Cocoa singletons wrapped in one class to allow mocking them.

ApplicationContext.h

//
//  ApplicationContext.h
//
//  Created by Michał Banasiak on 22.07.2013.
//  Copyright (c) 2013 Michał Banasiak. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface ApplicationContext : NSObject

@property(weak, readonly) UIApplication* application;
@property(weak, readonly) NSBundle* bundle;
@property(weak, readonly) NSNotificationCenter* notificationCenter;
@property(weak, readonly) UIScreen* screen;
@property(weak, readonly) UIDevice* device;
@property(weak, readonly) NSFileManager* fileManager;
@property(weak, readonly) NSUserDefaults* userDefaults;

@end

ApplicationContext.m

//
//  ApplicationContext.m
//
//  Created by Michał Banasiak on 22.07.2013.
//  Copyright (c) 2013 Michał Banasiak. All rights reserved.
//

#import "ApplicationContext.h"

@implementation ApplicationContext

- (id)init
{
    self = [super init];

    if (!self)
        return nil;

    _application = [UIApplication sharedApplication];
    _bundle = [NSBundle mainBundle];
    _notificationCenter = [NSNotificationCenter defaultCenter];
    _screen = [UIScreen mainScreen];
    _device = [UIDevice currentDevice];
    _fileManager = [NSFileManager defaultManager];
    _userDefaults = [NSUserDefaults standardUserDefaults];
    
    return self;
}

@end