fix(android): handling initially empty pages (#408)

Author: mikeqcp

android/src/main/java/com/reactnativepagerview/PagerViewViewManager.kt

@@ -101,6 +101,14 @@ class PagerViewViewManager : ViewGroupManager<ViewPager2>() {
       return
     }
     (parent.adapter as FragmentAdapter?)?.addFragment(child, index)
+
+    if (parent.currentItem == index) {
+      // Solves https://github.com/callstack/react-native-pager-view/issues/219
+      // Required so ViewPager actually displays first dynamically added child
+      // (otherwise a white screen is shown until the next user interaction).
+      // https://github.com/facebook/react-native/issues/17968#issuecomment-697136929
+      refreshViewChildrenLayout(parent)
+    }
   }
 
   override fun getChildCount(parent: ViewPager2): Int {

ios/ReactNativePageView.m

@@ -198,11 +198,11 @@ - (UIViewController *)findCachedControllerForView:(UIView *)view {
 }
 
 - (void)updateDataSource {
-    if (!self.currentView) {
+    if (!self.currentView && self.reactSubviews.count == 0) {
         return;
     }
 
-    NSInteger newIndex = [self.reactSubviews indexOfObject:self.currentView];
+    NSInteger newIndex = self.currentView ? [self.reactSubviews indexOfObject:self.currentView] : 0;
 
     if (newIndex == NSNotFound) {
         // Current view was removed