated:(BOOL)animated;
 
+/**
+ * Should be called by subclasses when ViewController is dismissed.
+ */
+- (void)didDismissViewController;
+
 @end

React/Fabric/Mounting/ComponentViews/Modal/RCTModalHostViewComponentView.mm

@@ -7,6 +7,8 @@
 
 #import "RCTModalHostViewComponentView.h"
 
+#import <React/RCTBridge+Private.h>
+#import <React/RCTModalManager.h>
 #import <React/UIView+React.h>
 #import <react/renderer/components/modal/ModalHostViewComponentDescriptor.h>
 #import <react/renderer/components/modal/ModalHostViewState.h>
@@ -137,7 +139,16 @@ - (void)presentViewController:(UIViewController *)modalViewController
 
 - (void)dismissViewController:(UIViewController *)modalViewController animated:(BOOL)animated
 {
-  [modalViewController dismissViewControllerAnimated:animated completion:nil];
+  [modalViewController dismissViewControllerAnimated:animated
+                                          completion:^{
+                                            [self didDismissViewController];
+                                          }];
+}
+
+- (void)didDismissViewController
+{
+  const auto &props = *std::static_pointer_cast<ModalHostViewProps const>(_props);
+  [[RCTBridge currentBridge].modalManager modalDismissed:@(props.identifier)];
 }
 
 - (void)ensurePresentedOnlyIfNeeded

React/Views/RCTModalManager.h

@@ -7,6 +7,7 @@
 
 #import <UIKit/UIKit.h>
 
+#import <React/RCTBridge.h>
 #import <React/RCTBridgeModule.h>
 #import <React/RCTEventEmitter.h>
 
@@ -15,3 +16,9 @@
 - (void)modalDismissed:(NSNumber *)modalID;
 
 @end
+
+@interface RCTBridge (RCTModalManager)
+
+@property (nonatomic, readonly) RCTModalManager *modalManager;
+
+@end

React/Views/RCTModalManager.m

@@ -40,3 +40,12 @@ - (void)modalDismissed:(NSNumber *)modalID
 }
 
 @end
+
+@implementation RCTBridge (RCTDevSettings)
+
+- (RCTModalManager *)modalManager
+{
+  return [self moduleForClass:[RCTModalManager class]];
+}
+
+@end