Lo siguiente es mi intento de usar el formulario de código drekka en el método delegate(UITabBarControllerDelegate)

- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController {

    NSArray *tabViewControllers = tabBarController.viewControllers;
    UIView * fromView = tabBarController.selectedViewController.view;
    UIView * toView = viewController.view;
    if (fromView == toView)
        return false;
    NSUInteger fromIndex = [tabViewControllers indexOfObject:tabBarController.selectedViewController];
    NSUInteger toIndex = [tabViewControllers indexOfObject:viewController];

    [UIView transitionFromView:fromView
                        toView:toView
                      duration:0.3
                       options: toIndex > fromIndex ? UIViewAnimationOptionTransitionFlipFromLeft : UIViewAnimationOptionTransitionFlipFromRight
                    completion:^(BOOL finished) {
                        if (finished) {
                            tabBarController.selectedIndex = toIndex;
                        }
                    }];
    return true;
}

Mi solución para iOS7.0 o superior.

Puede especificar un controlador de animación personalizado en el delegado de la barra de pestañas.

Implementa un controlador de animación como este:

@interface TabSwitchAnimationController : NSObject <UIViewControllerAnimatedTransitioning>

@end

@implementation TabSwitchAnimationController

- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext
{
    return 0.2;
}

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
{
    UIViewController* fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController* toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView* toView = toVC.view;
    UIView* fromView = fromVC.view;

    UIView* containerView = [transitionContext containerView];
    [containerView addSubview:toView];
    toView.frame = [transitionContext finalFrameForViewController:toVC];

    // Animate by fading
    toView.alpha = 0.0;
    [UIView animateWithDuration:[self transitionDuration:transitionContext]
                          delay:0.0
                        options:UIViewAnimationOptionCurveEaseOut | UIViewAnimationOptionAllowUserInteraction
                     animations:^{
                         toView.alpha = 1.0;
                     }
                     completion:^(BOOL finished) {
                         toView.alpha = 1.0;
                         [fromView removeFromSuperview];
                         [transitionContext completeTransition:YES];
                     }];
}

@end

Luego úsalo en tu UITabBarControllerDelegate:

- (id <UIViewControllerAnimatedTransitioning>)tabBarController:(UITabBarController *)tabBarController
            animationControllerForTransitionFromViewController:(UIViewController *)fromVC
                                              toViewController:(UIViewController *)toVC
{
    return [[TabSwitchAnimationController alloc] init];
}

Creo que puede lograr fácilmente transiciones para UITabBarControlelr usando CATransition; Esto también resolverá cualquier efecto secundario de usar transitionFromView: toView:

Use esto dentro de su clase personalizada TabBarController extendida desde UITabBarController.

- (void)tabBarController:(UITabBarController *)tabBarController didSelectViewController (UIViewController*)viewController {

    CATransition *animation = [CATransition animation];
    [animation setType:kCATransitionFade];
    [animation setDuration:0.25];
    [animation setTimingFunction:[CAMediaTimingFunction functionWithName:
                              kCAMediaTimingFunctionEaseIn]];
    [self.view.window.layer addAnimation:animation forKey:@"fadeTransition"];
}

Espero que esto ayude:)

Escribí un post después de probar las diversas respuestas aquí.

El código está en Swift, y puede cambiar programáticamente la pestaña con animación llamando a animateToTab.

func animateToTab(toIndex: Int) {
    let tabViewControllers = viewControllers!
    let fromView = selectedViewController!.view
    let toView = tabViewControllers[toIndex].view    
    let fromIndex = tabViewControllers.indexOf(selectedViewController!)

    guard fromIndex != toIndex else {return}

    // Add the toView to the tab bar view
    fromView.superview!.addSubview(toView)

    // Position toView off screen (to the left/right of fromView)
    let screenWidth = UIScreen.mainScreen().bounds.size.width;
    let scrollRight = toIndex > fromIndex;
    let offset = (scrollRight ? screenWidth : -screenWidth)
    toView.center = CGPoint(x: fromView.center.x + offset, y: toView.center.y)

    // Disable interaction during animation
    view.userInteractionEnabled = false

    UIView.animateWithDuration(0.5, delay: 0.0, usingSpringWithDamping: 1, initialSpringVelocity: 0, options: UIViewAnimationOptions.CurveEaseOut, animations: {

            // Slide the views by -offset
            fromView.center = CGPoint(x: fromView.center.x - offset, y: fromView.center.y);
            toView.center   = CGPoint(x: toView.center.x - offset, y: toView.center.y);

        }, completion: { finished in

            // Remove the old view from the tabbar view.
            fromView.removeFromSuperview()
            self.selectedIndex = toIndex
            self.view.userInteractionEnabled = true
        })
}

Si quieres que todos los cambios de tabulación tengan la animación, entonces enganchalo a UITabBarControllerDelegate como tal:

func tabBarController(tabBarController: UITabBarController, shouldSelectViewController viewController: UIViewController) -> Bool {
    let tabViewControllers = tabBarController.viewControllers!
    guard let toIndex = tabViewControllers.indexOf(viewController) else {
        return false
    }

    // Our method
    animateToTab(toIndex)

    return true
}

En lugar de usar tabBarController:shouldSelectViewController: es mejor implementar tabBarController:animationControllerForTransitionFromViewController:toViewController:

Objeto de transición.swift

import UIKit

class TransitioningObject: NSObject, UIViewControllerAnimatedTransitioning {

    func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
        let fromView: UIView = transitionContext.viewForKey(UITransitionContextFromViewKey)!
        let toView: UIView = transitionContext.viewForKey(UITransitionContextToViewKey)!

        transitionContext.containerView().addSubview(fromView)
        transitionContext.containerView().addSubview(toView)

        UIView.transitionFromView(fromView, toView: toView, duration: transitionDuration(transitionContext), options: UIViewAnimationOptions.TransitionCrossDissolve) { finished in
            transitionContext.completeTransition(true)
        }
    }

    func transitionDuration(transitionContext: UIViewControllerContextTransitioning) -> NSTimeInterval {
        return 0.25
    }
}

TabBarViewController.swift

import UIKit

    class TabBarViewController: UITabBarController, UITabBarControllerDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

        self.delegate = self
    }

    // MARK: - Tabbar delegate

    func tabBarController(tabBarController: UITabBarController, animationControllerForTransitionFromViewController fromVC: UIViewController, toViewController toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return TransitioningObject()
    }
}

Mi solución en Swift:

Cree una clase de barra de pestañas personalizada y configúrela en la barra de pestañas de storyboard

class MainTabBarController: UITabBarController, UITabBarControllerDelegate {

override func viewDidLoad() {
    super.viewDidLoad()
    self.delegate = self
    // Do any additional setup after loading the view.
}

func tabBarController(tabBarController: UITabBarController, shouldSelectViewController viewController: UIViewController) -> Bool {


    let tabViewControllers = tabBarController.viewControllers!
    let fromView = tabBarController.selectedViewController!.view
    let toView = viewController.view

    if (fromView == toView) {
        return false
    }

    let fromIndex = tabViewControllers.indexOf(tabBarController.selectedViewController!)
    let toIndex = tabViewControllers.indexOf(viewController)

    let offScreenRight = CGAffineTransformMakeTranslation(toView.frame.width, 0)
    let offScreenLeft = CGAffineTransformMakeTranslation(-toView.frame.width, 0)

    // start the toView to the right of the screen


    if (toIndex < fromIndex) {
        toView.transform = offScreenLeft
        fromView.transform = offScreenRight
    } else {
        toView.transform = offScreenRight
        fromView.transform = offScreenLeft
    }

    fromView.tag = 124
    toView.addSubview(fromView)

    self.view.userInteractionEnabled = false
    UIView.animateWithDuration(0.5, delay: 0.0, usingSpringWithDamping: 1, initialSpringVelocity: 0, options: UIViewAnimationOptions.CurveEaseOut, animations: {

        toView.transform = CGAffineTransformIdentity

        }, completion: { finished in

            let subViews = toView.subviews
            for subview in subViews{
                if (subview.tag == 124) {
                    subview.removeFromSuperview()
                }
            }
            tabBarController.selectedIndex = toIndex!
            self.view.userInteractionEnabled = true

    })

    return true
 }

}

Una corrección para la animación nerviosa...

UIView * fromView = self.view.superview;

Esto se puede resolver de dos maneras

1 - Escribe esto en tu AppDelegate.m archivo una vez. Recuerda incluir UITabBarControllerDelegate usando después de dos puntos (:) en tu AppDelegate.h

-(void)tabBarController:(UITabBarController *)tabBarControllerThis didSelectViewController:(UIViewController *)viewController
{
    [UIView transitionWithView:viewController.view
                      duration:0.1
                       options:UIViewAnimationOptionAllowUserInteraction | UIViewAnimationOptionTransitionCrossDissolve
                    animations:^(void){
                    } completion:^(BOOL finished){
                        [UIView beginAnimations:@"animation" context:nil];
                        [UIView setAnimationDuration:0.7];
                        [UIView setAnimationBeginsFromCurrentState:YES];
                        [UIView setAnimationTransition:UIViewAnimationTransitionFlipFromLeft
                                               forView:viewController.view
                                                 cache:NO];
                        [UIView commitAnimations];
                    }];
}

2 - Escribe esto en cada uno de tus ViewController.m file

-(void)viewWillAppear:(BOOL)animated
{
    [UIView transitionWithView:self.view
                      duration:1.0
                       options:UIViewAnimationOptionAllowUserInteraction | UIViewAnimationOptionTransitionCrossDissolve
                    animations:^(void){
                        [super viewWillAppear:YES];
                    } completion:^(BOOL finished){
                    }];
}

Espero que esto ayude...!

Usé la solución de @Mofumofu y la actualizé a Swift 1.2 y también implementé una animación arriba / abajo. Es decir, el nuevo ViewController aparece y empuja el antiguo hacia arriba si el índice del nuevo viewcontroller es mayor que el del antiguo.

class TabScrollPageAnimationController: NSObject, UIViewControllerAnimatedTransitioning {

    let tabBarController: UITabBarController

    init(tabBarController: UITabBarController) {
        self.tabBarController = tabBarController
    }

    func transitionDuration(transitionContext: UIViewControllerContextTransitioning) -> NSTimeInterval {
        return 0.5
    }

    func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
        if let fromVC = transitionContext.viewControllerForKey(UITransitionContextFromViewControllerKey),
            let toVC = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey) {
                let fromView = fromVC.view
                let toView = toVC.view

                let containerView = transitionContext.containerView()

                var directionUpwardMultiplier: CGFloat = 1.0
                if let vcs = tabBarController.viewControllers as? [UIViewController],
                    let fIndex = find(vcs, fromVC),
                    let tIndex = find(vcs, toVC) {
                        directionUpwardMultiplier = (fIndex < tIndex) ? +1.0 : -1.0
                }

                containerView.clipsToBounds = false
                containerView.addSubview(toView)

                var fromViewEndFrame = fromView.frame
                fromViewEndFrame.origin.y -= (containerView.frame.height * directionUpwardMultiplier)

                let toViewEndFrame = transitionContext.finalFrameForViewController(toVC)
                var toViewStartFrame = toViewEndFrame
                toViewStartFrame.origin.y += (containerView.frame.height * directionUpwardMultiplier)
                toView.frame = toViewStartFrame

                toView.alpha = 0.0
                UIView.animateWithDuration(transitionDuration(transitionContext), delay: 0.0, usingSpringWithDamping: 1.0, initialSpringVelocity: 0.0, options: UIViewAnimationOptions.CurveEaseInOut, animations: { () -> Void in
                    toView.alpha = 1.0
                    toView.frame = toViewEndFrame
                    fromView.alpha = 0.0
                    fromView.frame = fromViewEndFrame
                }, completion: { (completed) -> Void in
                    toView.alpha = 1.0
                    fromView.removeFromSuperview()
                    transitionContext.completeTransition(completed)
                    containerView.clipsToBounds = true
                })

        }
    }

}

En el Contenedor ViewController:

extension XYViewController: UITabBarControllerDelegate {

    func tabBarController(tabBarController: UITabBarController, animationControllerForTransitionFromViewController fromVC: UIViewController, toViewController toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return TabScrollPageAnimationController(tabBarController: tabBarController)
    }

}

La respuesta de Drekka es realmente genial. Ajusté un poco la transición de desplazamiento para que la animación se pareciera más a la animación push de Apple. Agregué una animación adicional al completar la primera animación para conseguir que el efecto deslizante se vea bien.

// Disable interaction during animation to avoids bugs.
self.tabBarController.view.userInteractionEnabled = NO;

// Get the views.
UIView * fromView = tabBarController.selectedViewController.view;
UIView * toView = [[tabBarController.viewControllers objectAtIndex:controllerIndex] view];

// Get the size of the view area.
CGRect viewSize = fromView.frame;
BOOL scrollRight = controllerIndex > tabBarController.selectedIndex;

// Add the to view to the tab bar view.
[fromView.superview addSubview:toView];
[fromView.superview addSubview:fromView];

self.tabBarController.selectedIndex = 0;

// Position it off screen.
toView.frame = CGRectMake((scrollRight ? (viewSize.size.width *.25) : -(viewSize.size.width * .25 )), viewSize.origin.y, viewSize.size.width, viewSize.size.height);

[UIView animateWithDuration:0.25 
             animations: ^{
                 // Animate the views on and off the screen.
                 [UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];
                 fromView.frame = CGRectMake(viewSize.size.width * .95, viewSize.origin.y, viewSize.size.width, viewSize.size.height);
                 toView.frame = CGRectMake((viewSize.origin.x * .90), viewSize.origin.y, viewSize.size.width, viewSize.size.height);
             }

             completion:^(BOOL finished) {
                 if (finished) {
                     // Being new animation.
                     [UIView animateWithDuration:0.2
                                          animations: ^{
                                              [UIView setAnimationCurve:UIViewAnimationCurveLinear];
                                              fromView.frame = CGRectMake(viewSize.size.width, viewSize.origin.y, viewSize.size.width, viewSize.size.height);
                                              toView.frame = CGRectMake((viewSize.origin.x), viewSize.origin.y, viewSize.size.width, viewSize.size.height);
                                          }
                                          completion:^(BOOL finished) {
                                              if (finished) {
                                                  // Remove the old view from the tabbar view.
                                                  [fromView removeFromSuperview];
                                                  // Restore interaction.
                                                  self.tabBarController.view.userInteractionEnabled = YES;
                                              }
                                          }];
                 }
             }];

Aquí está mi solución Swift 3:

Anulo selectedIndex de mi UITabBarViewController así:

override var selectedIndex: Int{
    get{
        return super.selectedIndex
    }
    set{
        animateToTab(toIndex: newValue)
        super.selectedIndex = newValue
    }
}

Entonces utilizo esta función que imita animación push/pop nativa:

func animateToTab(toIndex: Int) {
    guard let tabViewControllers = viewControllers, tabViewControllers.count > toIndex, let fromViewController = selectedViewController, let fromIndex = tabViewControllers.index(of: fromViewController), fromIndex != toIndex else {return}

    view.isUserInteractionEnabled = false

    let toViewController = tabViewControllers[toIndex]
    let push = toIndex > fromIndex
    let bounds = UIScreen.main.bounds

    let offScreenCenter = CGPoint(x: fromViewController.view.center.x + bounds.width, y: toViewController.view.center.y)
    let partiallyOffCenter = CGPoint(x: fromViewController.view.center.x - bounds.width*0.25, y: fromViewController.view.center.y)

    if push{
        fromViewController.view.superview?.addSubview(toViewController.view)
        toViewController.view.center = offScreenCenter
    }else{
        fromViewController.view.superview?.insertSubview(toViewController.view, belowSubview: fromViewController.view)
        toViewController.view.center = partiallyOffCenter
    }

    UIView.animate(withDuration: 0.5, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 0, options: .curveEaseIn, animations: {
        toViewController.view.center   = fromViewController.view.center
        fromViewController.view.center = push ? partiallyOffCenter : offScreenCenter
    }, completion: { finished in
        fromViewController.view.removeFromSuperview()
        self.view.isUserInteractionEnabled = true
    })
}

Espero que ayude :)

Puede animar dependiendo del Elemento punteado - En este ejemplo, hemos flipFromLeft si el roscado índice es > que el anterior índice seleccionado y nos flipFromRight si el roscado índice es

func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {

    let fromView: UIView = tabBarController.selectedViewController!.view
    let toView: UIView = viewController.view

    if fromView == toView {
        return false
    }

    if let tappedIndex = tabBarController.viewControllers?.index(of: viewController) {
        if tappedIndex > tabBarController.selectedIndex {
            UIView.transition(from: fromView, to: toView, duration: 0.5, options: UIViewAnimationOptions.transitionFlipFromLeft, completion: nil)
        } else {
            UIView.transition(from: fromView, to: toView, duration: 0.5, options: UIViewAnimationOptions.transitionFlipFromRight, completion: nil)
        }
    }
    return true
}

Quería usar una transición de volteo entre dos controladores de vista secundarios al presionar un botón y lo logré de la siguiente manera:

-(IBAction)flipViewControllers:(id)sender{
    NSUInteger index = self.selectedIndex;
    index++;
    if(index >= self.childViewControllers.count){
        index = 0;
    }

    self.selectedIndex = index;

    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:0.75];
    [UIView setAnimationTransition:index % 2 ? UIViewAnimationTransitionFlipFromLeft : UIViewAnimationTransitionFlipFromRight
                           forView:self.view
                             cache:YES];
    [UIView commitAnimations];
}

También establecí el color de fondo en negro, en mi caso lo hice configurando el NavigationController.vista.Color de fondo, pero en su caso podría ser la ventana.Color de fondo que se puede configurar fácilmente en el delegado de la aplicación.

Aquí está mi código de trabajo ( para 3 pestañas , no lo he probado en más!!) para animar transiciones entre pestañas. Se basa principalmente en la solución de drekka, pero ya está implementada en el método delegado de la barra de pestañas, por lo que debería hacer el trabajo si solo lo copia/pega.. (nunca se sabe!)

-(BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController {

// Important! We validate that the selected tab is not the current tab, to avoid misplacing views
if (tabBarController.selectedViewController == viewController) {
    return NO;
}

// Find the selected view's index
NSUInteger controllerIndex = 0;
for (UIViewController *vc in tabBarController.viewControllers) {
    if (vc == viewController) {
        controllerIndex = [tabBarController.viewControllers indexOfObject:vc];
    }
}

CGFloat screenWidth = SCREEN_SIZE.width;

// Note: We must invert the views according to the direction of the scrolling ( FROM Left TO right or FROM right TO left )
UIView * fromView = tabBarController.selectedViewController.view;
UIView * toView = viewController.view;

[fromView.superview addSubview:toView];
CGRect fromViewInitialFrame = fromView.frame;
CGRect fromViewNewframe = fromView.frame;

CGRect toViewInitialFrame = toView.frame;

if ( controllerIndex > tabBarController.selectedIndex ) {
// FROM left TO right ( tab0 to tab1 or tab2 )

    // The final frame for the current view. It will be displaced to the left
    fromViewNewframe.origin.x = -screenWidth;
    // The initial frame for the new view. It will be displaced to the left
    toViewInitialFrame.origin.x = screenWidth;
    toView.frame = toViewInitialFrame;

} else {
// FROM right TO left ( tab2 to tab1 or tab0 )

    // The final frame for the current view. It will be displaced to the right
    fromViewNewframe.origin.x = screenWidth;
    // The initial frame for the new view. It will be displaced to the right
    toViewInitialFrame.origin.x = -screenWidth;
    toView.frame = toViewInitialFrame;
}

[UIView animateWithDuration:0.2 animations:^{
    // The new view will be placed where the initial view was placed
    toView.frame = fromViewInitialFrame;
    // The initial view will be place outside the screen bounds
    fromView.frame = fromViewNewframe;

    tabBarController.selectedIndex = controllerIndex;

    // To prevent user interaction during the animation
    [[UIApplication sharedApplication] beginIgnoringInteractionEvents];

} completion:^(BOOL finished) {

    // Before removing the initial view, we adjust its frame to avoid visual lags
    fromView.frame = CGRectMake(0, 0, fromView.frame.size.width, fromView.frame.size.height);
    [fromView removeFromSuperview];

    [[UIApplication sharedApplication] endIgnoringInteractionEvents];
}];

return NO;

}

Esto funciona para mí en Swift 3:

func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {

    if let fromView = tabBarController.selectedViewController?.view, let toView = viewController.view {

        if fromView == toView {
            return false
        }

        UIView.transition(from: fromView, to: toView, duration: 0.2, options: .transitionCrossDissolve) { (finished) in
        }
    }

    return true
}

@samwize Respuesta traducida a Swift 3 - 2 pulgares hacia arriba en esta, crea un efecto de página de izquierda a wright:

func animateToTab(toIndex: Int) {
        let tabViewControllers = viewControllers!
        let fromView = selectedViewController!.view
        let toView = tabViewControllers[toIndex].view
        let fromIndex = tabViewControllers.index(of: selectedViewController!)

        guard fromIndex != toIndex else {return}

        // Add the toView to the tab bar view
        fromView?.superview!.addSubview(toView!)

        // Position toView off screen (to the left/right of fromView)
        let screenWidth = screenSize.width
        let scrollRight = toIndex > fromIndex!
        let offset = (scrollRight ? screenWidth : -screenWidth)
        toView?.center = CGPoint(x: (fromView?.center.x)! + offset, y: (toView?.center.y)!)

        // Disable interaction during animation
        view.isUserInteractionEnabled = false

        UIView.animate(withDuration: 0.5, delay: 0.0, usingSpringWithDamping: 1, initialSpringVelocity: 0, options: UIViewAnimationOptions.curveEaseOut, animations: {

            // Slide the views by -offset
            fromView?.center = CGPoint(x: (fromView?.center.x)! - offset, y: (fromView?.center.y)!);
            toView?.center   = CGPoint(x: (toView?.center.x)! - offset, y: (toView?.center.y)!);

        }, completion: { finished in

            // Remove the old view from the tabbar view.
            fromView?.removeFromSuperview()
            self.selectedIndex = toIndex
            self.view.isUserInteractionEnabled = true
        })
    }

Swift 4 +

Usted UITabBarControllerDelegate método debe ser así,

func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {

    animateToTab(toIndex: (tabBarController.viewControllers?.index(of: viewController))!)
    return true
}

Y el método es,

func animateToTab(toIndex: Int) {
    let tabViewControllers = viewControllers!
    let fromView = selectedViewController!.view
    let toView = tabViewControllers[toIndex].view
    let fromIndex = tabViewControllers.index(of: selectedViewController!)

    guard fromIndex != toIndex else {return}

    // Add the toView to the tab bar view
    fromView!.superview!.addSubview(toView!)

    // Position toView off screen (to the left/right of fromView)
    let screenWidth = UIScreen.main.bounds.size.width;
    let scrollRight = toIndex > fromIndex!;
    let offset = (scrollRight ? screenWidth : -screenWidth)
    toView!.center = CGPoint(x: fromView!.center.x + offset, y: toView!.center.y)

    // Disable interaction during animation
    view.isUserInteractionEnabled = false

    UIView.animate(withDuration: 0.5, delay: 0.0, usingSpringWithDamping: 1, initialSpringVelocity: 0, options: UIViewAnimationOptions.curveEaseOut, animations: {

        // Slide the views by -offset
        fromView!.center = CGPoint(x: fromView!.center.x - offset, y: fromView!.center.y);
        toView!.center   = CGPoint(x: toView!.center.x - offset, y: toView!.center.y);

    }, completion: { finished in

        // Remove the old view from the tabbar view.
        fromView!.removeFromSuperview()
        self.selectedIndex = toIndex
        self.view.isUserInteractionEnabled = true
    });

}