- (void)setContentViewController:(UIViewController *)viewController {
    for(UIViewController *childViewController in self.childViewControllers) {
        [childViewController willMoveToParentViewController:nil];
        [childViewController.view removeFromSuperview];
    }
    [self.childViewControllers makeObjectsPerformSelector:@selector(removeFromParentViewController)];

    [self addChildViewController:viewController];
    [viewController didMoveToParentViewController:self];

    CGRect rect = CGRectMake(0.0, 0.0, 300.0, 200.0);
    [viewController.view setFrame:rect];
    [viewController.view setCenter:self.view.center];
    [self.view addSubview:viewController.view];
}