Adds the ability to use UIManager to check if a node is an ancestor

Summary:
Sometimes is handy to check if a React node is a descendant of another node or not. For instance, I want to check if the focused `TextInput` is descendant of an specific `ScrollView`:

```js
const currentlyFocusedField = TextInput.State.currentlyFocusedField()
UIManager.viewIsAncestorOf(
  currentlyFocusedField,
  this.getInnerViewNode(),
  (isAncestor) => {
    if (isAncestor) {
      console.log('The focused field is a descendant of this ScrollView!')
    }
  }
)
```

This function uses the same strategy as the `measureLayout` method to check if one node is an ancestor of other node. As the `measureLayout` method, this is performed outside the main thread.

By now I've only implemented the iOS version and its tests, but if this function is going to be merged I'll implement the Android version too. I have objc experience but no Java or Android, so I prefer to validate this functionality before jumping into developing the Android part.
Closes https://github.com/facebook/react-native/pull/7876

Differential Revision: D3662045

Pulled By: javache

fbshipit-source-id: b9668e8ea94fd01db76651f16243926cf9c2566f

Examples/UIExplorer/UIExplorerUnitTests/RCTShadowViewTests.m

@@ -106,6 +106,29 @@
   XCTAssertTrue(CGRectEqualToRect([rightView measureLayoutRelativeToAncestor:self.parentView], CGRectMake(330, 120, 100, 200)));
 }
 
+- (void)testAncestorCheck
+{
+  RCTShadowView *centerView = [self _shadowViewWithConfig:^(CSSNodeRef node) {
+    CSSNodeStyleSetFlex(node, 1);
+  }];
+  
+  RCTShadowView *mainView = [self _shadowViewWithConfig:^(CSSNodeRef node) {
+    CSSNodeStyleSetFlex(node, 1);
+  }];
+  
+  [mainView insertReactSubview:centerView atIndex:0];
+
+  RCTShadowView *footerView = [self _shadowViewWithConfig:^(CSSNodeRef node) {
+    CSSNodeStyleSetFlex(node, 1);
+  }];
+  
+  [self.parentView insertReactSubview:mainView atIndex:0];
+  [self.parentView insertReactSubview:footerView atIndex:1];
+  
+  XCTAssertTrue([centerView viewIsDescendantOf:mainView]);
+  XCTAssertFalse([footerView viewIsDescendantOf:mainView]);
+}
+
 - (void)testAssignsSuggestedWidthDimension
 {
   [self _withShadowViewWithStyle:^(CSSNodeRef node) {

React/Modules/RCTUIManager.m

@@ -1226,6 +1226,26 @@ RCT_EXPORT_METHOD(measureInWindow:(nonnull NSNumber *)reactTag
   }];
 }
 
+/**
+ * Returs if the shadow view provided has the `ancestor` shadow view as
+ * an actual ancestor.
+ */
+RCT_EXPORT_METHOD(viewIsDescendantOf:(nonnull NSNumber *)reactTag
+                  ancestor:(nonnull NSNumber *)ancestorReactTag
+                  callback:(RCTResponseSenderBlock)callback)
+{
+  RCTShadowView *shadowView = _shadowViewRegistry[reactTag];
+  RCTShadowView *ancestorShadowView = _shadowViewRegistry[ancestorReactTag];
+  if (!shadowView) {
+    return;
+  }
+  if (!ancestorShadowView) {
+    return;
+  }
+  BOOL viewIsAncestor = [shadowView viewIsDescendantOf:ancestorShadowView];
+  callback(@[@(viewIsAncestor)]);
+}
+
 static void RCTMeasureLayout(RCTShadowView *view,
                              RCTShadowView *ancestor,
                              RCTResponseSenderBlock callback)

React/Views/RCTShadowView.h

@@ -212,4 +212,9 @@ typedef void (^RCTApplierBlock)(NSDictionary<NSNumber *, UIView *> *viewRegistry
  */
 - (CGRect)measureLayoutRelativeToAncestor:(RCTShadowView *)ancestor;
 
+/**
+ * Checks if the current shadow view is a descendant of the provided `ancestor`
+ */
+- (BOOL)viewIsDescendantOf:(RCTShadowView *)ancestor;
+
 @end

React/Views/RCTShadowView.m

@@ -279,6 +279,17 @@ DEFINE_PROCESS_META_PROPS(Border);
   return (CGRect){offset, self.frame.size};
 }
 
+- (BOOL)viewIsDescendantOf:(RCTShadowView *)ancestor
+{
+  NSInteger depth = 30; // max depth to search
+  RCTShadowView *shadowView = self;
+  while (depth && shadowView && shadowView != ancestor) {
+    shadowView = shadowView->_superview;
+    depth--;
+  }
+  return ancestor == shadowView;
+}
+
 - (instancetype)init
 {
   if ((self = [super init])) {