Introducing RCTWeakViewHolder [4/n]

Summary:
Problem: In paper, there is a handy API called `[uiManager viewForReactTag:]`. Fabric does not have this mapping. The Fabric interop layer still relies on this Paper mapping.

Solution: As a workaround, re-create this mapping in the Fabric interop layer. Therefore, whenever Fabric interop layer asks a paper view manager to create a view, store a weak reference to the view in a `NSMapTable`. NSMapTable allows us to customize the strong/weak relationship. I've added a comment explaining that `RCTWeakViewHolder` only needs to be used for this special circumstance.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D27438899

fbshipit-source-id: 94663ef06479a8c863ce58b0f36d42109fa1c4f3

React/Fabric/Mounting/ComponentViews/LegacyViewManagerInterop/RCTLegacyViewManagerInteropCoordinatorAdapter.mm

@@ -31,9 +31,7 @@
 - (UIView *)paperView
 {
   if (!_paperView) {
-    _paperView = _coordinator.paperView;
-    _paperView.reactTag = [NSNumber numberWithInteger:_tag];
-
+    _paperView = [_coordinator createPaperViewWithTag:_tag];
     __weak __typeof(self) weakSelf = self;
     [_coordinator addObserveForTag:_tag
                         usingBlock:^(std::string eventName, folly::dynamic event) {

React/Views/RCTWeakViewHolder.h

@@ -0,0 +1,19 @@
+/*
+ * 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.
+ */
+
+/*
+ * THIS IS A HACK
+ * This protocol should only be used for Paper ViewManagers which need to
+ * use the Fabric Interop layer, and also need to reference their created
+ * views. This protocol allows the view manager to store a weak reference to
+ * any created views. It will not affect view lifecycle in any way.
+ */
+@protocol RCTWeakViewHolder
+
+@property (nonatomic, strong) NSMapTable<NSNumber *, UIView *> *weakViews;
+
+@end

ReactCommon/react/renderer/components/legacyviewmanagerinterop/RCTLegacyViewManagerInteropCoordinator.h

@@ -20,7 +20,7 @@ typedef void (^InterceptorBlock)(std::string eventName, folly::dynamic event);
 
 - (instancetype)initWithComponentData:(RCTComponentData *)componentData bridge:(RCTBridge *)bridge;
 
-- (UIView *)paperView;
+- (UIView *)createPaperViewWithTag:(NSInteger)tag;
 
 - (void)addObserveForTag:(NSInteger)tag usingBlock:(InterceptorBlock)block;
 

ReactCommon/react/renderer/components/legacyviewmanagerinterop/RCTLegacyViewManagerInteropCoordinator.mm

@@ -16,6 +16,7 @@
 #include <React/RCTUIManager.h>
 #include <React/RCTUIManagerUtils.h>
 #include <React/RCTUtils.h>
+#include <React/RCTWeakViewHolder.h>
 #include <folly/json.h>
 #include <objc/runtime.h>
 
@@ -70,10 +71,17 @@ using namespace facebook::react;
   [_eventInterceptors removeObjectForKey:[NSNumber numberWithInteger:tag]];
 }
 
-- (UIView *)paperView
+- (UIView *)createPaperViewWithTag:(NSInteger)tag;
 {
-  // TODO: pass in the right tags?
-  return [_componentData createViewWithTag:NULL rootTag:NULL];
+  UIView *view = [_componentData createViewWithTag:[NSNumber numberWithInteger:tag] rootTag:NULL];
+  if ([_componentData.bridgelessViewManager conformsToProtocol:@protocol(RCTWeakViewHolder)]) {
+    id<RCTWeakViewHolder> weakViewHolder = (id<RCTWeakViewHolder>)_componentData.bridgelessViewManager;
+    if (!weakViewHolder.weakViews) {
+      weakViewHolder.weakViews = [NSMapTable strongToWeakObjectsMapTable];
+    }
+    [weakViewHolder.weakViews setObject:view forKey:[NSNumber numberWithInteger:tag]];
+  }
+  return view;
 }
 
 - (void)setProps:(folly::dynamic const &)props forView:(UIView *)view