fix(ios): fix onPageSelected event (#299)

Author: troZee

ios/ReactNativePageView.h

@@ -10,6 +10,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher;
 
 @property(nonatomic) NSInteger initialPage;
+@property(nonatomic) NSInteger previousIndex;
 @property(nonatomic) NSInteger currentIndex;
 @property(nonatomic) NSInteger pageMargin;
 @property(nonatomic, readonly) BOOL scrollEnabled;

ios/ReactNativePageView.m

@@ -36,6 +36,7 @@ - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher {
     if (self = [super init]) {
         _scrollEnabled = YES;
         _pageMargin = 0;
+        _previousIndex = -1;
         _transitionStyle = UIPageViewControllerTransitionStyleScroll;
         _orientation = UIPageViewControllerNavigationOrientationHorizontal;
         _currentIndex = 0;
@@ -173,7 +174,10 @@ - (void)setReactViewControllers:(NSInteger)index
         weakSelf.currentView = controller.view;
 
         if (weakSelf.eventDispatcher) {
-            [weakSelf.eventDispatcher sendEvent:[[RCTOnPageSelected alloc] initWithReactTag:weakSelf.reactTag position:@(index) coalescingKey:coalescingKey]];
+            if(_previousIndex != _currentIndex){
+                [weakSelf.eventDispatcher sendEvent:[[RCTOnPageSelected alloc] initWithReactTag:weakSelf.reactTag position:@(index) coalescingKey:coalescingKey]];
+            }
+            _previousIndex = _currentIndex;
         }
 
     }];
@@ -280,7 +284,7 @@ - (void)pageViewController:(UIPageViewController *)pageViewController
         NSUInteger currentIndex = [self.reactSubviews indexOfObject:currentVC.view];
 
         self.currentIndex = currentIndex;
-        
+        self.previousIndex = currentIndex - 1;
         self.currentView = currentVC.view;
         self.reactPageIndicatorView.currentPage = currentIndex;