UIViewController+Alerts Category for presenting a view controller with a transparency above another view controller

UIViewController+Alerts.h

//
//  UIViewController+Alerts.h
//
//  Created by Dave Wood on 2012-01-17.
//
//  Simplified BSD License
//  Copyright (c) 2012, Cerebral Gardens
//  All rights reserved.
//  
//  Redistribution and use in source and binary forms, with or without
//  modification, are permitted provided that the following conditions are met: 
//  
//  1. Redistributions of source code must retain the above copyright notice, this
//     list of conditions and the following disclaimer. 
//  2. Redistributions in binary form must reproduce the above copyright notice,
//     this list of conditions and the following disclaimer in the documentation
//     and/or other materials provided with the distribution. 
//  
//  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
//  ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
//  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
//  DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
//  ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
//  (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
//  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
//  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
//  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
//  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//  
//  The views and conclusions contained in the software and documentation are those
//  of the authors and should not be interpreted as representing official policies, 
//  either expressed or implied, of the FreeBSD Project.
// 

#import <UIKit/UIKit.h>

@interface UIViewController (Alerts)

- (void)presentAlertViewController:(UIViewController *)alertViewController named:(NSString *)key animated:(BOOL)animated;
- (void)dismissAlertViewControllerNamed:(NSString *)key animated:(BOOL)animated;

@end

UIViewController+Alerts.m

//
//  UIViewController+Alerts.m
//
//  Created by Dave Wood on 2012-01-17.
//
//  Simplified BSD License
//  Copyright (c) 2012, Cerebral Gardens
//  All rights reserved.
//  
//  Redistribution and use in source and binary forms, with or without
//  modification, are permitted provided that the following conditions are met: 
//  
//  1. Redistributions of source code must retain the above copyright notice, this
//     list of conditions and the following disclaimer. 
//  2. Redistributions in binary form must reproduce the above copyright notice,
//     this list of conditions and the following disclaimer in the documentation
//     and/or other materials provided with the distribution. 
//  
//  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
//  ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
//  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
//  DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
//  ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
//  (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
//  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
//  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
//  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
//  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//  
//  The views and conclusions contained in the software and documentation are those
//  of the authors and should not be interpreted as representing official policies, 
//  either expressed or implied, of the FreeBSD Project.
// 

#import "UIViewController+Alerts.h"
#import <objc/runtime.h>

static const char *alertViewControllersKey = "alertViewControllersKey";

@implementation UIViewController (Alerts)

- (void)presentAlertViewController:(UIViewController *)alertViewController named:(NSString *)key animated:(BOOL)animated
{
    NSMutableDictionary *alertViewControllers = objc_getAssociatedObject(self, alertViewControllersKey);
    if (!alertViewControllers)
    {
        alertViewControllers = [[NSMutableDictionary alloc] init];
        objc_setAssociatedObject(self, alertViewControllersKey, alertViewControllers, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    
    // Check for an existing object with the same key
    UIViewController *oldViewController = [alertViewControllers objectForKey:key];
    if (oldViewController)
    {
        [self dismissAlertViewControllerNamed:key animated:YES];
    }
    
    // store the alertViewController in our array
    [alertViewControllers setObject:alertViewController forKey:key];
    [self addChildViewController:alertViewController];

    // Add new view to our view stack
    CGFloat originalAlpha = alertViewController.view.alpha;
    alertViewController.view.alpha = 0;
    [self.view addSubview:alertViewController.view];

    [UIView animateWithDuration:(animated ? 0.3 : 0.0) animations:^{
        alertViewController.view.alpha = originalAlpha;
    }];
}

- (void)dismissAlertViewControllerNamed:(NSString *)key animated:(BOOL)animated
{
    NSMutableDictionary *alertViewControllers = objc_getAssociatedObject(self, alertViewControllersKey);
    if (alertViewControllers)
    {
        UIViewController *alertViewController = [alertViewControllers objectForKey:key];

        if (alertViewController)
        {
            // animate out of position
            CGFloat originalAlpha = alertViewController.view.alpha;
            [UIView animateWithDuration:(animated ? 0.3 : 0.0) animations:^{
                alertViewController.view.alpha = 0;
            } completion:^(BOOL finished) {
                // remove the view controller from the list
                [alertViewController.view removeFromSuperview];
                alertViewController.view.alpha = originalAlpha;
                [alertViewController removeFromParentViewController];
                [alertViewControllers removeObjectForKey:key];
            }];
        }
    }
}

@end