not call UIView.animate if no animation

According to UIView's document, completion block in UIView.animate is
performed at the beginning of the next run loop cycle.
This will cause an obvious pause when switching to a view controller not
next to the current one.

Author: wwwjfy

Pod/Classes/PagingMenuController.swift

@@ -175,16 +175,22 @@ open class PagingMenuController: UIViewController {
         pagingViewController.currentViewController = nextPagingViewController
 
         let duration = animated ? options.animationDuration : 0
-        UIView.animate(withDuration: duration, animations: {
-            () -> Void in
+        let animationClosure = {
             pagingViewController.positionMenuController()
-            }) { [weak self] (_) -> Void in
-                pagingViewController.relayoutPagingViewControllers()
-                
-                // show paging views
-                self?.showPagingMenuControllers()
-                
-                self?.onMove?(.didMoveController(to: nextPagingViewController, from: previousPagingViewController))
+        }
+        let completionClosure = { [weak self] (_: Bool) -> Void in
+            pagingViewController.relayoutPagingViewControllers()
+
+            // show paging views
+            self?.showPagingMenuControllers()
+
+            self?.onMove?(.didMoveController(to: nextPagingViewController, from: previousPagingViewController))
+        }
+        if duration > 0 {
+            UIView.animate(withDuration: duration, animations: animationClosure, completion: completionClosure)
+        } else {
+            animationClosure()
+            completionClosure(true)
         }
     }