Avoids compile time errors about the using of `[UIApplication sharedApplication]` in an app extension.

UIApplication+RSKSharedApplication.h

//
// UIApplication+RSKSharedApplication.h
//
// Copyright (c) 2015 Ruslan Skorb, http://ruslanskorb.com/
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//

#import <UIKit/UIKit.h>

/**
 The category `RSKSharedApplication` of the class `UIApplication` provides the method `rsk_sharedApplication` which returns `nil` in an application extension, otherwise it returns the singleton app instance.
 */
@interface UIApplication (RSKSharedApplication)

/**
 Returns `nil` in an application extension, otherwise returns the singleton app instance.
 
 @return `nil` in an application extension, otherwise the app instance is created in the `UIApplicationMain` function.
 */
+ (UIApplication *)rsk_sharedApplication;

@end

UIApplication+RSKSharedApplication.m

//
// UIApplication+RSKSharedApplication.m
//
// Copyright (c) 2015 Ruslan Skorb, http://ruslanskorb.com/
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//

#import "UIApplication+RSKSharedApplication.h"
#import <objc/runtime.h>

@implementation UIApplication (RSKSharedApplication)

+ (void)load
{
    // When you build an extension based on an Xcode template, you get an extension bundle that ends in .appex.
    // https://developer.apple.com/library/ios/documentation/General/Conceptual/ExtensibilityPG/ExtensionCreation.html
    if (![[[NSBundle mainBundle] bundlePath] hasSuffix:@".appex"]) {
        Method sharedApplicationMethod = class_getClassMethod([UIApplication class], @selector(sharedApplication));
        if (sharedApplicationMethod != NULL) {
            IMP sharedApplicationMethodImplementation = method_getImplementation(sharedApplicationMethod);
            Method rsk_sharedApplicationMethod = class_getClassMethod([UIApplication class], @selector(rsk_sharedApplication));
            method_setImplementation(rsk_sharedApplicationMethod, sharedApplicationMethodImplementation);
        }
    }
}

+ (UIApplication *)rsk_sharedApplication
{
    return nil;
}

@end

@shergin The method sharedApplication is available in the App Extension. It is only marked as unavailable for developers. Because of this UIApplication.performSelector("sharedApplication") will return UIApplication object. But its use is prohibited.
I think your method will work safely if you add a check of the suffix of the bundle path. As I mentioned earlier on stackoverflow when you build an extension based on an Xcode template, you get an extension bundle that ends in .appex.