Just a demo to show how to use runtime functions to implement UIAlertController in iOS 7 or before.

CHAlertViewController.m

//
//  AppDelegate.m
//  RuntimeDemo
//
//  Created by hangchen on 11/7/14.
//  Copyright (c) 2014 hangchen. All rights reserved.
//

#import "AppDelegate.h"
#import <objc/runtime.h>

@interface CHAlertViewController : UIViewController

/*
 * Add the same method declarations in UIAlertController here
 */

@end

@implementation CHAlertViewController

/*
 * Add the same method implementations in UIAlertController here
 */

@end

static void CHRegisterClasses() {
    NSDictionary *map = @{
                          @"UIAlertController": CHAlertViewController.class
                          
                          };
    
    [map enumerateKeysAndObjectsUsingBlock:^(NSString* UIClassName, id CHClass, BOOL *stop) {
        Class UIClass = NSClassFromString(UIClassName);
        if (!UIClass) {
            if ((UIClass = objc_allocateClassPair(CHClass, UIClassName.UTF8String, 0))) {
                objc_registerClassPair(UIClass);
            }
        }
    }];
    
}


@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    CHRegisterClasses();
    //Notice: We can only initialize a UIAlertController instance in this way, you can not use UIAlertController to initialize  instances directly.
    
    UIAlertController *vc = [[NSClassFromString(@"UIAlertController") alloc] init];
    NSLog(@"%@",vc);
    return YES;
}

@end