Exposed a new layouter method that moves the current page when inserting a new one at the same index (#18)

Author: stefanceriu

SCPageViewController/Layouters/SCPageLayouter.m

@@ -91,6 +91,11 @@ - (void)animatePageInsertionAtIndex:(NSUInteger)index
 	}];
 }
 
+- (BOOL)shouldPreserveOffsetForInsertionAtIndex:(NSUInteger)index pageViewController:(SCPageViewController *)pageViewController
+{
+    return YES;
+}
+
 - (void)animatePageDeletionAtIndex:(NSUInteger)index
 					viewController:(UIViewController *)viewController
 				pageViewController:(SCPageViewController *)pageViewController

SCPageViewController/Layouters/SCPageLayouterProtocol.h

@@ -160,6 +160,16 @@ typedef NS_OPTIONS(NSUInteger, SCPageLayouterNavigationContraintType) {
 				 pageViewController:(SCPageViewController *)pageViewController
 						 completion:(void(^)())completion;
 
+/** Called bue the pageViewController when inserting a new page that
+ * would affect the current content offset
+ *
+ * @param index The index where a new page will be inserted
+ * @param pageViewController the calling pageViewController
+ * @return a boolean indicating whether the current offset should be kept
+ */
+- (BOOL)shouldPreserveOffsetForInsertionAtIndex:(NSUInteger)index
+                             pageViewController:(SCPageViewController *)pageViewController;
+
 
 /** Called by the pageViewController when it receives a delete page animated
  * request.

SCPageViewController/SCPageViewController.m

@@ -1101,7 +1101,13 @@ - (void)insertPagesAtIndexes:(NSIndexSet *)indexes animated:(BOOL)animated compl
 
 	__block BOOL shouldAdjustOffset = NO;
 	[indexes enumerateIndexesUsingBlock:^(NSUInteger pageIndex, BOOL *stop) {
-		if(pageIndex <= self.currentPage) {
+        
+        BOOL shouldKeepCurrentPage = YES;
+        if([self.layouter respondsToSelector:@selector(shouldPreserveOffsetForInsertionAtIndex:pageViewController:)]) {
+            shouldKeepCurrentPage = [self.layouter shouldPreserveOffsetForInsertionAtIndex:pageIndex pageViewController:self];
+        }
+        
+		if((shouldKeepCurrentPage && pageIndex <= self.currentPage) || (!shouldKeepCurrentPage && pageIndex < self.currentPage)) {
 			shouldAdjustOffset = YES;
 			*stop = YES;
 		}