Apple TV: RCTTabBar selection controlled by native after render (fix facebook#15081)

Summary:
**Motivation**

Fix flickering in TabBarIOS on Apple TV... issue facebook#15081

After this change, on Apple TV, TabBarIOS item selections will be controlled purely from the native side after initial render with the `selected` prop.  This is necessary because the `UITabBar` implementation in tvOS moves the selection before calling `shouldSelectViewController:`; this issue does not occur on iOS.

**Test plan**

Existing CI should still pass. Issue is resolved when testing the example code in facebook#15081 .
Closes facebook#15220

Differential Revision: D5601671

Pulled By: javache

fbshipit-source-id: c18e7d3482d6c07d534ff40a443a6f642d4267bb

Author: Douglas Lowder

React/Views/RCTTabBar.m

@@ -115,9 +115,17 @@ - (void)reactBridgeDidFinishTransaction
     [tab.barItem setTitleTextAttributes:@{NSForegroundColorAttributeName: self.tintColor} forState:UIControlStateSelected];
 
     controller.tabBarItem = tab.barItem;
+#if TARGET_OS_TV
+// On Apple TV, disable JS control of selection after initial render
+    if (tab.selected && !tab.wasSelectedInJS) {
+      self->_tabController.selectedViewController = controller;
+    }
+    tab.wasSelectedInJS = YES;
+#else
     if (tab.selected) {
       self->_tabController.selectedViewController = controller;
     }
+#endif
   }];
 }
 
@@ -175,6 +183,18 @@ - (void)setItemPositioning:(UITabBarItemPositioning)itemPositioning
 
 #pragma mark - UITabBarControllerDelegate
 
+#if TARGET_OS_TV
+
+- (void)tabBarController:(UITabBarController *)tabBarController didSelectViewController:(nonnull UIViewController *)viewController
+{
+  NSUInteger index = [tabBarController.viewControllers indexOfObject:viewController];
+  RCTTabBarItem *tab = (RCTTabBarItem *)self.reactSubviews[index];
+  if (tab.onPress) tab.onPress(nil);
+  return;
+}
+
+#else
+
 - (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController
 {
   NSUInteger index = [tabBarController.viewControllers indexOfObject:viewController];
@@ -183,6 +203,8 @@ - (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectView
   return NO;
 }
 
+#endif
+
 #if TARGET_OS_TV
 
 - (BOOL)isUserInteractionEnabled

React/Views/RCTTabBarItem.h

@@ -29,4 +29,8 @@
 @property (nonatomic, readonly) UITabBarItem *barItem;
 @property (nonatomic, copy) RCTBubblingEventBlock onPress;
 
+#if TARGET_OS_TV
+@property (nonatomic, assign) BOOL wasSelectedInJS;
+#endif
+
 @end

React/Views/RCTTabBarItem.m

@@ -42,6 +42,9 @@ - (instancetype)initWithFrame:(CGRect)frame
 {
   if ((self = [super initWithFrame:frame])) {
     _systemIcon = NSNotFound;
+#if TARGET_OS_TV
+    _wasSelectedInJS = NO;
+#endif
   }
   return self;
 }
@@ -118,4 +121,16 @@ - (UIViewController *)reactViewController
   return self.superview.reactViewController;
 }
 
+#if TARGET_OS_TV
+
+// On Apple TV, we let native control the tab bar selection after initial render
+- (void)setSelected:(BOOL)selected
+{
+  if (!_wasSelectedInJS) {
+    _selected = selected;
+  }
+}
+
+#endif
+
 @end

docs/BuildingForAppleTV.md

@@ -89,6 +89,8 @@ class Game2048 extends React.Component {
 
 - *Back navigation with the TV remote menu button*: The `BackHandler` component, originally written to support the Android back button, now also supports back navigation on the Apple TV using the menu button on the TV remote.
 
+- *TabBarIOS behavior*: The `TabBarIOS` component wraps the native `UITabBar` API, which works differently on Apple TV.  To avoid jittery rerendering of the tab bar in tvOS (see [this issue](https://github.com/facebook/react-native/issues/15081)), the selected tab bar item can only be set from Javascript on initial render, and is controlled after that by the user through native code.
+
 - *Known issues*:
 
   - [ListView scrolling](https://github.com/facebook/react-native/issues/12793).  The issue can be easily worked around by setting `removeClippedSubviews` to false in ListView and similar components.  For more discussion of this issue, see [this PR](https://github.com/facebook/react-native/pull/12944).