iOS flip: cover the page with a screenshot so the user doesnt see cha…

…nges to the webview before the flip. Also, halfway during the flip, remove the screenshot to reveal the changes.

src/ios/NativePageTransitions.m

@@ -229,6 +229,24 @@ - (void) flip:(CDVInvokedUrlCommand*)command {
   // duration is passed in ms, but needs to be in sec here
   duration = duration / 1000;
 
+  // overlay the webview with a screenshot to prevent the user from seeing changes in the webview before the flip kicks in
+  CGSize viewSize = self.viewController.view.bounds.size;
+
+  UIGraphicsBeginImageContextWithOptions(viewSize, YES, 0.0);
+  [self.viewController.view.layer renderInContext:UIGraphicsGetCurrentContext()];
+
+  // Read the UIImage object
+  UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
+  UIGraphicsEndImageContext();
+
+  CGFloat width = self.viewController.view.frame.size.width;
+  CGFloat height = self.viewController.view.frame.size.height;
+  [_screenShotImageView setFrame:CGRectMake(0, 0, width, height)];
+
+  _screenShotImageView = [[UIImageView alloc]initWithFrame:[self.viewController.view.window frame]];
+  [_screenShotImageView setImage:image];
+  [UIApplication.sharedApplication.keyWindow.subviews.lastObject insertSubview:_screenShotImageView aboveSubview:self.webView];
+
   UIViewAnimationOptions animationOptions;
   if ([direction isEqualToString:@"right"]) {
     animationOptions = UIViewAnimationOptionTransitionFlipFromLeft;
@@ -246,9 +264,13 @@ - (void) flip:(CDVInvokedUrlCommand*)command {
 
   if ([self loadHrefIfPassed:href]) {
     dispatch_after(dispatch_time(DISPATCH_TIME_NOW, delay * NSEC_PER_MSEC), dispatch_get_main_queue(), ^{
+      // remove the screenshot halfway during the transition
+      dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (duration/2) * NSEC_PER_MSEC), dispatch_get_main_queue(), ^{
+        [_screenShotImageView removeFromSuperview];
+      });
       [UIView transitionWithView:self.viewController.view
                         duration:duration
-                         options:animationOptions | UIViewAnimationOptionAllowAnimatedContent // that last bit prevents screenshot-based animation (https://developer.apple.com/library/ios/documentation/windowsviews/conceptual/viewpg_iphoneos/animatingviews/animatingviews.html)
+                         options:animationOptions | UIViewAnimationOptionAllowAnimatedContent
                       animations:^{}
                       completion:^(BOOL finished) {
                         CDVPluginResult* pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK];